README - platform/external/u-boot - Gitiles

 # SPDX-License-Identifier: GPL-2.0+
 #
 # (C) Copyright 2000 - 2013
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.

 Summary:
 ========

 This directory contains the source code for U-Boot, a boot loader for
 Embedded boards based on PowerPC, ARM, MIPS and several other
 processors, which can be installed in a boot ROM and used to
 initialize and test the hardware or to download and run application
 code.

 The development of U-Boot is closely related to Linux: some parts of
 the source code originate in the Linux source tree, we have some
 header files in common, and special provision has been made to
 support booting of Linux images.

 Some attention has been paid to make this software easily
 configurable and extendable. For instance, all monitor commands are
 implemented with the same call interface, so that it's very easy to
 add new commands. Also, instead of permanently adding rarely used
 code (for instance hardware test utilities) to the monitor, you can
 load and run it dynamically.


 Status:
 =======

 In general, all boards for which a configuration option exists in the
 Makefile have been tested to some extent and can be considered
 "working". In fact, many of them are used in production systems.

 In case of problems see the CHANGELOG file to find out who contributed
 the specific port. In addition, there are various MAINTAINERS files
 scattered throughout the U-Boot source identifying the people or
 companies responsible for various boards and subsystems.

 Note: As of August, 2010, there is no longer a CHANGELOG file in the
 actual U-Boot source tree; however, it can be created dynamically
 from the Git log using:

 	make CHANGELOG


 Where to get help:
 ==================

 In case you have questions about, problems with or contributions for
 U-Boot, you should send a message to the U-Boot mailing list at
 <u-boot@lists.denx.de>. There is also an archive of previous traffic
 on the mailing list - please search the archive before asking FAQ's.
 Please see https://lists.denx.de/pipermail/u-boot and
 https://marc.info/?l=u-boot

 Where to get source code:
 =========================

 The U-Boot source code is maintained in the Git repository at
 https://source.denx.de/u-boot/u-boot.git ; you can browse it online at
 https://source.denx.de/u-boot/u-boot

 The "Tags" links on this page allow you to download tarballs of
 any version you might be interested in. Official releases are also
 available from the DENX file server through HTTPS or FTP.
 https://ftp.denx.de/pub/u-boot/
 ftp://ftp.denx.de/pub/u-boot/


 Where we come from:
 ===================

 - start from 8xxrom sources
 - create PPCBoot project (https://sourceforge.net/projects/ppcboot)
 - clean up code
 - make it easier to add custom boards
 - make it possible to add other [PowerPC] CPUs
 - extend functions, especially:
   * Provide extended interface to Linux boot loader
   * S-Record download
   * network boot
   * ATA disk / SCSI ... boot
 - create ARMBoot project (https://sourceforge.net/projects/armboot)
 - add other CPU families (starting with ARM)
 - create U-Boot project (https://sourceforge.net/projects/u-boot)
 - current project page: see https://www.denx.de/wiki/U-Boot


 Names and Spelling:
 ===================

 The "official" name of this project is "Das U-Boot". The spelling
 "U-Boot" shall be used in all written text (documentation, comments
 in source files etc.). Example:

 	This is the README file for the U-Boot project.

 File names etc. shall be based on the string "u-boot". Examples:

 	include/asm-ppc/u-boot.h

 	#include <asm/u-boot.h>

 Variable names, preprocessor constants etc. shall be either based on
 the string "u_boot" or on "U_BOOT". Example:

 	U_BOOT_VERSION		u_boot_logo
 	IH_OS_U_BOOT		u_boot_hush_start


 Versioning:
 ===========

 Starting with the release in October 2008, the names of the releases
 were changed from numerical release numbers without deeper meaning
 into a time stamp based numbering. Regular releases are identified by
 names consisting of the calendar year and month of the release date.
 Additional fields (if present) indicate release candidates or bug fix
 releases in "stable" maintenance trees.

 Examples:
 	U-Boot v2009.11	    - Release November 2009
 	U-Boot v2009.11.1   - Release 1 in version November 2009 stable tree
 	U-Boot v2010.09-rc1 - Release candidate 1 for September 2010 release


 Directory Hierarchy:
 ====================

 /arch			Architecture-specific files
   /arc			Files generic to ARC architecture
   /arm			Files generic to ARM architecture
   /m68k			Files generic to m68k architecture
   /microblaze		Files generic to microblaze architecture
   /mips			Files generic to MIPS architecture
   /nios2		Files generic to Altera NIOS2 architecture
   /powerpc		Files generic to PowerPC architecture
   /riscv		Files generic to RISC-V architecture
   /sandbox		Files generic to HW-independent "sandbox"
   /sh			Files generic to SH architecture
   /x86			Files generic to x86 architecture
   /xtensa		Files generic to Xtensa architecture
 /api			Machine/arch-independent API for external apps
 /board			Board-dependent files
 /boot			Support for images and booting
 /cmd			U-Boot commands functions
 /common			Misc architecture-independent functions
 /configs		Board default configuration files
 /disk			Code for disk drive partition handling
 /doc			Documentation (a mix of ReST and READMEs)
 /drivers		Device drivers
 /dts			Makefile for building internal U-Boot fdt.
 /env			Environment support
 /examples		Example code for standalone applications, etc.
 /fs			Filesystem code (cramfs, ext2, jffs2, etc.)
 /include		Header Files
 /lib			Library routines generic to all architectures
 /Licenses		Various license files
 /net			Networking code
 /post			Power On Self Test
 /scripts		Various build scripts and Makefiles
 /test			Various unit test files
 /tools			Tools to build and sign FIT images, etc.

 Software Configuration:
 =======================

 Selection of Processor Architecture and Board Type:
 ---------------------------------------------------

 For all supported boards there are ready-to-use default
 configurations available; just type "make <board_name>_defconfig".

 Example: For a TQM823L module type:

 	cd u-boot
 	make TQM823L_defconfig

 Note: If you're looking for the default configuration file for a board
 you're sure used to be there but is now missing, check the file
 doc/README.scrapyard for a list of no longer supported boards.

 Sandbox Environment:
 --------------------

 U-Boot can be built natively to run on a Linux host using the 'sandbox'
 board. This allows feature development which is not board- or architecture-
 specific to be undertaken on a native platform. The sandbox is also used to
 run some of U-Boot's tests.

 See doc/arch/sandbox.rst for more details.


 Board Initialisation Flow:
 --------------------------

 This is the intended start-up flow for boards. This should apply for both
 SPL and U-Boot proper (i.e. they both follow the same rules).

 Note: "SPL" stands for "Secondary Program Loader," which is explained in
 more detail later in this file.

 At present, SPL mostly uses a separate code path, but the function names
 and roles of each function are the same. Some boards or architectures
 may not conform to this.  At least most ARM boards which use
 CONFIG_SPL_FRAMEWORK conform to this.

 Execution typically starts with an architecture-specific (and possibly
 CPU-specific) start.S file, such as:

 	- arch/arm/cpu/armv7/start.S
 	- arch/powerpc/cpu/mpc83xx/start.S
 	- arch/mips/cpu/start.S

 and so on. From there, three functions are called; the purpose and
 limitations of each of these functions are described below.

 lowlevel_init():
 	- purpose: essential init to permit execution to reach board_init_f()
 	- no global_data or BSS
 	- there is no stack (ARMv7 may have one but it will soon be removed)
 	- must not set up SDRAM or use console
 	- must only do the bare minimum to allow execution to continue to
 		board_init_f()
 	- this is almost never needed
 	- return normally from this function

 board_init_f():
 	- purpose: set up the machine ready for running board_init_r():
 		i.e. SDRAM and serial UART
 	- global_data is available
 	- stack is in SRAM
 	- BSS is not available, so you cannot use global/static variables,
 		only stack variables and global_data

 	Non-SPL-specific notes:
 	- dram_init() is called to set up DRAM. If already done in SPL this
 		can do nothing

 	SPL-specific notes:
 	- you can override the entire board_init_f() function with your own
 		version as needed.
 	- preloader_console_init() can be called here in extremis
 	- should set up SDRAM, and anything needed to make the UART work
 	- there is no need to clear BSS, it will be done by crt0.S
 	- for specific scenarios on certain architectures an early BSS *can*
 	  be made available (via CONFIG_SPL_EARLY_BSS by moving the clearing
 	  of BSS prior to entering board_init_f()) but doing so is discouraged.
 	  Instead it is strongly recommended to architect any code changes
 	  or additions such to not depend on the availability of BSS during
 	  board_init_f() as indicated in other sections of this README to
 	  maintain compatibility and consistency across the entire code base.
 	- must return normally from this function (don't call board_init_r()
 		directly)

 Here the BSS is cleared. For SPL, if CONFIG_SPL_STACK_R is defined, then at
 this point the stack and global_data are relocated to below
 CONFIG_SPL_STACK_R_ADDR. For non-SPL, U-Boot is relocated to run at the top of
 memory.

 board_init_r():
 	- purpose: main execution, common code
 	- global_data is available
 	- SDRAM is available
 	- BSS is available, all static/global variables can be used
 	- execution eventually continues to main_loop()

 	Non-SPL-specific notes:
 	- U-Boot is relocated to the top of memory and is now running from
 		there.

 	SPL-specific notes:
 	- stack is optionally in SDRAM, if CONFIG_SPL_STACK_R is defined and
 		CONFIG_SYS_FSL_HAS_CCI400

 		Defined For SoC that has cache coherent interconnect
 		CCN-400

 		CONFIG_SYS_FSL_HAS_CCN504

 		Defined for SoC that has cache coherent interconnect CCN-504

 The following options need to be configured:

 - CPU Type:	Define exactly one, e.g. CONFIG_MPC85XX.

 - Board Type:	Define exactly one, e.g. CONFIG_MPC8540ADS.

 - 85xx CPU Options:
 		CONFIG_SYS_PPC64

 		Specifies that the core is a 64-bit PowerPC implementation (implements
 		the "64" category of the Power ISA). This is necessary for ePAPR
 		compliance, among other possible reasons.

 		CONFIG_SYS_FSL_ERRATUM_A004510

 		Enables a workaround for erratum A004510.  If set,
 		then CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV and
 		CFG_SYS_FSL_CORENET_SNOOPVEC_COREONLY must be set.

 		CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV
 		CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV2 (optional)

 		Defines one or two SoC revisions (low 8 bits of SVR)
 		for which the A004510 workaround should be applied.

 		The rest of SVR is either not relevant to the decision
 		of whether the erratum is present (e.g. p2040 versus
 		p2041) or is implied by the build target, which controls
 		whether CONFIG_SYS_FSL_ERRATUM_A004510 is set.

 		See Freescale App Note 4493 for more information about
 		this erratum.

 		CFG_SYS_FSL_CORENET_SNOOPVEC_COREONLY

 		This is the value to write into CCSR offset 0x18600
 		according to the A004510 workaround.

 		CONFIG_SYS_FSL_SINGLE_SOURCE_CLK
 		Single Source Clock is clocking mode present in some of FSL SoC's.
 		In this mode, a single differential clock is used to supply
 		clocks to the sysclock, ddrclock and usbclock.

 - Generic CPU options:

 		CONFIG_SYS_FSL_DDR
 		Freescale DDR driver in use. This type of DDR controller is
 		found in mpc83xx, mpc85xx as well as some ARM core SoCs.

 		CFG_SYS_FSL_DDR_ADDR
 		Freescale DDR memory-mapped register base.

 		CONFIG_SYS_FSL_IFC_CLK_DIV
 		Defines divider of platform clock(clock input to IFC controller).

 		CONFIG_SYS_FSL_LBC_CLK_DIV
 		Defines divider of platform clock(clock input to eLBC controller).

 		CFG_SYS_FSL_DDR_SDRAM_BASE_PHY
 		Physical address from the view of DDR controllers. It is the
 		same as CFG_SYS_DDR_SDRAM_BASE for  all Power SoCs. But
 		it could be different for ARM SoCs.

 - MIPS CPU options:
 		CONFIG_XWAY_SWAP_BYTES

 		Enable compilation of tools/xway-swap-bytes needed for Lantiq
 		XWAY SoCs for booting from NOR flash. The U-Boot image needs to
 		be swapped if a flash programmer is used.

 - ARM options:
 		CFG_SYS_EXCEPTION_VECTORS_HIGH

 		Select high exception vectors of the ARM core, e.g., do not
 		clear the V bit of the c1 register of CP15.

 		COUNTER_FREQUENCY
 		Generic timer clock source frequency.

 		COUNTER_FREQUENCY_REAL
 		Generic timer clock source frequency if the real clock is
 		different from COUNTER_FREQUENCY, and can only be determined
 		at run time.

 - Tegra SoC options:
 		CONFIG_TEGRA_SUPPORT_NON_SECURE

 		Support executing U-Boot in non-secure (NS) mode. Certain
 		impossible actions will be skipped if the CPU is in NS mode,
 		such as ARM architectural timer initialization.

 - Linux Kernel Interface:
 		CONFIG_OF_LIBFDT

 		New kernel versions are expecting firmware settings to be
 		passed using flattened device trees (based on open firmware
 		concepts).

 		CONFIG_OF_LIBFDT
 		 * New libfdt-based support
 		 * Adds the "fdt" command
 		 * The bootm command automatically updates the fdt

 		OF_TBCLK - The timebase frequency.

 		boards with QUICC Engines require OF_QE to set UCC MAC
 		addresses

 		CONFIG_OF_IDE_FIXUP

 		U-Boot can detect if an IDE device is present or not.
 		If not, and this new config option is activated, U-Boot
 		removes the ATA node from the DTS before booting Linux,
 		so the Linux IDE driver does not probe the device and
 		crash. This is needed for buggy hardware (uc101) where
 		no pull down resistor is connected to the signal IDE5V_DD7.

 - vxWorks boot parameters:

 		bootvx constructs a valid bootline using the following
 		environments variables: bootdev, bootfile, ipaddr, netmask,
 		serverip, gatewayip, hostname, othbootargs.
 		It loads the vxWorks image pointed bootfile.

 		Note: If a "bootargs" environment is defined, it will override
 		the defaults discussed just above.

 - Cache Configuration for ARM:
 		CFG_SYS_PL310_BASE - Physical base address of PL310
 					controller register space

 - Serial Ports:
 		CFG_PL011_CLOCK

 		If you have Amba PrimeCell PL011 UARTs, set this variable to
 		the clock speed of the UARTs.

 		CFG_PL01x_PORTS

 		If you have Amba PrimeCell PL010 or PL011 UARTs on your board,
 		define this to a list of base addresses for each (supported)
 		port. See e.g. include/configs/versatile.h

 		CONFIG_SERIAL_HW_FLOW_CONTROL

 		Define this variable to enable hw flow control in serial driver.
 		Current user of this option is drivers/serial/nsl16550.c driver

 - Removal of commands
 		If no commands are needed to boot, you can disable
 		CONFIG_CMDLINE to remove them. In this case, the command line
 		will not be available, and when U-Boot wants to execute the
 		boot command (on start-up) it will call board_run_command()
 		instead. This can reduce image size significantly for very
 		simple boot procedures.

 - Regular expression support:
 		CONFIG_REGEX
 		If this variable is defined, U-Boot is linked against
 		the SLRE (Super Light Regular Expression) library,
 		which adds regex support to some commands, as for
 		example "env grep" and "setexpr".

 - Watchdog:
 		CONFIG_SYS_WATCHDOG_FREQ
 		Some platforms automatically call WATCHDOG_RESET()
 		from the timer interrupt handler every
 		CONFIG_SYS_WATCHDOG_FREQ interrupts. If not set by the
 		board configuration file, a default of CONFIG_SYS_HZ/2
 		(i.e. 500) is used. Setting CONFIG_SYS_WATCHDOG_FREQ
 		to 0 disables calling WATCHDOG_RESET() from the timer
 		interrupt.

 - GPIO Support:
 		The CFG_SYS_I2C_PCA953X_WIDTH option specifies a list of
 		chip-ngpio pairs that tell the PCA953X driver the number of
 		pins supported by a particular chip.

 		Note that if the GPIO device uses I2C, then the I2C interface
 		must also be configured. See I2C Support, below.

 - I/O tracing:
 		When CONFIG_IO_TRACE is selected, U-Boot intercepts all I/O
 		accesses and can checksum them or write a list of them out
 		to memory. See the 'iotrace' command for details. This is
 		useful for testing device drivers since it can confirm that
 		the driver behaves the same way before and after a code
 		change. Currently this is supported on sandbox and arm. To
 		add support for your architecture, add '#include <iotrace.h>'
 		to the bottom of arch/<arch>/include/asm/io.h and test.

 		Example output from the 'iotrace stats' command is below.
 		Note that if the trace buffer is exhausted, the checksum will
 		still continue to operate.

 			iotrace is enabled
 			Start:  10000000	(buffer start address)
 			Size:   00010000	(buffer size)
 			Offset: 00000120	(current buffer offset)
 			Output: 10000120	(start + offset)
 			Count:  00000018	(number of trace records)
 			CRC32:  9526fb66	(CRC32 of all trace records)

 - Timestamp Support:

 		When CONFIG_TIMESTAMP is selected, the timestamp
 		(date and time) of an image is printed by image
 		commands like bootm or iminfo. This option is
 		automatically enabled when you select CONFIG_CMD_DATE .

 - Partition Labels (disklabels) Supported:
 		Zero or more of the following:
 		CONFIG_MAC_PARTITION   Apple's MacOS partition table.
 		CONFIG_ISO_PARTITION   ISO partition table, used on CDROM etc.
 		CONFIG_EFI_PARTITION   GPT partition table, common when EFI is the
 				       bootloader.  Note 2TB partition limit; see
 				       disk/part_efi.c
 		CONFIG_SCSI) you must configure support for at
 		least one non-MTD partition type as well.

 - NETWORK Support (PCI):
 		CONFIG_E1000_SPI
 		Utility code for direct access to the SPI bus on Intel 8257x.
 		This does not do anything useful unless you set at least one
 		of CONFIG_CMD_E1000 or CONFIG_E1000_SPI_GENERIC.

 		CONFIG_NATSEMI
 		Support for National dp83815 chips.

 		CONFIG_NS8382X
 		Support for National dp8382[01] gigabit chips.

 - NETWORK Support (other):
 		CONFIG_CALXEDA_XGMAC
 		Support for the Calxeda XGMAC device

 		CONFIG_LAN91C96
 		Support for SMSC's LAN91C96 chips.

 			CONFIG_LAN91C96_USE_32_BIT
 			Define this to enable 32 bit addressing

 			CONFIG_SYS_DAVINCI_EMAC_PHY_COUNT
 			Define this if you have more then 3 PHYs.

 		CONFIG_FTGMAC100
 		Support for Faraday's FTGMAC100 Gigabit SoC Ethernet

 			CONFIG_FTGMAC100_EGIGA
 			Define this to use GE link update with gigabit PHY.
 			Define this if FTGMAC100 is connected to gigabit PHY.
 			If your system has 10/100 PHY only, it might not occur
 			wrong behavior. Because PHY usually return timeout or
 			useless data when polling gigabit status and gigabit
 			control registers. This behavior won't affect the
 			correctnessof 10/100 link speed update.

 		CONFIG_SH_ETHER
 		Support for Renesas on-chip Ethernet controller

 			CONFIG_SH_ETHER_USE_PORT
 			Define the number of ports to be used

 			CFG_SH_ETHER_PHY_ADDR
 			Define the ETH PHY's address

 			CFG_SH_ETHER_CACHE_WRITEBACK
 			If this option is set, the driver enables cache flush.

 - TPM Support:
 		CONFIG_TPM
 		Support TPM devices.

 		CONFIG_TPM_TIS_INFINEON
 		Support for Infineon i2c bus TPM devices. Only one device
 		per system is supported at this time.

 			CONFIG_TPM_TIS_I2C_BURST_LIMITATION
 			Define the burst count bytes upper limit

 		CONFIG_TPM_ST33ZP24
 		Support for STMicroelectronics TPM devices. Requires DM_TPM support.

 			CONFIG_TPM_ST33ZP24_I2C
 			Support for STMicroelectronics ST33ZP24 I2C devices.
 			Requires TPM_ST33ZP24 and I2C.

 			CONFIG_TPM_ST33ZP24_SPI
 			Support for STMicroelectronics ST33ZP24 SPI devices.
 			Requires TPM_ST33ZP24 and SPI.

 		CONFIG_TPM_ATMEL_TWI
 		Support for Atmel TWI TPM device. Requires I2C support.

 		CONFIG_TPM_TIS_LPC
 		Support for generic parallel port TPM devices. Only one device
 		per system is supported at this time.

 		CONFIG_TPM
 		Define this to enable the TPM support library which provides
 		functional interfaces to some TPM commands.
 		Requires support for a TPM device.

 		CONFIG_TPM_AUTH_SESSIONS
 		Define this to enable authorized functions in the TPM library.
 		Requires CONFIG_TPM and CONFIG_SHA1.

 - USB Support:
 		At the moment only the UHCI host controller is
 		supported (PIP405, MIP405); define
 		CONFIG_USB_UHCI to enable it.
 		define CONFIG_USB_KEYBOARD to enable the USB Keyboard
 		and define CONFIG_USB_STORAGE to enable the USB
 		storage devices.
 		Note:
 		Supported are USB Keyboards and USB Floppy drives
 		(TEAC FD-05PUB).

 		CONFIG_USB_DWC2_REG_ADDR the physical CPU address of the DWC2
 		HW module registers.

 - USB Device:
 		Define the below if you wish to use the USB console.
 		Once firmware is rebuilt from a serial console issue the
 		command "setenv stdin usbtty; setenv stdout usbtty" and
 		attach your USB cable. The Unix command "dmesg" should print
 		it has found a new device. The environment variable usbtty
 		can be set to gserial or cdc_acm to enable your device to
 		appear to a USB host as a Linux gserial device or a
 		Common Device Class Abstract Control Model serial device.
 		If you select usbtty = gserial you should be able to enumerate
 		a Linux host by
 		# modprobe usbserial vendor=0xVendorID product=0xProductID
 		else if using cdc_acm, simply setting the environment
 		variable usbtty to be cdc_acm should suffice. The following
 		might be defined in YourBoardName.h

 		If you have a USB-IF assigned VendorID then you may wish to
 		define your own vendor specific values either in BoardName.h
 		or directly in usbd_vendor_info.h. If you don't define
 		CONFIG_USBD_MANUFACTURER, CONFIG_USBD_PRODUCT_NAME,
 		CONFIG_USBD_VENDORID and CONFIG_USBD_PRODUCTID, then U-Boot
 		should pretend to be a Linux device to it's target host.

 			CONFIG_USBD_MANUFACTURER
 			Define this string as the name of your company for
 			- CONFIG_USBD_MANUFACTURER "my company"

 			CONFIG_USBD_PRODUCT_NAME
 			Define this string as the name of your product
 			- CONFIG_USBD_PRODUCT_NAME "acme usb device"

 			CONFIG_USBD_VENDORID
 			Define this as your assigned Vendor ID from the USB
 			Implementors Forum. This *must* be a genuine Vendor ID
 			to avoid polluting the USB namespace.
 			- CONFIG_USBD_VENDORID 0xFFFF

 			CONFIG_USBD_PRODUCTID
 			Define this as the unique Product ID
 			for your device
 			- CONFIG_USBD_PRODUCTID 0xFFFF

 - ULPI Layer Support:
 		The ULPI (UTMI Low Pin (count) Interface) PHYs are supported via
 		the generic ULPI layer. The generic layer accesses the ULPI PHY
 		via the platform viewport, so you need both the genric layer and
 		the viewport enabled. Currently only Chipidea/ARC based
 		viewport is supported.
 		To enable the ULPI layer support, define CONFIG_USB_ULPI and
 		CONFIG_USB_ULPI_VIEWPORT in your board configuration file.
 		If your ULPI phy needs a different reference clock than the
 		standard 24 MHz then you have to define CONFIG_ULPI_REF_CLK to
 		the appropriate value in Hz.

 - MMC Support:
 		CONFIG_SH_MMCIF
 		Support for Renesas on-chip MMCIF controller

 			CONFIG_SH_MMCIF_ADDR
 			Define the base address of MMCIF registers

 			CONFIG_SH_MMCIF_CLK
 			Define the clock frequency for MMCIF

 - USB Device Firmware Update (DFU) class support:
 		CONFIG_DFU_OVER_USB
 		This enables the USB portion of the DFU USB class

 		CONFIG_DFU_NAND
 		This enables support for exposing NAND devices via DFU.

 		CONFIG_DFU_RAM
 		This enables support for exposing RAM via DFU.
 		Note: DFU spec refer to non-volatile memory usage, but
 		allow usages beyond the scope of spec - here RAM usage,
 		one that would help mostly the developer.

 		CONFIG_SYS_DFU_DATA_BUF_SIZE
 		Dfu transfer uses a buffer before writing data to the
 		raw storage device. Make the size (in bytes) of this buffer
 		configurable. The size of this buffer is also configurable
 		through the "dfu_bufsiz" environment variable.

 		CONFIG_SYS_DFU_MAX_FILE_SIZE
 		When updating files rather than the raw storage device,
 		we use a static buffer to copy the file into and then write
 		the buffer once we've been given the whole file.  Define
 		this to the maximum filesize (in bytes) for the buffer.
 		Default is 4 MiB if undefined.

 		DFU_DEFAULT_POLL_TIMEOUT
 		Poll timeout [ms], is the timeout a device can send to the
 		host. The host must wait for this timeout before sending
 		a subsequent DFU_GET_STATUS request to the device.

 		DFU_MANIFEST_POLL_TIMEOUT
 		Poll timeout [ms], which the device sends to the host when
 		entering dfuMANIFEST state. Host waits this timeout, before
 		sending again an USB request to the device.

 - Keyboard Support:
 		See Kconfig help for available keyboard drivers.

 - MII/PHY support:
 		CONFIG_PHY_CLOCK_FREQ (ppc4xx)

 		The clock frequency of the MII bus

 		CONFIG_PHY_CMD_DELAY (ppc4xx)

 		Some PHY like Intel LXT971A need extra delay after
 		command issued before MII status register can be read

 - BOOTP Recovery Mode:
 		CONFIG_BOOTP_RANDOM_DELAY

 		If you have many targets in a network that try to
 		boot using BOOTP, you may want to avoid that all
 		systems send out BOOTP requests at precisely the same
 		moment (which would happen for instance at recovery
 		from a power failure, when all systems will try to
 		boot, thus flooding the BOOTP server. Defining
 		CONFIG_BOOTP_RANDOM_DELAY causes a random delay to be
 		inserted before sending out BOOTP requests. The
 		following delays are inserted then:

 		1st BOOTP request:	delay 0 ... 1 sec
 		2nd BOOTP request:	delay 0 ... 2 sec
 		3rd BOOTP request:	delay 0 ... 4 sec
 		4th and following
 		BOOTP requests:		delay 0 ... 8 sec

 		CONFIG_BOOTP_ID_CACHE_SIZE

 		BOOTP packets are uniquely identified using a 32-bit ID. The
 		server will copy the ID from client requests to responses and
 		U-Boot will use this to determine if it is the destination of
 		an incoming response. Some servers will check that addresses
 		aren't in use before handing them out (usually using an ARP
 		ping) and therefore take up to a few hundred milliseconds to
 		respond. Network congestion may also influence the time it
 		takes for a response to make it back to the client. If that
 		time is too long, U-Boot will retransmit requests. In order
 		to allow earlier responses to still be accepted after these
 		retransmissions, U-Boot's BOOTP client keeps a small cache of
 		IDs. The CONFIG_BOOTP_ID_CACHE_SIZE controls the size of this
 		cache. The default is to keep IDs for up to four outstanding
 		requests. Increasing this will allow U-Boot to accept offers
 		from a BOOTP client in networks with unusually high latency.

 - DHCP Advanced Options:

  - Link-local IP address negotiation:
 		Negotiate with other link-local clients on the local network
 		for an address that doesn't require explicit configuration.
 		This is especially useful if a DHCP server cannot be guaranteed
 		to exist in all environments that the device must operate.

 		See doc/README.link-local for more information.

  - MAC address from environment variables

 		FDT_SEQ_MACADDR_FROM_ENV

 		Fix-up device tree with MAC addresses fetched sequentially from
 		environment variables. This config work on assumption that
 		non-usable ethernet node of device-tree are either not present
 		or their status has been marked as "disabled".

  - CDP Options:
 		CONFIG_CDP_DEVICE_ID

 		The device id used in CDP trigger frames.

 		CONFIG_CDP_DEVICE_ID_PREFIX

 		A two character string which is prefixed to the MAC address
 		of the device.

 		CONFIG_CDP_PORT_ID

 		A printf format string which contains the ascii name of
 		the port. Normally is set to "eth%d" which sets
 		eth0 for the first Ethernet, eth1 for the second etc.

 		CONFIG_CDP_CAPABILITIES

 		A 32bit integer which indicates the device capabilities;
 		0x00000010 for a normal host which does not forwards.

 		CONFIG_CDP_VERSION

 		An ascii string containing the version of the software.

 		CONFIG_CDP_PLATFORM

 		An ascii string containing the name of the platform.

 		CONFIG_CDP_TRIGGER

 		A 32bit integer sent on the trigger.

 		CONFIG_CDP_POWER_CONSUMPTION

 		A 16bit integer containing the power consumption of the
 		device in .1 of milliwatts.

 		CONFIG_CDP_APPLIANCE_VLAN_TYPE

 		A byte containing the id of the VLAN.

 - Status LED:	CONFIG_LED_STATUS

 		Several configurations allow to display the current
 		status using a LED. For instance, the LED will blink
 		fast while running U-Boot code, stop blinking as
 		soon as a reply to a BOOTP request was received, and
 		start blinking slow once the Linux kernel is running
 		(supported by a status LED driver in the Linux
 		kernel). Defining CONFIG_LED_STATUS enables this
 		feature in U-Boot.

 		Additional options:

 		CONFIG_LED_STATUS_GPIO
 		The status LED can be connected to a GPIO pin.
 		In such cases, the gpio_led driver can be used as a
 		status LED backend implementation. Define CONFIG_LED_STATUS_GPIO
 		to include the gpio_led driver in the U-Boot binary.

 		CONFIG_GPIO_LED_INVERTED_TABLE
 		Some GPIO connected LEDs may have inverted polarity in which
 		case the GPIO high value corresponds to LED off state and
 		GPIO low value corresponds to LED on state.
 		In such cases CONFIG_GPIO_LED_INVERTED_TABLE may be defined
 		with a list of GPIO LEDs that have inverted polarity.

 - I2C Support:
 		CFG_SYS_NUM_I2C_BUSES
 		Hold the number of i2c buses you want to use.

 		CONFIG_SYS_I2C_DIRECT_BUS
 		define this, if you don't use i2c muxes on your hardware.
 		if CFG_SYS_I2C_MAX_HOPS is not defined or == 0 you can
 		omit this define.

 		CFG_SYS_I2C_MAX_HOPS
 		define how many muxes are maximal consecutively connected
 		on one i2c bus. If you not use i2c muxes, omit this
 		define.

 		CFG_SYS_I2C_BUSES
 		hold a list of buses you want to use, only used if
 		CONFIG_SYS_I2C_DIRECT_BUS is not defined, for example
 		a board with CFG_SYS_I2C_MAX_HOPS = 1 and
 		CFG_SYS_NUM_I2C_BUSES = 9:

 		 CFG_SYS_I2C_BUSES	{{0, {I2C_NULL_HOP}}, \
 					{0, {{I2C_MUX_PCA9547, 0x70, 1}}}, \
 					{0, {{I2C_MUX_PCA9547, 0x70, 2}}}, \
 					{0, {{I2C_MUX_PCA9547, 0x70, 3}}}, \
 					{0, {{I2C_MUX_PCA9547, 0x70, 4}}}, \
 					{0, {{I2C_MUX_PCA9547, 0x70, 5}}}, \
 					{1, {I2C_NULL_HOP}}, \
 					{1, {{I2C_MUX_PCA9544, 0x72, 1}}}, \
 					{1, {{I2C_MUX_PCA9544, 0x72, 2}}}, \
 					}

 		which defines
 			bus 0 on adapter 0 without a mux
 			bus 1 on adapter 0 with a PCA9547 on address 0x70 port 1
 			bus 2 on adapter 0 with a PCA9547 on address 0x70 port 2
 			bus 3 on adapter 0 with a PCA9547 on address 0x70 port 3
 			bus 4 on adapter 0 with a PCA9547 on address 0x70 port 4
 			bus 5 on adapter 0 with a PCA9547 on address 0x70 port 5
 			bus 6 on adapter 1 without a mux
 			bus 7 on adapter 1 with a PCA9544 on address 0x72 port 1
 			bus 8 on adapter 1 with a PCA9544 on address 0x72 port 2

 		If you do not have i2c muxes on your board, omit this define.

 - Legacy I2C Support:
 		If you use the software i2c interface (CONFIG_SYS_I2C_SOFT)
 		then the following macros need to be defined (examples are
 		from include/configs/lwmon.h):

 		I2C_INIT

 		(Optional). Any commands necessary to enable the I2C
 		controller or configure ports.

 		eg: #define I2C_INIT (immr->im_cpm.cp_pbdir |=	PB_SCL)

 		I2C_ACTIVE

 		The code necessary to make the I2C data line active
 		(driven).  If the data line is open collector, this
 		define can be null.

 		eg: #define I2C_ACTIVE (immr->im_cpm.cp_pbdir |=  PB_SDA)

 		I2C_TRISTATE

 		The code necessary to make the I2C data line tri-stated
 		(inactive).  If the data line is open collector, this
 		define can be null.

 		eg: #define I2C_TRISTATE (immr->im_cpm.cp_pbdir &= ~PB_SDA)

 		I2C_READ

 		Code that returns true if the I2C data line is high,
 		false if it is low.

 		eg: #define I2C_READ ((immr->im_cpm.cp_pbdat & PB_SDA) != 0)

 		I2C_SDA(bit)

 		If <bit> is true, sets the I2C data line high. If it
 		is false, it clears it (low).

 		eg: #define I2C_SDA(bit) \
 			if(bit) immr->im_cpm.cp_pbdat |=  PB_SDA; \
 			else	immr->im_cpm.cp_pbdat &= ~PB_SDA

 		I2C_SCL(bit)

 		If <bit> is true, sets the I2C clock line high. If it
 		is false, it clears it (low).

 		eg: #define I2C_SCL(bit) \
 			if(bit) immr->im_cpm.cp_pbdat |=  PB_SCL; \
 			else	immr->im_cpm.cp_pbdat &= ~PB_SCL

 		I2C_DELAY

 		This delay is invoked four times per clock cycle so this
 		controls the rate of data transfer.  The data rate thus
 		is 1 / (I2C_DELAY * 4). Often defined to be something
 		like:

 		#define I2C_DELAY  udelay(2)

 		CONFIG_SOFT_I2C_GPIO_SCL / CONFIG_SOFT_I2C_GPIO_SDA

 		If your arch supports the generic GPIO framework (asm/gpio.h),
 		then you may alternatively define the two GPIOs that are to be
 		used as SCL / SDA.  Any of the previous I2C_xxx macros will
 		have GPIO-based defaults assigned to them as appropriate.

 		You should define these to the GPIO value as given directly to
 		the generic GPIO functions.

 		CFG_I2C_MULTI_BUS

 		This option allows the use of multiple I2C buses, each of which
 		must have a controller.	 At any point in time, only one bus is
 		active.	 To switch to a different bus, use the 'i2c dev' command.
 		Note that bus numbering is zero-based.

 		CFG_SYS_I2C_NOPROBES

 		This option specifies a list of I2C devices that will be skipped
 		when the 'i2c probe' command is issued.

 		e.g.
 			#define CFG_SYS_I2C_NOPROBES {0x50,0x68}

 		will skip addresses 0x50 and 0x68 on a board with one I2C bus

 		CFG_SYS_RTC_BUS_NUM

 		If defined, then this indicates the I2C bus number for the RTC.
 		If not defined, then U-Boot assumes that RTC is on I2C bus 0.

 		CONFIG_SOFT_I2C_READ_REPEATED_START

 		defining this will force the i2c_read() function in
 		the soft_i2c driver to perform an I2C repeated start
 		between writing the address pointer and reading the
 		data.  If this define is omitted the default behaviour
 		of doing a stop-start sequence will be used.  Most I2C
 		devices can use either method, but some require one or
 		the other.

 - SPI Support:	CONFIG_SPI

 		Enables SPI driver (so far only tested with
 		SPI EEPROM, also an instance works with Crystal A/D and
 		D/As on the SACSng board)

 		CONFIG_SYS_SPI_MXC_WAIT
 		Timeout for waiting until spi transfer completed.
 		default: (CONFIG_SYS_HZ/100)     /* 10 ms */

 - FPGA Support: CONFIG_FPGA

 		Enables FPGA subsystem.

 		CONFIG_FPGA_<vendor>

 		Enables support for specific chip vendors.
 		(ALTERA, XILINX)

 		CONFIG_FPGA_<family>

 		Enables support for FPGA family.
 		(SPARTAN2, SPARTAN3, VIRTEX2, CYCLONE2, ACEX1K, ACEX)

 		CONFIG_SYS_FPGA_CHECK_BUSY

 		Enable checks on FPGA configuration interface busy
 		status by the configuration function. This option
 		will require a board or device specific function to
 		be written.

 		CFG_FPGA_DELAY

 		If defined, a function that provides delays in the FPGA
 		configuration driver.

 		CONFIG_SYS_FPGA_CHECK_ERROR

 		Check for configuration errors during FPGA bitfile
 		loading. For example, abort during Virtex II
 		configuration if the INIT_B line goes low (which
 		indicated a CRC error).

 		CFG_SYS_FPGA_WAIT_INIT

 		Maximum time to wait for the INIT_B line to de-assert
 		after PROB_B has been de-asserted during a Virtex II
 		FPGA configuration sequence. The default time is 500
 		ms.

 		CFG_SYS_FPGA_WAIT_BUSY

 		Maximum time to wait for BUSY to de-assert during
 		Virtex II FPGA configuration. The default is 5 ms.

 		CFG_SYS_FPGA_WAIT_CONFIG

 		Time to wait after FPGA configuration. The default is
 		200 ms.

 - Vendor Parameter Protection:

 		U-Boot considers the values of the environment
 		variables "serial#" (Board Serial Number) and
 		"ethaddr" (Ethernet Address) to be parameters that
 		are set once by the board vendor / manufacturer, and
 		protects these variables from casual modification by
 		the user. Once set, these variables are read-only,
 		and write or delete attempts are rejected. You can
 		change this behaviour:

 		If CONFIG_ENV_OVERWRITE is #defined in your config
 		file, the write protection for vendor parameters is
 		completely disabled. Anybody can change or delete
 		these parameters.

 		The same can be accomplished in a more flexible way
 		for any variable by configuring the type of access
 		to allow for those variables in the ".flags" variable
 		or define CFG_ENV_FLAGS_LIST_STATIC.

 - Protected RAM:
 		CFG_PRAM

 		Define this variable to enable the reservation of
 		"protected RAM", i. e. RAM which is not overwritten
 		by U-Boot. Define CFG_PRAM to hold the number of
 		kB you want to reserve for pRAM. You can overwrite
 		this default value by defining an environment
 		variable "pram" to the number of kB you want to
 		reserve. Note that the board info structure will
 		still show the full amount of RAM. If pRAM is
 		reserved, a new environment variable "mem" will
 		automatically be defined to hold the amount of
 		remaining RAM in a form that can be passed as boot
 		argument to Linux, for instance like that:

 			setenv bootargs ... mem=\${mem}
 			saveenv

 		This way you can tell Linux not to use this memory,
 		either, which results in a memory region that will
 		not be affected by reboots.

 		*WARNING* If your board configuration uses automatic
 		detection of the RAM size, you must make sure that
 		this memory test is non-destructive. So far, the
 		following board configurations are known to be
 		"pRAM-clean":

 			IVMS8, IVML24, SPD8xx,
 			HERMES, IP860, RPXlite, LWMON,
 			FLAGADM

 - Error Recovery:
 	Note:

 		In the current implementation, the local variables
 		space and global environment variables space are
 		separated. Local variables are those you define by
 		simply typing `name=value'. To access a local
 		variable later on, you have write `$name' or
 		`${name}'; to execute the contents of a variable
 		directly type `$name' at the command prompt.

 		Global environment variables are those you use
 		setenv/printenv to work with. To run a command stored
 		in such a variable, you need to use the run command,
 		and you must not use the '$' sign to access them.

 		To store commands and special characters in a
 		variable, please use double quotation marks
 		surrounding the whole text of the variable, instead
 		of the backslashes before semicolons and special
 		symbols.

 - Default Environment:
 		CFG_EXTRA_ENV_SETTINGS

 		Define this to contain any number of null terminated
 		strings (variable = value pairs) that will be part of
 		the default environment compiled into the boot image.

 		For example, place something like this in your
 		board's config file:

 		#define CFG_EXTRA_ENV_SETTINGS \
 			"myvar1=value1\0" \
 			"myvar2=value2\0"

 		Warning: This method is based on knowledge about the
 		internal format how the environment is stored by the
 		U-Boot code. This is NOT an official, exported
 		interface! Although it is unlikely that this format
 		will change soon, there is no guarantee either.
 		You better know what you are doing here.

 		Note: overly (ab)use of the default environment is
 		discouraged. Make sure to check other ways to preset
 		the environment like the "source" command or the
 		boot command first.

 		CONFIG_DELAY_ENVIRONMENT

 		Normally the environment is loaded when the board is
 		initialised so that it is available to U-Boot. This inhibits
 		that so that the environment is not available until
 		explicitly loaded later by U-Boot code. With CONFIG_OF_CONTROL
 		this is instead controlled by the value of
 		/config/load-environment.

 - Automatic software updates via TFTP server
 		CONFIG_UPDATE_TFTP
 		CONFIG_UPDATE_TFTP_CNT_MAX
 		CONFIG_UPDATE_TFTP_MSEC_MAX

 		These options enable and control the auto-update feature;
 		for a more detailed description refer to doc/README.update.

 - MTD Support (mtdparts command, UBI support)
 		CONFIG_MTD_UBI_WL_THRESHOLD
 		This parameter defines the maximum difference between the highest
 		erase counter value and the lowest erase counter value of eraseblocks
 		of UBI devices. When this threshold is exceeded, UBI starts performing
 		wear leveling by means of moving data from eraseblock with low erase
 		counter to eraseblocks with high erase counter.

 		The default value should be OK for SLC NAND flashes, NOR flashes and
 		other flashes which have eraseblock life-cycle 100000 or more.
 		However, in case of MLC NAND flashes which typically have eraseblock
 		life-cycle less than 10000, the threshold should be lessened (e.g.,
 		to 128 or 256, although it does not have to be power of 2).

 		default: 4096

 		CONFIG_MTD_UBI_BEB_LIMIT
 		This option specifies the maximum bad physical eraseblocks UBI
 		expects on the MTD device (per 1024 eraseblocks). If the
 		underlying flash does not admit of bad eraseblocks (e.g. NOR
 		flash), this value is ignored.

 		NAND datasheets often specify the minimum and maximum NVM
 		(Number of Valid Blocks) for the flashes' endurance lifetime.
 		The maximum expected bad eraseblocks per 1024 eraseblocks
 		then can be calculated as "1024 * (1 - MinNVB / MaxNVB)",
 		which gives 20 for most NANDs (MaxNVB is basically the total
 		count of eraseblocks on the chip).

 		To put it differently, if this value is 20, UBI will try to
 		reserve about 1.9% of physical eraseblocks for bad blocks
 		handling. And that will be 1.9% of eraseblocks on the entire
 		NAND chip, not just the MTD partition UBI attaches. This means
 		that if you have, say, a NAND flash chip admits maximum 40 bad
 		eraseblocks, and it is split on two MTD partitions of the same
 		size, UBI will reserve 40 eraseblocks when attaching a
 		partition.

 		default: 20

 		CONFIG_MTD_UBI_FASTMAP
 		Fastmap is a mechanism which allows attaching an UBI device
 		in nearly constant time. Instead of scanning the whole MTD device it
 		only has to locate a checkpoint (called fastmap) on the device.
 		The on-flash fastmap contains all information needed to attach
 		the device. Using fastmap makes only sense on large devices where
 		attaching by scanning takes long. UBI will not automatically install
 		a fastmap on old images, but you can set the UBI parameter
 		CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT to 1 if you want so. Please note
 		that fastmap-enabled images are still usable with UBI implementations
 		without	fastmap support. On typical flash devices the whole fastmap
 		fits into one PEB. UBI will reserve PEBs to hold two fastmaps.

 		CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT
 		Set this parameter to enable fastmap automatically on images
 		without a fastmap.
 		default: 0

 		CONFIG_MTD_UBI_FM_DEBUG
 		Enable UBI fastmap debug
 		default: 0

 - SPL framework
 		CONFIG_SPL
 		Enable building of SPL globally.

 		CONFIG_SPL_PANIC_ON_RAW_IMAGE
 		When defined, SPL will panic() if the image it has
 		loaded does not have a signature.
 		Defining this is useful when code which loads images
 		in SPL cannot guarantee that absolutely all read errors
 		will be caught.
 		An example is the LPC32XX MLC NAND driver, which will
 		consider that a completely unreadable NAND block is bad,
 		and thus should be skipped silently.

 		CONFIG_SPL_DISPLAY_PRINT
 		For ARM, enable an optional function to print more information
 		about the running system.

 		CONFIG_SPL_MPC83XX_WAIT_FOR_NAND
 		Set this for NAND SPL on PPC mpc83xx targets, so that
 		start.S waits for the rest of the SPL to load before
 		continuing (the hardware starts execution after just
 		loading the first page rather than the full 4K).

 		CONFIG_SPL_UBI
 		Support for a lightweight UBI (fastmap) scanner and
 		loader

 		CONFIG_SYS_NAND_5_ADDR_CYCLE, CONFIG_SYS_NAND_PAGE_COUNT,
 		CONFIG_SYS_NAND_PAGE_SIZE, CONFIG_SYS_NAND_OOBSIZE,
 		CONFIG_SYS_NAND_BLOCK_SIZE, CONFIG_SYS_NAND_BAD_BLOCK_POS,
 		CFG_SYS_NAND_ECCPOS, CFG_SYS_NAND_ECCSIZE,
 		CFG_SYS_NAND_ECCBYTES
 		Defines the size and behavior of the NAND that SPL uses
 		to read U-Boot

 		CFG_SYS_NAND_U_BOOT_DST
 		Location in memory to load U-Boot to

 		CFG_SYS_NAND_U_BOOT_SIZE
 		Size of image to load

 		CFG_SYS_NAND_U_BOOT_START
 		Entry point in loaded image to jump to

 		CONFIG_SPL_RAM_DEVICE
 		Support for running image already present in ram, in SPL binary

 		CONFIG_SPL_FIT_PRINT
 		Printing information about a FIT image adds quite a bit of
 		code to SPL. So this is normally disabled in SPL. Use this
 		option to re-enable it. This will affect the output of the
 		bootm command when booting a FIT image.

 - Interrupt support (PPC):

 		There are common interrupt_init() and timer_interrupt()
 		for all PPC archs. interrupt_init() calls interrupt_init_cpu()
 		for CPU specific initialization. interrupt_init_cpu()
 		should set decrementer_count to appropriate value. If
 		CPU resets decrementer automatically after interrupt
 		(ppc4xx) it should set decrementer_count to zero.
 		timer_interrupt() calls timer_interrupt_cpu() for CPU
 		specific handling. If board has watchdog / status_led
 		/ other_activity_monitor it works automatically from
 		general timer_interrupt().


 Board initialization settings:
 ------------------------------

 During Initialization u-boot calls a number of board specific functions
 to allow the preparation of board specific prerequisites, e.g. pin setup
 before drivers are initialized. To enable these callbacks the
 following configuration macros have to be defined. Currently this is
 architecture specific, so please check arch/your_architecture/lib/board.c
 typically in board_init_f() and board_init_r().

 - CONFIG_BOARD_EARLY_INIT_F: Call board_early_init_f()
 - CONFIG_BOARD_EARLY_INIT_R: Call board_early_init_r()
 - CONFIG_BOARD_LATE_INIT: Call board_late_init()

 Configuration Settings:
 -----------------------

 - MEM_SUPPORT_64BIT_DATA: Defined automatically if compiled as 64-bit.
 		Optionally it can be defined to support 64-bit memory commands.

 - CONFIG_SYS_LONGHELP: Defined when you want long help messages included;
 		undefine this when you're short of memory.

 - CONFIG_SYS_HELP_CMD_WIDTH: Defined when you want to override the default
 		width of the commands listed in the 'help' command output.

 - CONFIG_SYS_PROMPT:	This is what U-Boot prints on the console to
 		prompt for user input.

 - CFG_SYS_BAUDRATE_TABLE:
 		List of legal baudrate settings for this board.

 - CFG_SYS_MEM_RESERVE_SECURE
 		Only implemented for ARMv8 for now.
 		If defined, the size of CFG_SYS_MEM_RESERVE_SECURE memory
 		is substracted from total RAM and won't be reported to OS.
 		This memory can be used as secure memory. A variable
 		gd->arch.secure_ram is used to track the location. In systems
 		the RAM base is not zero, or RAM is divided into banks,
 		this variable needs to be recalcuated to get the address.

 - CFG_SYS_SDRAM_BASE:
 		Physical start address of SDRAM. _Must_ be 0 here.

 - CFG_SYS_FLASH_BASE:
 		Physical start address of Flash memory.

 - CONFIG_SYS_MALLOC_LEN:
 		Size of DRAM reserved for malloc() use.

 - CONFIG_SYS_MALLOC_F_LEN
 		Size of the malloc() pool for use before relocation. If
 		this is defined, then a very simple malloc() implementation
 		will become available before relocation. The address is just
 		below the global data, and the stack is moved down to make
 		space.

 		This feature allocates regions with increasing addresses
 		within the region. calloc() is supported, but realloc()
 		is not available. free() is supported but does nothing.
 		The memory will be freed (or in fact just forgotten) when
 		U-Boot relocates itself.

 - CONFIG_SYS_MALLOC_SIMPLE
 		Provides a simple and small malloc() and calloc() for those
 		boards which do not use the full malloc in SPL (which is
 		enabled with CONFIG_SYS_SPL_MALLOC).

 - CFG_SYS_BOOTMAPSZ:
 		Maximum size of memory mapped by the startup code of
 		the Linux kernel; all data that must be processed by
 		the Linux kernel (bd_info, boot arguments, FDT blob if
 		used) must be put below this limit, unless "bootm_low"
 		environment variable is defined and non-zero. In such case
 		all data for the Linux kernel must be between "bootm_low"
 		and "bootm_low" + CFG_SYS_BOOTMAPSZ.	 The environment
 		variable "bootm_mapsize" will override the value of
 		CFG_SYS_BOOTMAPSZ.  If CFG_SYS_BOOTMAPSZ is undefined,
 		then the value in "bootm_size" will be used instead.

 - CONFIG_SYS_BOOT_GET_CMDLINE:
 		Enables allocating and saving kernel cmdline in space between
 		"bootm_low" and "bootm_low" + BOOTMAPSZ.

 - CONFIG_SYS_BOOT_GET_KBD:
 		Enables allocating and saving a kernel copy of the bd_info in
 		space between "bootm_low" and "bootm_low" + BOOTMAPSZ.

 - CONFIG_SYS_FLASH_PROTECTION
 		If defined, hardware flash sectors protection is used
 		instead of U-Boot software protection.

 - CONFIG_SYS_FLASH_CFI:
 		Define if the flash driver uses extra elements in the
 		common flash structure for storing flash geometry.

 - CONFIG_FLASH_CFI_DRIVER
 		This option also enables the building of the cfi_flash driver
 		in the drivers directory

 - CONFIG_FLASH_CFI_MTD
 		This option enables the building of the cfi_mtd driver
 		in the drivers directory. The driver exports CFI flash
 		to the MTD layer.

 - CONFIG_SYS_FLASH_USE_BUFFER_WRITE
 		Use buffered writes to flash.

 - CONFIG_ENV_FLAGS_LIST_DEFAULT
 - CFG_ENV_FLAGS_LIST_STATIC
 	Enable validation of the values given to environment variables when
 	calling env set.  Variables can be restricted to only decimal,
 	hexadecimal, or boolean.  If CONFIG_CMD_NET is also defined,
 	the variables can also be restricted to IP address or MAC address.

 	The format of the list is:
 		type_attribute = [s|d|x|b|i|m]
 		access_attribute = [a|r|o|c]
 		attributes = type_attribute[access_attribute]
 		entry = variable_name[:attributes]
 		list = entry[,list]

 	The type attributes are:
 		s - String (default)
 		d - Decimal
 		x - Hexadecimal
 		b - Boolean ([1yYtT|0nNfF])
 		i - IP address
 		m - MAC address

 	The access attributes are:
 		a - Any (default)
 		r - Read-only
 		o - Write-once
 		c - Change-default

 	- CONFIG_ENV_FLAGS_LIST_DEFAULT
 		Define this to a list (string) to define the ".flags"
 		environment variable in the default or embedded environment.

 	- CFG_ENV_FLAGS_LIST_STATIC
 		Define this to a list (string) to define validation that
 		should be done if an entry is not found in the ".flags"
 		environment variable.  To override a setting in the static
 		list, simply add an entry for the same variable name to the
 		".flags" variable.

 	If CONFIG_REGEX is defined, the variable_name above is evaluated as a
 	regular expression. This allows multiple variables to define the same
 	flags without explicitly listing them for each variable.

 The following definitions that deal with the placement and management
 of environment data (variable area); in general, we support the
 following configurations:

 BE CAREFUL! The first access to the environment happens quite early
 in U-Boot initialization (when we try to get the setting of for the
 console baudrate). You *MUST* have mapped your NVRAM area then, or
 U-Boot will hang.

 Please note that even with NVRAM we still use a copy of the
 environment in RAM: we could work on NVRAM directly, but we want to
 keep settings there always unmodified except somebody uses "saveenv"
 to save the current settings.

 BE CAREFUL! For some special cases, the local device can not use
 "saveenv" command. For example, the local device will get the
 environment stored in a remote NOR flash by SRIO or PCIE link,
 but it can not erase, write this NOR flash by SRIO or PCIE interface.

 - CONFIG_NAND_ENV_DST

 	Defines address in RAM to which the nand_spl code should copy the
 	environment. If redundant environment is used, it will be copied to
 	CONFIG_NAND_ENV_DST + CONFIG_ENV_SIZE.

 Please note that the environment is read-only until the monitor
 has been relocated to RAM and a RAM copy of the environment has been
 created; also, when using EEPROM you will have to use env_get_f()
 until then to read environment variables.

 The environment is protected by a CRC32 checksum. Before the monitor
 is relocated into RAM, as a result of a bad CRC you will be working
 with the compiled-in default environment - *silently*!!! [This is
 necessary, because the first environment variable we need is the
 "baudrate" setting for the console - if we have a bad CRC, we don't
 have any device yet where we could complain.]

 Note: once the monitor has been relocated, then it will complain if
 the default environment is used; a new CRC is computed as soon as you
 use the "saveenv" command to store a valid environment.

 - CONFIG_SYS_FAULT_MII_ADDR:
 		MII address of the PHY to check for the Ethernet link state.

 - CONFIG_DISPLAY_BOARDINFO
 		Display information about the board that U-Boot is running on
 		when U-Boot starts up. The board function checkboard() is called
 		to do this.

 - CONFIG_DISPLAY_BOARDINFO_LATE
 		Similar to the previous option, but display this information
 		later, once stdio is running and output goes to the LCD, if
 		present.

 Low Level (hardware related) configuration options:
 ---------------------------------------------------

 - CONFIG_SYS_CACHELINE_SIZE:
 		Cache Line Size of the CPU.

 - CONFIG_SYS_CCSRBAR_DEFAULT:
 		Default (power-on reset) physical address of CCSR on Freescale
 		PowerPC SOCs.

 - CFG_SYS_CCSRBAR:
 		Virtual address of CCSR.  On a 32-bit build, this is typically
 		the same value as CONFIG_SYS_CCSRBAR_DEFAULT.

 - CFG_SYS_CCSRBAR_PHYS:
 		Physical address of CCSR.  CCSR can be relocated to a new
 		physical address, if desired.  In this case, this macro should
 		be set to that address.	 Otherwise, it should be set to the
 		same value as CONFIG_SYS_CCSRBAR_DEFAULT.  For example, CCSR
 		is typically relocated on 36-bit builds.  It is recommended
 		that this macro be defined via the _HIGH and _LOW macros:

 		#define CFG_SYS_CCSRBAR_PHYS ((CFG_SYS_CCSRBAR_PHYS_HIGH
 			* 1ull) << 32 | CFG_SYS_CCSRBAR_PHYS_LOW)

 - CFG_SYS_CCSRBAR_PHYS_HIGH:
 		Bits 33-36 of CFG_SYS_CCSRBAR_PHYS.	This value is typically
 		either 0 (32-bit build) or 0xF (36-bit build).	This macro is
 		used in assembly code, so it must not contain typecasts or
 		integer size suffixes (e.g. "ULL").

 - CFG_SYS_CCSRBAR_PHYS_LOW:
 		Lower 32-bits of CFG_SYS_CCSRBAR_PHYS.  This macro is
 		used in assembly code, so it must not contain typecasts or
 		integer size suffixes (e.g. "ULL").

 - CONFIG_SYS_IMMR:	Physical address of the Internal Memory.
 		DO NOT CHANGE unless you know exactly what you're
 		doing! (11-4) [MPC8xx systems only]

 - CFG_SYS_INIT_RAM_ADDR:

 		Start address of memory area that can be used for
 		initial data and stack; please note that this must be
 		writable memory that is working WITHOUT special
 		initialization, i. e. you CANNOT use normal RAM which
 		will become available only after programming the
 		memory controller and running certain initialization
 		sequences.

 		U-Boot uses the following memory types:
 		- MPC8xx: IMMR (internal memory of the CPU)

 - CONFIG_SYS_SCCR:	System Clock and reset Control Register (15-27)

 - CONFIG_SYS_OR_TIMING_SDRAM:
 		SDRAM timing

 - CONFIG_SYS_SRIOn_MEM_VIRT:
 		Virtual Address of SRIO port 'n' memory region

 - CONFIG_SYS_SRIOn_MEM_PHYxS:
 		Physical Address of SRIO port 'n' memory region

 - CONFIG_SYS_SRIOn_MEM_SIZE:
 		Size of SRIO port 'n' memory region

 - CONFIG_SYS_NAND_BUSWIDTH_16BIT
 		Defined to tell the NAND controller that the NAND chip is using
 		a 16 bit bus.
 		Not all NAND drivers use this symbol.
 		Example of drivers that use it:
 		- drivers/mtd/nand/raw/ndfc.c
 		- drivers/mtd/nand/raw/mxc_nand.c

 - CONFIG_SYS_NDFC_EBC0_CFG
 		Sets the EBC0_CFG register for the NDFC. If not defined
 		a default value will be used.

 - CONFIG_SYS_SPD_BUS_NUM
 		If SPD EEPROM is on an I2C bus other than the first
 		one, specify here. Note that the value must resolve
 		to something your driver can deal with.

 - CONFIG_FSL_DDR_INTERACTIVE
 		Enable interactive DDR debugging. See doc/README.fsl-ddr.

 - CONFIG_FSL_DDR_SYNC_REFRESH
 		Enable sync of refresh for multiple controllers.

 - CONFIG_FSL_DDR_BIST
 		Enable built-in memory test for Freescale DDR controllers.

 - CONFIG_RMII
 		Enable RMII mode for all FECs.
 		Note that this is a global option, we can't
 		have one FEC in standard MII mode and another in RMII mode.

 - CONFIG_CRC32_VERIFY
 		Add a verify option to the crc32 command.
 		The syntax is:

 		=> crc32 -v <address> <count> <crc32>

 		Where address/count indicate a memory area
 		and crc32 is the correct crc32 which the
 		area should have.

 - CONFIG_LOOPW
 		Add the "loopw" memory command. This only takes effect if
 		the memory commands are activated globally (CONFIG_CMD_MEMORY).

 - CONFIG_CMD_MX_CYCLIC
 		Add the "mdc" and "mwc" memory commands. These are cyclic
 		"md/mw" commands.
 		Examples:

 		=> mdc.b 10 4 500
 		This command will print 4 bytes (10,11,12,13) each 500 ms.

 		=> mwc.l 100 12345678 10
 		This command will write 12345678 to address 100 all 10 ms.

 		This only takes effect if the memory commands are activated
 		globally (CONFIG_CMD_MEMORY).

 - CONFIG_SPL_BUILD
 		Set when the currently-running compilation is for an artifact
 		that will end up in the SPL (as opposed to the TPL or U-Boot
 		proper). Code that needs stage-specific behavior should check
 		this.

 - CONFIG_TPL_BUILD
 		Set when the currently-running compilation is for an artifact
 		that will end up in the TPL (as opposed to the SPL or U-Boot
 		proper). Code that needs stage-specific behavior should check
 		this.

 - CONFIG_ARCH_MAP_SYSMEM
 		Generally U-Boot (and in particular the md command) uses
 		effective address. It is therefore not necessary to regard
 		U-Boot address as virtual addresses that need to be translated
 		to physical addresses. However, sandbox requires this, since
 		it maintains its own little RAM buffer which contains all
 		addressable memory. This option causes some memory accesses
 		to be mapped through map_sysmem() / unmap_sysmem().

 - CONFIG_X86_RESET_VECTOR
 		If defined, the x86 reset vector code is included. This is not
 		needed when U-Boot is running from Coreboot.

 Freescale QE/FMAN Firmware Support:
 -----------------------------------

 The Freescale QUICCEngine (QE) and Frame Manager (FMAN) both support the
 loading of "firmware", which is encoded in the QE firmware binary format.
 This firmware often needs to be loaded during U-Boot booting, so macros
 are used to identify the storage device (NOR flash, SPI, etc) and the address
 within that device.

 - CONFIG_SYS_FMAN_FW_ADDR
 	The address in the storage device where the FMAN microcode is located.  The
 	meaning of this address depends on which CONFIG_SYS_QE_FMAN_FW_IN_xxx macro
 	is also specified.

 - CONFIG_SYS_QE_FW_ADDR
 	The address in the storage device where the QE microcode is located.  The
 	meaning of this address depends on which CONFIG_SYS_QE_FMAN_FW_IN_xxx macro
 	is also specified.

 - CONFIG_SYS_QE_FMAN_FW_LENGTH
 	The maximum possible size of the firmware.  The firmware binary format
 	has a field that specifies the actual size of the firmware, but it
 	might not be possible to read any part of the firmware unless some
 	local storage is allocated to hold the entire firmware first.

 - CONFIG_SYS_QE_FMAN_FW_IN_NOR
 	Specifies that QE/FMAN firmware is located in NOR flash, mapped as
 	normal addressable memory via the LBC.  CONFIG_SYS_FMAN_FW_ADDR is the
 	virtual address in NOR flash.

 - CONFIG_SYS_QE_FMAN_FW_IN_NAND
 	Specifies that QE/FMAN firmware is located in NAND flash.
 	CONFIG_SYS_FMAN_FW_ADDR is the offset within NAND flash.

 - CONFIG_SYS_QE_FMAN_FW_IN_MMC
 	Specifies that QE/FMAN firmware is located on the primary SD/MMC
 	device.  CONFIG_SYS_FMAN_FW_ADDR is the byte offset on that device.

 - CONFIG_SYS_QE_FMAN_FW_IN_REMOTE
 	Specifies that QE/FMAN firmware is located in the remote (master)
 	memory space.	CONFIG_SYS_FMAN_FW_ADDR is a virtual address which
 	can be mapped from slave TLB->slave LAW->slave SRIO or PCIE outbound
 	window->master inbound window->master LAW->the ucode address in
 	master's memory space.

 Freescale Layerscape Management Complex Firmware Support:
 ---------------------------------------------------------
 The Freescale Layerscape Management Complex (MC) supports the loading of
 "firmware".
 This firmware often needs to be loaded during U-Boot booting, so macros
 are used to identify the storage device (NOR flash, SPI, etc) and the address
 within that device.

 - CONFIG_FSL_MC_ENET
 	Enable the MC driver for Layerscape SoCs.

 Freescale Layerscape Debug Server Support:
 -------------------------------------------
 The Freescale Layerscape Debug Server Support supports the loading of
 "Debug Server firmware" and triggering SP boot-rom.
 This firmware often needs to be loaded during U-Boot booting.

 - CONFIG_SYS_MC_RSV_MEM_ALIGN
 	Define alignment of reserved memory MC requires

 Reproducible builds
 -------------------

 In order to achieve reproducible builds, timestamps used in the U-Boot build
 process have to be set to a fixed value.

 This is done using the SOURCE_DATE_EPOCH environment variable.
 SOURCE_DATE_EPOCH is to be set on the build host's shell, not as a configuration
 option for U-Boot or an environment variable in U-Boot.

 SOURCE_DATE_EPOCH should be set to a number of seconds since the epoch, in UTC.

 Building the Software:
 ======================

 Building U-Boot has been tested in several native build environments
 and in many different cross environments. Of course we cannot support
 all possibly existing versions of cross development tools in all
 (potentially obsolete) versions. In case of tool chain problems we
 recommend to use the ELDK (see https://www.denx.de/wiki/DULG/ELDK)
 which is extensively used to build and test U-Boot.

 If you are not using a native environment, it is assumed that you
 have GNU cross compiling tools available in your path. In this case,
 you must set the environment variable CROSS_COMPILE in your shell.
 Note that no changes to the Makefile or any other source files are
 necessary. For example using the ELDK on a 4xx CPU, please enter:

 	$ CROSS_COMPILE=ppc_4xx-
 	$ export CROSS_COMPILE

 U-Boot is intended to be simple to build. After installing the
 sources you must configure U-Boot for one specific board type. This
 is done by typing:

 	make NAME_defconfig

 where "NAME_defconfig" is the name of one of the existing configu-
 rations; see configs/*_defconfig for supported names.

 Note: for some boards special configuration names may exist; check if
       additional information is available from the board vendor; for
       instance, the TQM823L systems are available without (standard)
       or with LCD support. You can select such additional "features"
       when choosing the configuration, i. e.

       make TQM823L_defconfig
 	- will configure for a plain TQM823L, i. e. no LCD support

       make TQM823L_LCD_defconfig
 	- will configure for a TQM823L with U-Boot console on LCD

       etc.


 Finally, type "make all", and you should get some working U-Boot
 images ready for download to / installation on your system:

 - "u-boot.bin" is a raw binary image
 - "u-boot" is an image in ELF binary format
 - "u-boot.srec" is in Motorola S-Record format

 By default the build is performed locally and the objects are saved
 in the source directory. One of the two methods can be used to change
 this behavior and build U-Boot to some external directory:

 1. Add O= to the make command line invocations:

 	make O=/tmp/build distclean
 	make O=/tmp/build NAME_defconfig
 	make O=/tmp/build all

 2. Set environment variable KBUILD_OUTPUT to point to the desired location:

 	export KBUILD_OUTPUT=/tmp/build
 	make distclean
 	make NAME_defconfig
 	make all

 Note that the command line "O=" setting overrides the KBUILD_OUTPUT environment
 variable.

 User specific CPPFLAGS, AFLAGS and CFLAGS can be passed to the compiler by
 setting the according environment variables KCPPFLAGS, KAFLAGS and KCFLAGS.
 For example to treat all compiler warnings as errors:

 	make KCFLAGS=-Werror

 Please be aware that the Makefiles assume you are using GNU make, so
 for instance on NetBSD you might need to use "gmake" instead of
 native "make".


 If the system board that you have is not listed, then you will need
 to port U-Boot to your hardware platform. To do this, follow these
 steps:

 1.  Create a new directory to hold your board specific code. Add any
     files you need. In your board directory, you will need at least
     the "Makefile" and a "<board>.c".
 2.  Create a new configuration file "include/configs/<board>.h" for
     your board.
 3.  If you're porting U-Boot to a new CPU, then also create a new
     directory to hold your CPU specific code. Add any files you need.
 4.  Run "make <board>_defconfig" with your new name.
 5.  Type "make", and you should get a working "u-boot.srec" file
     to be installed on your target system.
 6.  Debug and solve any problems that might arise.
     [Of course, this last step is much harder than it sounds.]


 Testing of U-Boot Modifications, Ports to New Hardware, etc.:
 ==============================================================

 If you have modified U-Boot sources (for instance added a new board
 or support for new devices, a new CPU, etc.) you are expected to
 provide feedback to the other developers. The feedback normally takes
 the form of a "patch", i.e. a context diff against a certain (latest
 official or latest in the git repository) version of U-Boot sources.

 But before you submit such a patch, please verify that your modifi-
 cation did not break existing code. At least make sure that *ALL* of
 the supported boards compile WITHOUT ANY compiler warnings. To do so,
 just run the buildman script (tools/buildman/buildman), which will
 configure and build U-Boot for ALL supported system. Be warned, this
 will take a while. Please see the buildman README, or run 'buildman -H'
 for documentation.


 See also "U-Boot Porting Guide" below.


 Monitor Commands - Overview:
 ============================

 go	- start application at address 'addr'
 run	- run commands in an environment variable
 bootm	- boot application image from memory
 bootp	- boot image via network using BootP/TFTP protocol
 bootz   - boot zImage from memory
 tftpboot- boot image via network using TFTP protocol
 	       and env variables "ipaddr" and "serverip"
 	       (and eventually "gatewayip")
 tftpput - upload a file via network using TFTP protocol
 rarpboot- boot image via network using RARP/TFTP protocol
 diskboot- boot from IDE devicebootd   - boot default, i.e., run 'bootcmd'
 loads	- load S-Record file over serial line
 loadb	- load binary file over serial line (kermit mode)
 loadm   - load binary blob from source address to destination address
 md	- memory display
 mm	- memory modify (auto-incrementing)
 nm	- memory modify (constant address)
 mw	- memory write (fill)
 ms	- memory search
 cp	- memory copy
 cmp	- memory compare
 crc32	- checksum calculation
 i2c	- I2C sub-system
 sspi	- SPI utility commands
 base	- print or set address offset
 printenv- print environment variables
 pwm	- control pwm channels
 setenv	- set environment variables
 saveenv - save environment variables to persistent storage
 protect - enable or disable FLASH write protection
 erase	- erase FLASH memory
 flinfo	- print FLASH memory information
 nand	- NAND memory operations (see doc/README.nand)
 bdinfo	- print Board Info structure
 iminfo	- print header information for application image
 coninfo - print console devices and informations
 ide	- IDE sub-system
 loop	- infinite loop on address range
 loopw	- infinite write loop on address range
 mtest	- simple RAM test
 icache	- enable or disable instruction cache
 dcache	- enable or disable data cache
 reset	- Perform RESET of the CPU
 echo	- echo args to console
 version - print monitor version
 help	- print online help
 ?	- alias for 'help'


 Monitor Commands - Detailed Description:
 ========================================

 TODO.

 For now: just type "help <command>".


 Note for Redundant Ethernet Interfaces:
 =======================================

 Some boards come with redundant Ethernet interfaces; U-Boot supports
 such configurations and is capable of automatic selection of a
 "working" interface when needed. MAC assignment works as follows:

 Network interfaces are numbered eth0, eth1, eth2, ... Corresponding
 MAC addresses can be stored in the environment as "ethaddr" (=>eth0),
 "eth1addr" (=>eth1), "eth2addr", ...

 If the network interface stores some valid MAC address (for instance
 in SROM), this is used as default address if there is NO correspon-
 ding setting in the environment; if the corresponding environment
 variable is set, this overrides the settings in the card; that means:

 o If the SROM has a valid MAC address, and there is no address in the
   environment, the SROM's address is used.

 o If there is no valid address in the SROM, and a definition in the
   environment exists, then the value from the environment variable is
   used.

 o If both the SROM and the environment contain a MAC address, and
   both addresses are the same, this MAC address is used.

 o If both the SROM and the environment contain a MAC address, and the
   addresses differ, the value from the environment is used and a
   warning is printed.

 o If neither SROM nor the environment contain a MAC address, an error
   is raised. If CONFIG_NET_RANDOM_ETHADDR is defined, then in this case
   a random, locally-assigned MAC is used.

 If Ethernet drivers implement the 'write_hwaddr' function, valid MAC addresses
 will be programmed into hardware as part of the initialization process.	 This
 may be skipped by setting the appropriate 'ethmacskip' environment variable.
 The naming convention is as follows:
 "ethmacskip" (=>eth0), "eth1macskip" (=>eth1) etc.

 Image Formats:
 ==============

 U-Boot is capable of booting (and performing other auxiliary operations on)
 images in two formats:

 New uImage format (FIT)
 -----------------------

 Flexible and powerful format based on Flattened Image Tree -- FIT (similar
 to Flattened Device Tree). It allows the use of images with multiple
 components (several kernels, ramdisks, etc.), with contents protected by
 SHA1, MD5 or CRC32. More details are found in the doc/uImage.FIT directory.


 Old uImage format
 -----------------

 Old image format is based on binary files which can be basically anything,
 preceded by a special header; see the definitions in include/image.h for
 details; basically, the header defines the following image properties:

 * Target Operating System (Provisions for OpenBSD, NetBSD, FreeBSD,
   4.4BSD, Linux, SVR4, Esix, Solaris, Irix, SCO, Dell, NCR, VxWorks,
   LynxOS, pSOS, QNX, RTEMS, INTEGRITY;
   Currently supported: Linux, NetBSD, VxWorks, QNX, RTEMS, INTEGRITY).
 * Target CPU Architecture (Provisions for Alpha, ARM, Intel x86,
   IA64, MIPS, Nios II, PowerPC, IBM S390, SuperH, Sparc, Sparc 64 Bit;
   Currently supported: ARM, Intel x86, MIPS, Nios II, PowerPC).
 * Compression Type (uncompressed, gzip, bzip2)
 * Load Address
 * Entry Point
 * Image Name
 * Image Timestamp

 The header is marked by a special Magic Number, and both the header
 and the data portions of the image are secured against corruption by
 CRC32 checksums.


 Linux Support:
 ==============

 Although U-Boot should support any OS or standalone application
 easily, the main focus has always been on Linux during the design of
 U-Boot.

 U-Boot includes many features that so far have been part of some
 special "boot loader" code within the Linux kernel. Also, any
 "initrd" images to be used are no longer part of one big Linux image;
 instead, kernel and "initrd" are separate images. This implementation
 serves several purposes:

 - the same features can be used for other OS or standalone
   applications (for instance: using compressed images to reduce the
   Flash memory footprint)

 - it becomes much easier to port new Linux kernel versions because
   lots of low-level, hardware dependent stuff are done by U-Boot

 - the same Linux kernel image can now be used with different "initrd"
   images; of course this also means that different kernel images can
   be run with the same "initrd". This makes testing easier (you don't
   have to build a new "zImage.initrd" Linux image when you just
   change a file in your "initrd"). Also, a field-upgrade of the
   software is easier now.


 Linux HOWTO:
 ============

 Porting Linux to U-Boot based systems:
 ---------------------------------------

 U-Boot cannot save you from doing all the necessary modifications to
 configure the Linux device drivers for use with your target hardware
 (no, we don't intend to provide a full virtual machine interface to
 Linux :-).

 But now you can ignore ALL boot loader code (in arch/powerpc/mbxboot).

 Just make sure your machine specific header file (for instance
 include/asm-ppc/tqm8xx.h) includes the same definition of the Board
 Information structure as we define in include/asm-<arch>/u-boot.h,
 and make sure that your definition of IMAP_ADDR uses the same value
 as your U-Boot configuration in CONFIG_SYS_IMMR.

 Note that U-Boot now has a driver model, a unified model for drivers.
 If you are adding a new driver, plumb it into driver model. If there
 is no uclass available, you are encouraged to create one. See
 doc/driver-model.


 Configuring the Linux kernel:
 -----------------------------

 No specific requirements for U-Boot. Make sure you have some root
 device (initial ramdisk, NFS) for your target system.


 Building a Linux Image:
 -----------------------

 With U-Boot, "normal" build targets like "zImage" or "bzImage" are
 not used. If you use recent kernel source, a new build target
 "uImage" will exist which automatically builds an image usable by
 U-Boot. Most older kernels also have support for a "pImage" target,
 which was introduced for our predecessor project PPCBoot and uses a
 100% compatible format.

 Example:

 	make TQM850L_defconfig
 	make oldconfig
 	make dep
 	make uImage

 The "uImage" build target uses a special tool (in 'tools/mkimage') to
 encapsulate a compressed Linux kernel image with header	 information,
 CRC32 checksum etc. for use with U-Boot. This is what we are doing:

 * build a standard "vmlinux" kernel image (in ELF binary format):

 * convert the kernel into a raw binary image:

 	${CROSS_COMPILE}-objcopy -O binary \
 				 -R .note -R .comment \
 				 -S vmlinux linux.bin

 * compress the binary image:

 	gzip -9 linux.bin

 * package compressed binary image for U-Boot:

 	mkimage -A ppc -O linux -T kernel -C gzip \
 		-a 0 -e 0 -n "Linux Kernel Image" \
 		-d linux.bin.gz uImage


 The "mkimage" tool can also be used to create ramdisk images for use
 with U-Boot, either separated from the Linux kernel image, or
 combined into one file. "mkimage" encapsulates the images with a 64
 byte header containing information about target architecture,
 operating system, image type, compression method, entry points, time
 stamp, CRC32 checksums, etc.

 "mkimage" can be called in two ways: to verify existing images and
 print the header information, or to build new images.

 In the first form (with "-l" option) mkimage lists the information
 contained in the header of an existing U-Boot image; this includes
 checksum verification:

 	tools/mkimage -l image
 	  -l ==> list image header information

 The second form (with "-d" option) is used to build a U-Boot image
 from a "data file" which is used as image payload:

 	tools/mkimage -A arch -O os -T type -C comp -a addr -e ep \
 		      -n name -d data_file image
 	  -A ==> set architecture to 'arch'
 	  -O ==> set operating system to 'os'
 	  -T ==> set image type to 'type'
 	  -C ==> set compression type 'comp'
 	  -a ==> set load address to 'addr' (hex)
 	  -e ==> set entry point to 'ep' (hex)
 	  -n ==> set image name to 'name'
 	  -d ==> use image data from 'datafile'

 Right now, all Linux kernels for PowerPC systems use the same load
 address (0x00000000), but the entry point address depends on the
 kernel version:

 - 2.2.x kernels have the entry point at 0x0000000C,
 - 2.3.x and later kernels have the entry point at 0x00000000.

 So a typical call to build a U-Boot image would read:

 	-> tools/mkimage -n '2.4.4 kernel for TQM850L' \
 	> -A ppc -O linux -T kernel -C gzip -a 0 -e 0 \
 	> -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux.gz \
 	> examples/uImage.TQM850L
 	Image Name:   2.4.4 kernel for TQM850L
 	Created:      Wed Jul 19 02:34:59 2000
 	Image Type:   PowerPC Linux Kernel Image (gzip compressed)
 	Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
 	Load Address: 0x00000000
 	Entry Point:  0x00000000

 To verify the contents of the image (or check for corruption):

 	-> tools/mkimage -l examples/uImage.TQM850L
 	Image Name:   2.4.4 kernel for TQM850L
 	Created:      Wed Jul 19 02:34:59 2000
 	Image Type:   PowerPC Linux Kernel Image (gzip compressed)
 	Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
 	Load Address: 0x00000000
 	Entry Point:  0x00000000

 NOTE: for embedded systems where boot time is critical you can trade
 speed for memory and install an UNCOMPRESSED image instead: this
 needs more space in Flash, but boots much faster since it does not
 need to be uncompressed:

 	-> gunzip /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux.gz
 	-> tools/mkimage -n '2.4.4 kernel for TQM850L' \
 	> -A ppc -O linux -T kernel -C none -a 0 -e 0 \
 	> -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux \
 	> examples/uImage.TQM850L-uncompressed
 	Image Name:   2.4.4 kernel for TQM850L
 	Created:      Wed Jul 19 02:34:59 2000
 	Image Type:   PowerPC Linux Kernel Image (uncompressed)
 	Data Size:    792160 Bytes = 773.59 kB = 0.76 MB
 	Load Address: 0x00000000
 	Entry Point:  0x00000000


 Similar you can build U-Boot images from a 'ramdisk.image.gz' file
 when your kernel is intended to use an initial ramdisk:

 	-> tools/mkimage -n 'Simple Ramdisk Image' \
 	> -A ppc -O linux -T ramdisk -C gzip \
 	> -d /LinuxPPC/images/SIMPLE-ramdisk.image.gz examples/simple-initrd
 	Image Name:   Simple Ramdisk Image
 	Created:      Wed Jan 12 14:01:50 2000
 	Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
 	Data Size:    566530 Bytes = 553.25 kB = 0.54 MB
 	Load Address: 0x00000000
 	Entry Point:  0x00000000

 The "dumpimage" tool can be used to disassemble or list the contents of images
 built by mkimage. See dumpimage's help output (-h) for details.

 Installing a Linux Image:
 -------------------------

 To downloading a U-Boot image over the serial (console) interface,
 you must convert the image to S-Record format:

 	objcopy -I binary -O srec examples/image examples/image.srec

 The 'objcopy' does not understand the information in the U-Boot
 image header, so the resulting S-Record file will be relative to
 address 0x00000000. To load it to a given address, you need to
 specify the target address as 'offset' parameter with the 'loads'
 command.

 Example: install the image to address 0x40100000 (which on the
 TQM8xxL is in the first Flash bank):

 	=> erase 40100000 401FFFFF

 	.......... done
 	Erased 8 sectors

 	=> loads 40100000
 	## Ready for S-Record download ...
 	~>examples/image.srec
 	1 2 3 4 5 6 7 8 9 10 11 12 13 ...
 	...
 	15989 15990 15991 15992
 	[file transfer complete]
 	[connected]
 	## Start Addr = 0x00000000


 You can check the success of the download using the 'iminfo' command;
 this includes a checksum verification so you can be sure no data
 corruption happened:

 	=> imi 40100000

 	## Checking Image at 40100000 ...
 	   Image Name:	 2.2.13 for initrd on TQM850L
 	   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
 	   Data Size:	 335725 Bytes = 327 kB = 0 MB
 	   Load Address: 00000000
 	   Entry Point:	 0000000c
 	   Verifying Checksum ... OK


 Boot Linux:
 -----------

 The "bootm" command is used to boot an application that is stored in
 memory (RAM or Flash). In case of a Linux kernel image, the contents
 of the "bootargs" environment variable is passed to the kernel as
 parameters. You can check and modify this variable using the
 "printenv" and "setenv" commands:


 	=> printenv bootargs
 	bootargs=root=/dev/ram

 	=> setenv bootargs root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2

 	=> printenv bootargs
 	bootargs=root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2

 	=> bootm 40020000
 	## Booting Linux kernel at 40020000 ...
 	   Image Name:	 2.2.13 for NFS on TQM850L
 	   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
 	   Data Size:	 381681 Bytes = 372 kB = 0 MB
 	   Load Address: 00000000
 	   Entry Point:	 0000000c
 	   Verifying Checksum ... OK
 	   Uncompressing Kernel Image ... OK
 	Linux version 2.2.13 (wd@denx.local.net) (gcc version 2.95.2 19991024 (release)) #1 Wed Jul 19 02:35:17 MEST 2000
 	Boot arguments: root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2
 	time_init: decrementer frequency = 187500000/60
 	Calibrating delay loop... 49.77 BogoMIPS
 	Memory: 15208k available (700k kernel code, 444k data, 32k init) [c0000000,c1000000]
 	...

 If you want to boot a Linux kernel with initial RAM disk, you pass
 the memory addresses of both the kernel and the initrd image (PPBCOOT
 format!) to the "bootm" command:

 	=> imi 40100000 40200000

 	## Checking Image at 40100000 ...
 	   Image Name:	 2.2.13 for initrd on TQM850L
 	   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
 	   Data Size:	 335725 Bytes = 327 kB = 0 MB
 	   Load Address: 00000000
 	   Entry Point:	 0000000c
 	   Verifying Checksum ... OK

 	## Checking Image at 40200000 ...
 	   Image Name:	 Simple Ramdisk Image
 	   Image Type:	 PowerPC Linux RAMDisk Image (gzip compressed)
 	   Data Size:	 566530 Bytes = 553 kB = 0 MB
 	   Load Address: 00000000
 	   Entry Point:	 00000000
 	   Verifying Checksum ... OK

 	=> bootm 40100000 40200000
 	## Booting Linux kernel at 40100000 ...
 	   Image Name:	 2.2.13 for initrd on TQM850L
 	   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
 	   Data Size:	 335725 Bytes = 327 kB = 0 MB
 	   Load Address: 00000000
 	   Entry Point:	 0000000c
 	   Verifying Checksum ... OK
 	   Uncompressing Kernel Image ... OK
 	## Loading RAMDisk Image at 40200000 ...
 	   Image Name:	 Simple Ramdisk Image
 	   Image Type:	 PowerPC Linux RAMDisk Image (gzip compressed)
 	   Data Size:	 566530 Bytes = 553 kB = 0 MB
 	   Load Address: 00000000
 	   Entry Point:	 00000000
 	   Verifying Checksum ... OK
 	   Loading Ramdisk ... OK
 	Linux version 2.2.13 (wd@denx.local.net) (gcc version 2.95.2 19991024 (release)) #1 Wed Jul 19 02:32:08 MEST 2000
 	Boot arguments: root=/dev/ram
 	time_init: decrementer frequency = 187500000/60
 	Calibrating delay loop... 49.77 BogoMIPS
 	...
 	RAMDISK: Compressed image found at block 0
 	VFS: Mounted root (ext2 filesystem).

 	bash#

 Boot Linux and pass a flat device tree:
 -----------

 First, U-Boot must be compiled with the appropriate defines. See the section
 titled "Linux Kernel Interface" above for a more in depth explanation. The
 following is an example of how to start a kernel and pass an updated
 flat device tree:

 => print oftaddr
 oftaddr=0x300000
 => print oft
 oft=oftrees/mpc8540ads.dtb
 => tftp $oftaddr $oft
 Speed: 1000, full duplex
 Using TSEC0 device
 TFTP from server 192.168.1.1; our IP address is 192.168.1.101
 Filename 'oftrees/mpc8540ads.dtb'.
 Load address: 0x300000
 Loading: #
 done
 Bytes transferred = 4106 (100a hex)
 => tftp $loadaddr $bootfile
 Speed: 1000, full duplex
 Using TSEC0 device
 TFTP from server 192.168.1.1; our IP address is 192.168.1.2
 Filename 'uImage'.
 Load address: 0x200000
 Loading:############
 done
 Bytes transferred = 1029407 (fb51f hex)
 => print loadaddr
 loadaddr=200000
 => print oftaddr
 oftaddr=0x300000
 => bootm $loadaddr - $oftaddr
 ## Booting image at 00200000 ...
    Image Name:	 Linux-2.6.17-dirty
    Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
    Data Size:	 1029343 Bytes = 1005.2 kB
    Load Address: 00000000
    Entry Point:	 00000000
    Verifying Checksum ... OK
    Uncompressing Kernel Image ... OK
 Booting using flat device tree at 0x300000
 Using MPC85xx ADS machine description
 Memory CAM mapping: CAM0=256Mb, CAM1=256Mb, CAM2=0Mb residual: 0Mb
 [snip]


 More About U-Boot Image Types:
 ------------------------------

 U-Boot supports the following image types:

    "Standalone Programs" are directly runnable in the environment
 	provided by U-Boot; it is expected that (if they behave
 	well) you can continue to work in U-Boot after return from
 	the Standalone Program.
    "OS Kernel Images" are usually images of some Embedded OS which
 	will take over control completely. Usually these programs
 	will install their own set of exception handlers, device
 	drivers, set up the MMU, etc. - this means, that you cannot
 	expect to re-enter U-Boot except by resetting the CPU.
    "RAMDisk Images" are more or less just data blocks, and their
 	parameters (address, size) are passed to an OS kernel that is
 	being started.
    "Multi-File Images" contain several images, typically an OS
 	(Linux) kernel image and one or more data images like
 	RAMDisks. This construct is useful for instance when you want
 	to boot over the network using BOOTP etc., where the boot
 	server provides just a single image file, but you want to get
 	for instance an OS kernel and a RAMDisk image.

 	"Multi-File Images" start with a list of image sizes, each
 	image size (in bytes) specified by an "uint32_t" in network
 	byte order. This list is terminated by an "(uint32_t)0".
 	Immediately after the terminating 0 follow the images, one by
 	one, all aligned on "uint32_t" boundaries (size rounded up to
 	a multiple of 4 bytes).

    "Firmware Images" are binary images containing firmware (like
 	U-Boot or FPGA images) which usually will be programmed to
 	flash memory.

    "Script files" are command sequences that will be executed by
 	U-Boot's command interpreter; this feature is especially
 	useful when you configure U-Boot to use a real shell (hush)
 	as command interpreter.

 Booting the Linux zImage:
 -------------------------

 On some platforms, it's possible to boot Linux zImage. This is done
 using the "bootz" command. The syntax of "bootz" command is the same
 as the syntax of "bootm" command.

 Note, defining the CONFIG_SUPPORT_RAW_INITRD allows user to supply
 kernel with raw initrd images. The syntax is slightly different, the
 address of the initrd must be augmented by it's size, in the following
 format: "<initrd addres>:<initrd size>".


 Standalone HOWTO:
 =================

 One of the features of U-Boot is that you can dynamically load and
 run "standalone" applications, which can use some resources of
 U-Boot like console I/O functions or interrupt services.

 Two simple examples are included with the sources:

 "Hello World" Demo:
 -------------------

 'examples/hello_world.c' contains a small "Hello World" Demo
 application; it is automatically compiled when you build U-Boot.
 It's configured to run at address 0x00040004, so you can play with it
 like that:

 	=> loads
 	## Ready for S-Record download ...
 	~>examples/hello_world.srec
 	1 2 3 4 5 6 7 8 9 10 11 ...
 	[file transfer complete]
 	[connected]
 	## Start Addr = 0x00040004

 	=> go 40004 Hello World! This is a test.
 	## Starting application at 0x00040004 ...
 	Hello World
 	argc = 7
 	argv[0] = "40004"
 	argv[1] = "Hello"
 	argv[2] = "World!"
 	argv[3] = "This"
 	argv[4] = "is"
 	argv[5] = "a"
 	argv[6] = "test."
 	argv[7] = "<NULL>"
 	Hit any key to exit ...

 	## Application terminated, rc = 0x0

 Another example, which demonstrates how to register a CPM interrupt
 handler with the U-Boot code, can be found in 'examples/timer.c'.
 Here, a CPM timer is set up to generate an interrupt every second.
 The interrupt service routine is trivial, just printing a '.'
 character, but this is just a demo program. The application can be
 controlled by the following keys:

 	? - print current values og the CPM Timer registers
 	b - enable interrupts and start timer
 	e - stop timer and disable interrupts
 	q - quit application

 	=> loads
 	## Ready for S-Record download ...
 	~>examples/timer.srec
 	1 2 3 4 5 6 7 8 9 10 11 ...
 	[file transfer complete]
 	[connected]
 	## Start Addr = 0x00040004

 	=> go 40004
 	## Starting application at 0x00040004 ...
 	TIMERS=0xfff00980
 	Using timer 1
 	  tgcr @ 0xfff00980, tmr @ 0xfff00990, trr @ 0xfff00994, tcr @ 0xfff00998, tcn @ 0xfff0099c, ter @ 0xfff009b0

 Hit 'b':
 	[q, b, e, ?] Set interval 1000000 us
 	Enabling timer
 Hit '?':
 	[q, b, e, ?] ........
 	tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0xef6, ter=0x0
 Hit '?':
 	[q, b, e, ?] .
 	tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x2ad4, ter=0x0
 Hit '?':
 	[q, b, e, ?] .
 	tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x1efc, ter=0x0
 Hit '?':
 	[q, b, e, ?] .
 	tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x169d, ter=0x0
 Hit 'e':
 	[q, b, e, ?] ...Stopping timer
 Hit 'q':
 	[q, b, e, ?] ## Application terminated, rc = 0x0


 Minicom warning:
 ================

 Over time, many people have reported problems when trying to use the
 "minicom" terminal emulation program for serial download. I (wd)
 consider minicom to be broken, and recommend not to use it. Under
 Unix, I recommend to use C-Kermit for general purpose use (and
 especially for kermit binary protocol download ("loadb" command), and
 use "cu" for S-Record download ("loads" command).  See
 https://www.denx.de/wiki/view/DULG/SystemSetup#Section_4.3.
 for help with kermit.


 Nevertheless, if you absolutely want to use it try adding this
 configuration to your "File transfer protocols" section:

 	   Name	   Program			Name U/D FullScr IO-Red. Multi
 	X  kermit  /usr/bin/kermit -i -l %l -s	 Y    U	   Y	   N	  N
 	Y  kermit  /usr/bin/kermit -i -l %l -r	 N    D	   Y	   N	  N


 NetBSD Notes:
 =============

 Starting at version 0.9.2, U-Boot supports NetBSD both as host
 (build U-Boot) and target system (boots NetBSD/mpc8xx).

 Building requires a cross environment; it is known to work on
 NetBSD/i386 with the cross-powerpc-netbsd-1.3 package (you will also
 need gmake since the Makefiles are not compatible with BSD make).
 Note that the cross-powerpc package does not install include files;
 attempting to build U-Boot will fail because <machine/ansi.h> is
 missing.  This file has to be installed and patched manually:

 	# cd /usr/pkg/cross/powerpc-netbsd/include
 	# mkdir powerpc
 	# ln -s powerpc machine
 	# cp /usr/src/sys/arch/powerpc/include/ansi.h powerpc/ansi.h
 	# ${EDIT} powerpc/ansi.h	## must remove __va_list, _BSD_VA_LIST

 Native builds *don't* work due to incompatibilities between native
 and U-Boot include files.

 Booting assumes that (the first part of) the image booted is a
 stage-2 loader which in turn loads and then invokes the kernel
 proper. Loader sources will eventually appear in the NetBSD source
 tree (probably in sys/arc/mpc8xx/stand/u-boot_stage2/); in the
 meantime, see ftp://ftp.denx.de/pub/u-boot/ppcboot_stage2.tar.gz


 Implementation Internals:
 =========================

 The following is not intended to be a complete description of every
 implementation detail. However, it should help to understand the
 inner workings of U-Boot and make it easier to port it to custom
 hardware.


 Initial Stack, Global Data:
 ---------------------------

 The implementation of U-Boot is complicated by the fact that U-Boot
 starts running out of ROM (flash memory), usually without access to
 system RAM (because the memory controller is not initialized yet).
 This means that we don't have writable Data or BSS segments, and BSS
 is not initialized as zero. To be able to get a C environment working
 at all, we have to allocate at least a minimal stack. Implementation
 options for this are defined and restricted by the CPU used: Some CPU
 models provide on-chip memory (like the IMMR area on MPC8xx and
 MPC826x processors), on others (parts of) the data cache can be
 locked as (mis-) used as memory, etc.

 	Chris Hallinan posted a good summary of these issues to the
 	U-Boot mailing list:

 	Subject: RE: [U-Boot-Users] RE: More On Memory Bank x (nothingness)?
 	From: "Chris Hallinan" <clh@net1plus.com>
 	Date: Mon, 10 Feb 2003 16:43:46 -0500 (22:43 MET)
 	...

 	Correct me if I'm wrong, folks, but the way I understand it
 	is this: Using DCACHE as initial RAM for Stack, etc, does not
 	require any physical RAM backing up the cache. The cleverness
 	is that the cache is being used as a temporary supply of
 	necessary storage before the SDRAM controller is setup. It's
 	beyond the scope of this list to explain the details, but you
 	can see how this works by studying the cache architecture and
 	operation in the architecture and processor-specific manuals.

 	OCM is On Chip Memory, which I believe the 405GP has 4K. It
 	is another option for the system designer to use as an
 	initial stack/RAM area prior to SDRAM being available. Either
 	option should work for you. Using CS 4 should be fine if your
 	board designers haven't used it for something that would
 	cause you grief during the initial boot! It is frequently not
 	used.

 	CFG_SYS_INIT_RAM_ADDR should be somewhere that won't interfere
 	with your processor/board/system design. The default value
 	you will find in any recent u-boot distribution in
 	walnut.h should work for you. I'd set it to a value larger
 	than your SDRAM module. If you have a 64MB SDRAM module, set
 	it above 400_0000. Just make sure your board has no resources
 	that are supposed to respond to that address! That code in
 	start.S has been around a while and should work as is when
 	you get the config right.

 	-Chris Hallinan
 	DS4.COM, Inc.

 It is essential to remember this, since it has some impact on the C
 code for the initialization procedures:

 * Initialized global data (data segment) is read-only. Do not attempt
   to write it.

 * Do not use any uninitialized global data (or implicitly initialized
   as zero data - BSS segment) at all - this is undefined, initiali-
   zation is performed later (when relocating to RAM).

 * Stack space is very limited. Avoid big data buffers or things like
   that.

 Having only the stack as writable memory limits means we cannot use
 normal global data to share information between the code. But it
 turned out that the implementation of U-Boot can be greatly
 simplified by making a global data structure (gd_t) available to all
 functions. We could pass a pointer to this data as argument to _all_
 functions, but this would bloat the code. Instead we use a feature of
 the GCC compiler (Global Register Variables) to share the data: we
 place a pointer (gd) to the global data into a register which we
 reserve for this purpose.

 When choosing a register for such a purpose we are restricted by the
 relevant  (E)ABI  specifications for the current architecture, and by
 GCC's implementation.

 For PowerPC, the following registers have specific use:
 	R1:	stack pointer
 	R2:	reserved for system use
 	R3-R4:	parameter passing and return values
 	R5-R10: parameter passing
 	R13:	small data area pointer
 	R30:	GOT pointer
 	R31:	frame pointer

 	(U-Boot also uses R12 as internal GOT pointer. r12
 	is a volatile register so r12 needs to be reset when
 	going back and forth between asm and C)

     ==> U-Boot will use R2 to hold a pointer to the global data

     Note: on PPC, we could use a static initializer (since the
     address of the global data structure is known at compile time),
     but it turned out that reserving a register results in somewhat
     smaller code - although the code savings are not that big (on
     average for all boards 752 bytes for the whole U-Boot image,
     624 text + 127 data).

 On ARM, the following registers are used:

 	R0:	function argument word/integer result
 	R1-R3:	function argument word
 	R9:	platform specific
 	R10:	stack limit (used only if stack checking is enabled)
 	R11:	argument (frame) pointer
 	R12:	temporary workspace
 	R13:	stack pointer
 	R14:	link register
 	R15:	program counter

     ==> U-Boot will use R9 to hold a pointer to the global data

     Note: on ARM, only R_ARM_RELATIVE relocations are supported.

 On Nios II, the ABI is documented here:
 	https://www.altera.com/literature/hb/nios2/n2cpu_nii51016.pdf

     ==> U-Boot will use gp to hold a pointer to the global data

     Note: on Nios II, we give "-G0" option to gcc and don't use gp
     to access small data sections, so gp is free.

 On RISC-V, the following registers are used:

 	x0: hard-wired zero (zero)
 	x1: return address (ra)
 	x2:	stack pointer (sp)
 	x3:	global pointer (gp)
 	x4:	thread pointer (tp)
 	x5:	link register (t0)
 	x8:	frame pointer (fp)
 	x10-x11:	arguments/return values (a0-1)
 	x12-x17:	arguments (a2-7)
 	x28-31:	 temporaries (t3-6)
 	pc:	program counter (pc)

     ==> U-Boot will use gp to hold a pointer to the global data

 Memory Management:
 ------------------

 U-Boot runs in system state and uses physical addresses, i.e. the
 MMU is not used either for address mapping nor for memory protection.

 The available memory is mapped to fixed addresses using the memory
 controller. In this process, a contiguous block is formed for each
 memory type (Flash, SDRAM, SRAM), even when it consists of several
 physical memory banks.

 U-Boot is installed in the first 128 kB of the first Flash bank (on
 TQM8xxL modules this is the range 0x40000000 ... 0x4001FFFF). After
 booting and sizing and initializing DRAM, the code relocates itself
 to the upper end of DRAM. Immediately below the U-Boot code some
 memory is reserved for use by malloc() [see CONFIG_SYS_MALLOC_LEN
 configuration setting]. Below that, a structure with global Board
 Info data is placed, followed by the stack (growing downward).

 Additionally, some exception handler code is copied to the low 8 kB
 of DRAM (0x00000000 ... 0x00001FFF).

 So a typical memory configuration with 16 MB of DRAM could look like
 this:

 	0x0000 0000	Exception Vector code
 	      :
 	0x0000 1FFF
 	0x0000 2000	Free for Application Use
 	      :
 	      :

 	      :
 	      :
 	0x00FB FF20	Monitor Stack (Growing downward)
 	0x00FB FFAC	Board Info Data and permanent copy of global data
 	0x00FC 0000	Malloc Arena
 	      :
 	0x00FD FFFF
 	0x00FE 0000	RAM Copy of Monitor Code
 	...		eventually: LCD or video framebuffer
 	...		eventually: pRAM (Protected RAM - unchanged by reset)
 	0x00FF FFFF	[End of RAM]


 System Initialization:
 ----------------------

 In the reset configuration, U-Boot starts at the reset entry point
 (on most PowerPC systems at address 0x00000100). Because of the reset
 configuration for CS0# this is a mirror of the on board Flash memory.
 To be able to re-map memory U-Boot then jumps to its link address.
 To be able to implement the initialization code in C, a (small!)
 initial stack is set up in the internal Dual Ported RAM (in case CPUs
 which provide such a feature like), or in a locked part of the data
 cache. After that, U-Boot initializes the CPU core, the caches and
 the SIU.

 Next, all (potentially) available memory banks are mapped using a
 preliminary mapping. For example, we put them on 512 MB boundaries
 (multiples of 0x20000000: SDRAM on 0x00000000 and 0x20000000, Flash
 on 0x40000000 and 0x60000000, SRAM on 0x80000000). Then UPM A is
 programmed for SDRAM access. Using the temporary configuration, a
 simple memory test is run that determines the size of the SDRAM
 banks.

 When there is more than one SDRAM bank, and the banks are of
 different size, the largest is mapped first. For equal size, the first
 bank (CS2#) is mapped first. The first mapping is always for address
 0x00000000, with any additional banks following immediately to create
 contiguous memory starting from 0.

 Then, the monitor installs itself at the upper end of the SDRAM area
 and allocates memory for use by malloc() and for the global Board
 Info data; also, the exception vector code is copied to the low RAM
 pages, and the final stack is set up.

 Only after this relocation will you have a "normal" C environment;
 until that you are restricted in several ways, mostly because you are
 running from ROM, and because the code will have to be relocated to a
 new address in RAM.


 U-Boot Porting Guide:
 ----------------------

 [Based on messages by Jerry Van Baren in the U-Boot-Users mailing
 list, October 2002]


 int main(int argc, char *argv[])
 {
 	sighandler_t no_more_time;

 	signal(SIGALRM, no_more_time);
 	alarm(PROJECT_DEADLINE - toSec (3 * WEEK));

 	if (available_money > available_manpower) {
 		Pay consultant to port U-Boot;
 		return 0;
 	}

 	Download latest U-Boot source;

 	Subscribe to u-boot mailing list;

 	if (clueless)
 		email("Hi, I am new to U-Boot, how do I get started?");

 	while (learning) {
 		Read the README file in the top level directory;
 		Read https://www.denx.de/wiki/bin/view/DULG/Manual;
 		Read applicable doc/README.*;
 		Read the source, Luke;
 		/* find . -name "*.[chS]" | xargs grep -i <keyword> */
 	}

 	if (available_money > toLocalCurrency ($2500))
 		Buy a BDI3000;
 	else
 		Add a lot of aggravation and time;

 	if (a similar board exists) {	/* hopefully... */
 		cp -a board/<similar> board/<myboard>
 		cp include/configs/<similar>.h include/configs/<myboard>.h
 	} else {
 		Create your own board support subdirectory;
 		Create your own board include/configs/<myboard>.h file;
 	}
 	Edit new board/<myboard> files
 	Edit new include/configs/<myboard>.h

 	while (!accepted) {
 		while (!running) {
 			do {
 				Add / modify source code;
 			} until (compiles);
 			Debug;
 			if (clueless)
 				email("Hi, I am having problems...");
 		}
 		Send patch file to the U-Boot email list;
 		if (reasonable critiques)
 			Incorporate improvements from email list code review;
 		else
 			Defend code as written;
 	}

 	return 0;
 }

 void no_more_time (int sig)
 {
       hire_a_guru();
 }


 Coding Standards:
 -----------------

 All contributions to U-Boot should conform to the Linux kernel
 coding style; see the kernel coding style guide at
 https://www.kernel.org/doc/html/latest/process/coding-style.html, and the
 script "scripts/Lindent" in your Linux kernel source directory.

 Source files originating from a different project (for example the
 MTD subsystem) are generally exempt from these guidelines and are not
 reformatted to ease subsequent migration to newer versions of those
 sources.

 Please note that U-Boot is implemented in C (and to some small parts in
 Assembler); no C++ is used, so please do not use C++ style comments (//)
 in your code.

 Please also stick to the following formatting rules:
 - remove any trailing white space
 - use TAB characters for indentation and vertical alignment, not spaces
 - make sure NOT to use DOS '\r\n' line feeds
 - do not add more than 2 consecutive empty lines to source files
 - do not add trailing empty lines to source files

 Submissions which do not conform to the standards may be returned
 with a request to reformat the changes.


 Submitting Patches:
 -------------------

 Since the number of patches for U-Boot is growing, we need to
 establish some rules. Submissions which do not conform to these rules
 may be rejected, even when they contain important and valuable stuff.

 Please see https://www.denx.de/wiki/U-Boot/Patches for details.

 Patches shall be sent to the u-boot mailing list <u-boot@lists.denx.de>;
 see https://lists.denx.de/listinfo/u-boot

 When you send a patch, please include the following information with
 it:

 * For bug fixes: a description of the bug and how your patch fixes
   this bug. Please try to include a way of demonstrating that the
   patch actually fixes something.

 * For new features: a description of the feature and your
   implementation.

 * For major contributions, add a MAINTAINERS file with your
   information and associated file and directory references.

 * When you add support for a new board, don't forget to add a
   maintainer e-mail address to the boards.cfg file, too.

 * If your patch adds new configuration options, don't forget to
   document these in the README file.

 * The patch itself. If you are using git (which is *strongly*
   recommended) you can easily generate the patch using the
   "git format-patch". If you then use "git send-email" to send it to
   the U-Boot mailing list, you will avoid most of the common problems
   with some other mail clients.

   If you cannot use git, use "diff -purN OLD NEW". If your version of
   diff does not support these options, then get the latest version of
   GNU diff.

   The current directory when running this command shall be the parent
   directory of the U-Boot source tree (i. e. please make sure that
   your patch includes sufficient directory information for the
   affected files).

   We prefer patches as plain text. MIME attachments are discouraged,
   and compressed attachments must not be used.

 * If one logical set of modifications affects or creates several
   files, all these changes shall be submitted in a SINGLE patch file.

 * Changesets that contain different, unrelated modifications shall be
   submitted as SEPARATE patches, one patch per changeset.


 Notes:

 * Before sending the patch, run the buildman script on your patched
   source tree and make sure that no errors or warnings are reported
   for any of the boards.

 * Keep your modifications to the necessary minimum: A patch
   containing several unrelated changes or arbitrary reformats will be
   returned with a request to re-formatting / split it.

 * If you modify existing code, make sure that your new code does not
   add to the memory footprint of the code ;-) Small is beautiful!
   When adding new features, these should compile conditionally only
   (using #ifdef), and the resulting code with the new feature
   disabled must not need more memory than the old code without your
   modification.

 * Remember that there is a size limit of 100 kB per message on the
   u-boot mailing list. Bigger patches will be moderated. If they are
   reasonable and not too big, they will be acknowledged. But patches
   bigger than the size limit should be avoided.