doc/usage/environment.rst - platform/external/u-boot - Gitiles

 .. SPDX-License-Identifier: GPL-2.0+

 Environment Variables
 =====================

 U-Boot supports user configuration using environment variables which
 can be made persistent by saving to persistent storage, for example flash
 memory.

 Environment variables are set using "env set" (alias "setenv"), printed using
 "env print" (alias "printenv"), and saved to persistent storage using
 "env save" (alias "saveenv"). Using "env set"
 without a value can be used to delete a variable from the
 environment. As long as you don't save the environment, you are
 working with an in-memory copy. In case the Flash area containing the
 environment is erased by accident, a default environment is provided.

 See :doc:`cmd/env` for details.

 Some configuration is controlled by Environment Variables, so that setting the
 variable can adjust the behaviour of U-Boot (e.g. autoboot delay, autoloading
 from tftp).

 Text-based Environment
 ----------------------

 The default environment for a board is created using a `.env` environment file
 using a simple text format. The base filename for this is defined by
 `CONFIG_ENV_SOURCE_FILE`, or `CONFIG_SYS_BOARD` if that is empty.

 The file must be in the board directory and have a .env extension, so
 assuming that there is a board vendor, the resulting filename is therefore::

    board/<vendor>/<board>/<CONFIG_ENV_SOURCE_FILE>.env

 or::

    board/<vendor>/<board>/<CONFIG_SYS_BOARD>.env

 This is a plain text file where you can type your environment variables in
 the form `var=value`. Blank lines and multi-line variables are supported.
 The conversion script looks for a line that starts in column 1 with a string
 and has an equals sign immediately afterwards. Spaces before the = are not
 permitted. It is a good idea to indent your scripts so that only the 'var='
 appears at the start of a line.

 To add additional text to a variable you can use `var+=value`. This text is
 merged into the variable during the make process and made available as a
 single value to U-Boot. Variables can contain `+` characters but in the unlikely
 event that you want to have a variable name ending in plus, put a backslash
 before the `+` so that the script knows you are not adding to an existing
 variable but assigning to a new one::

     maximum\+=value

 This file can include C-style comments. Blank lines and multi-line
 variables are supported, and you can use normal C preprocessor directives
 and CONFIG defines from your board config also.

 For example, for snapper9260 you would create a text file called
 `board/bluewater/snapper9260.env` containing the environment text.

 Example::

     stdout=serial
     #ifdef CONFIG_LCD
     stdout+=,lcd
     #endif
     bootcmd=
         /* U-Boot script for booting */

         if [ -z ${tftpserverip} ]; then
             echo "Use 'setenv tftpserverip a.b.c.d' to set IP address."
         fi

         usb start; setenv autoload n; bootp;
         tftpboot ${tftpserverip}:
         bootm
     failed=
         /* Print a message when boot fails */
         echo CONFIG_SYS_BOARD boot failed - please check your image
         echo Load address is CONFIG_SYS_LOAD_ADDR

 If CONFIG_ENV_SOURCE_FILE is empty and the default filename is not present, then
 the old-style C environment is used instead. See below.

 Old-style C environment
 -----------------------

 Traditionally, the default environment is created in `include/env_default.h`,
 and can be augmented by various `CONFIG` defines. See that file for details. In
 particular you can define `CONFIG_EXTRA_ENV_SETTINGS` in your board file
 to add environment variables.

 Board maintainers are encouraged to migrate to the text-based environment as it
 is easier to maintain. The distro-board script still requires the old-style
 environment but work is underway to address this.


 List of environment variables
 -----------------------------

 Some device configuration options can be set using environment variables. In
 many cases the value in the default environment comes from a CONFIG option - see
 `include/env_default.h`) for this.

 This is most-likely not complete:

 baudrate
     Used to set the baudrate of the UART - it defaults to CONFIG_BAUDRATE (which
     defaults to 115200).

 bootdelay
     Delay before automatically running bootcmd. During this time the user
     can choose to enter the shell (or the boot menu if
     CONFIG_AUTOBOOT_MENU_SHOW=y):

     - 0 to autoboot with no delay, but you can stop it by key input.
     - -1 to disable autoboot.
     - -2 to autoboot with no delay and not check for abort

     The default value is defined by CONFIG_BOOTDELAY.
     The value of 'bootdelay' is overridden by the /config/bootdelay value in
     the device-tree if CONFIG_OF_CONTROL=y.

 bootcmd
     The command that is run if the user does not enter the shell during the
     boot delay.

 bootargs
     Command line arguments passed when booting an operating system or binary
     image

 bootfile
     Name of the image to load with TFTP

 bootm_low
     Memory range available for image processing in the bootm
     command can be restricted. This variable is given as
     a hexadecimal number and defines lowest address allowed
     for use by the bootm command. See also "bootm_size"
     environment variable. Address defined by "bootm_low" is
     also the base of the initial memory mapping for the Linux
     kernel -- see the description of CONFIG_SYS_BOOTMAPSZ and
     bootm_mapsize.

 bootm_mapsize
     Size of the initial memory mapping for the Linux kernel.
     This variable is given as a hexadecimal number and it
     defines the size of the memory region starting at base
     address bootm_low that is accessible by the Linux kernel
     during early boot.  If unset, CONFIG_SYS_BOOTMAPSZ is used
     as the default value if it is defined, and bootm_size is
     used otherwise.

 bootm_size
     Memory range available for image processing in the bootm
     command can be restricted. This variable is given as
     a hexadecimal number and defines the size of the region
     allowed for use by the bootm command. See also "bootm_low"
     environment variable.

 bootstopkeysha256, bootdelaykey, bootstopkey
     See README.autoboot

 updatefile
     Location of the software update file on a TFTP server, used
     by the automatic software update feature. Please refer to
     documentation in doc/README.update for more details.

 autoload
     if set to "no" (any string beginning with 'n'),
     "bootp" and "dhcp" will just load perform a lookup of the
     configuration from the BOOTP server, but not try to
     load any image.

 autostart
     if set to "yes", an image loaded using the "bootp", "dhcp",
     "rarpboot", "tftpboot" or "diskboot" commands will
     be automatically started (by internally calling
     "bootm")

     If unset, or set to "1"/"yes"/"true" (case insensitive, just the first
     character is enough), a standalone image
     passed to the "bootm" command will be copied to the load address
     (and eventually uncompressed), but NOT be started.
     This can be used to load and uncompress arbitrary
     data.

 fdt_high
     if set this restricts the maximum address that the
     flattened device tree will be copied into upon boot.
     For example, if you have a system with 1 GB memory
     at physical address 0x10000000, while Linux kernel
     only recognizes the first 704 MB as low memory, you
     may need to set fdt_high as 0x3C000000 to have the
     device tree blob be copied to the maximum address
     of the 704 MB low memory, so that Linux kernel can
     access it during the boot procedure.

     If this is set to the special value 0xffffffff (32-bit machines) or
     0xffffffffffffffff (64-bit machines) then
     the fdt will not be copied at all on boot.  For this
     to work it must reside in writable memory, have
     sufficient padding on the end of it for u-boot to
     add the information it needs into it, and the memory
     must be accessible by the kernel.

 fdtcontroladdr
     if set this is the address of the control flattened
     device tree used by U-Boot when CONFIG_OF_CONTROL is
     defined.

 initrd_high
     restrict positioning of initrd images:
     If this variable is not set, initrd images will be
     copied to the highest possible address in RAM; this
     is usually what you want since it allows for
     maximum initrd size. If for some reason you want to
     make sure that the initrd image is loaded below the
     CONFIG_SYS_BOOTMAPSZ limit, you can set this environment
     variable to a value of "no" or "off" or "0".
     Alternatively, you can set it to a maximum upper
     address to use (U-Boot will still check that it
     does not overwrite the U-Boot stack and data).

     For instance, when you have a system with 16 MB
     RAM, and want to reserve 4 MB from use by Linux,
     you can do this by adding "mem=12M" to the value of
     the "bootargs" variable. However, now you must make
     sure that the initrd image is placed in the first
     12 MB as well - this can be done with::

         setenv initrd_high 00c00000

     If you set initrd_high to 0xffffffff (32-bit machines) or
     0xffffffffffffffff (64-bit machines), this is an
     indication to U-Boot that all addresses are legal
     for the Linux kernel, including addresses in flash
     memory. In this case U-Boot will NOT COPY the
     ramdisk at all. This may be useful to reduce the
     boot time on your system, but requires that this
     feature is supported by your Linux kernel.

 ipaddr
     IP address; needed for tftpboot command

 loadaddr
     Default load address for commands like "bootp",
     "rarpboot", "tftpboot", "loadb" or "diskboot"

 loads_echo
     see CONFIG_LOADS_ECHO

 serverip
     TFTP server IP address; needed for tftpboot command

 bootretry
     see CONFIG_BOOT_RETRY_TIME

 bootdelaykey
     see CONFIG_AUTOBOOT_DELAY_STR

 bootstopkey
     see CONFIG_AUTOBOOT_STOP_STR

 ethprime
     controls which network interface is used first.

 ethact
     controls which interface is currently active.
     For example you can do the following::

     => setenv ethact FEC
     => ping 192.168.0.1 # traffic sent on FEC
     => setenv ethact SCC
     => ping 10.0.0.1 # traffic sent on SCC

 ethrotate
     When set to "no" U-Boot does not go through all
     available network interfaces.
     It just stays at the currently selected interface. When unset or set to
     anything other than "no", U-Boot does go through all
     available network interfaces.

 netretry
     When set to "no" each network operation will
     either succeed or fail without retrying.
     When set to "once" the network operation will
     fail when all the available network interfaces
     are tried once without success.
     Useful on scripts which control the retry operation
     themselves.

 silent_linux
     If set then Linux will be told to boot silently, by
     adding 'console=' to its command line. If "yes" it will be
     made silent. If "no" it will not be made silent. If
     unset, then it will be made silent if the U-Boot console
     is silent.

 tftpsrcp
     If this is set, the value is used for TFTP's
     UDP source port.

 tftpdstp
     If this is set, the value is used for TFTP's UDP
     destination port instead of the default port 69.

 tftpblocksize
     Block size to use for TFTP transfers; if not set,
     we use the TFTP server's default block size

 tftptimeout
     Retransmission timeout for TFTP packets (in milli-
     seconds, minimum value is 1000 = 1 second). Defines
     when a packet is considered to be lost so it has to
     be retransmitted. The default is 5000 = 5 seconds.
     Lowering this value may make downloads succeed
     faster in networks with high packet loss rates or
     with unreliable TFTP servers.

 tftptimeoutcountmax
     maximum count of TFTP timeouts (no
     unit, minimum value = 0). Defines how many timeouts
     can happen during a single file transfer before that
     transfer is aborted. The default is 10, and 0 means
     'no timeouts allowed'. Increasing this value may help
     downloads succeed with high packet loss rates, or with
     unreliable TFTP servers or client hardware.

 tftpwindowsize
     if this is set, the value is used for TFTP's
     window size as described by RFC 7440.
     This means the count of blocks we can receive before
     sending ack to server.

 vlan
     When set to a value < 4095 the traffic over
     Ethernet is encapsulated/received over 802.1q
     VLAN tagged frames.

     Note: This appears not to be used in U-Boot. See `README.VLAN`.

 bootpretryperiod
     Period during which BOOTP/DHCP sends retries.
     Unsigned value, in milliseconds. If not set, the period will
     be either the default (28000), or a value based on
     CONFIG_NET_RETRY_COUNT, if defined. This value has
     precedence over the value based on CONFIG_NET_RETRY_COUNT.

 memmatches
     Number of matches found by the last 'ms' command, in hex

 memaddr
     Address of the last match found by the 'ms' command, in hex,
     or 0 if none

 mempos
     Index position of the last match found by the 'ms' command,
     in units of the size (.b, .w, .l) of the search

 zbootbase
     (x86 only) Base address of the bzImage 'setup' block

 zbootaddr
     (x86 only) Address of the loaded bzImage, typically
     BZIMAGE_LOAD_ADDR which is 0x100000


 Image locations
 ---------------

 The following image location variables contain the location of images
 used in booting. The "Image" column gives the role of the image and is
 not an environment variable name. The other columns are environment
 variable names. "File Name" gives the name of the file on a TFTP
 server, "RAM Address" gives the location in RAM the image will be
 loaded to, and "Flash Location" gives the image's address in NOR
 flash or offset in NAND flash.

 *Note* - these variables don't have to be defined for all boards, some
 boards currently use other variables for these purposes, and some
 boards use these variables for other purposes.

 Also note that most of these variables are just a commonly used set of variable
 names, used in some other variable definitions, but are not hard-coded anywhere
 in U-Boot code.

 ================= ============== ================ ==============
 Image             File Name      RAM Address      Flash Location
 ================= ============== ================ ==============
 u-boot            u-boot         u-boot_addr_r    u-boot_addr
 Linux kernel      bootfile       kernel_addr_r    kernel_addr
 device tree blob  fdtfile        fdt_addr_r       fdt_addr
 ramdisk           ramdiskfile    ramdisk_addr_r   ramdisk_addr
 ================= ============== ================ ==============


 Automatically updated variables
 -------------------------------

 The following environment variables may be used and automatically
 updated by the network boot commands ("bootp" and "rarpboot"),
 depending the information provided by your boot server:

 =========  ===================================================
 Variable   Notes
 =========  ===================================================
 bootfile   see above
 dnsip      IP address of your Domain Name Server
 dnsip2     IP address of your secondary Domain Name Server
 gatewayip  IP address of the Gateway (Router) to use
 hostname   Target hostname
 ipaddr     See above
 netmask    Subnet Mask
 rootpath   Pathname of the root filesystem on the NFS server
 serverip   see above
 =========  ===================================================


 Special environment variables
 -----------------------------

 There are two special Environment Variables:

 serial#
     contains hardware identification information such as type string and/or
     serial number
 ethaddr
     Ethernet address. If CONFIG_REGEX=y, also eth*addr (where * is an integer).

 These variables can be set only once (usually during manufacturing of
 the board). U-Boot refuses to delete or overwrite these variables
 once they have been set, unless CONFIG_ENV_OVERWRITE is enabled in the board
 configuration.

 Also:

 ver
     Contains the U-Boot version string as printed
     with the "version" command. This variable is
     readonly (see CONFIG_VERSION_VARIABLE).

 Please note that changes to some configuration parameters may take
 only effect after the next boot (yes, that's just like Windows).


 External environment file
 -------------------------

 The `CONFIG_USE_DEFAULT_ENV_FILE` option provides a way to bypass the
 environment generation in U-Boot. If enabled, then `CONFIG_DEFAULT_ENV_FILE`
 provides the name of a file which is converted into the environment,
 completely bypassing the standard environment variables in `env_default.h`.

 The format is the same as accepted by the mkenvimage tool, with lines containing
 key=value pairs. Blank lines and lines beginning with # are ignored.

 Future work may unify this feature with the text-based environment, perhaps
 moving the contents of `env_default.h` to a text file.

 Implementation
 --------------

 See :doc:`../develop/environment` for internal development details.
	.. SPDX-License-Identifier: GPL-2.0+

	Environment Variables
	=====================

	U-Boot supports user configuration using environment variables which
	can be made persistent by saving to persistent storage, for example flash
	memory.

	Environment variables are set using "env set" (alias "setenv"), printed using
	"env print" (alias "printenv"), and saved to persistent storage using
	"env save" (alias "saveenv"). Using "env set"
	without a value can be used to delete a variable from the
	environment. As long as you don't save the environment, you are
	working with an in-memory copy. In case the Flash area containing the
	environment is erased by accident, a default environment is provided.

	See :doc:`cmd/env` for details.

	Some configuration is controlled by Environment Variables, so that setting the
	variable can adjust the behaviour of U-Boot (e.g. autoboot delay, autoloading
	from tftp).

	Text-based Environment
	----------------------

	The default environment for a board is created using a `.env` environment file
	using a simple text format. The base filename for this is defined by
	`CONFIG_ENV_SOURCE_FILE`, or `CONFIG_SYS_BOARD` if that is empty.

	The file must be in the board directory and have a .env extension, so
	assuming that there is a board vendor, the resulting filename is therefore::

	board/<vendor>/<board>/<CONFIG_ENV_SOURCE_FILE>.env

	or::

	board/<vendor>/<board>/<CONFIG_SYS_BOARD>.env

	This is a plain text file where you can type your environment variables in
	the form `var=value`. Blank lines and multi-line variables are supported.
	The conversion script looks for a line that starts in column 1 with a string
	and has an equals sign immediately afterwards. Spaces before the = are not
	permitted. It is a good idea to indent your scripts so that only the 'var='
	appears at the start of a line.

	To add additional text to a variable you can use `var+=value`. This text is
	merged into the variable during the make process and made available as a
	single value to U-Boot. Variables can contain `+` characters but in the unlikely
	event that you want to have a variable name ending in plus, put a backslash
	before the `+` so that the script knows you are not adding to an existing
	variable but assigning to a new one::

	maximum\+=value

	This file can include C-style comments. Blank lines and multi-line
	variables are supported, and you can use normal C preprocessor directives
	and CONFIG defines from your board config also.

	For example, for snapper9260 you would create a text file called
	`board/bluewater/snapper9260.env` containing the environment text.

	Example::

	stdout=serial
	#ifdef CONFIG_LCD
	stdout+=,lcd
	#endif
	bootcmd=
	/* U-Boot script for booting */

	if [ -z ${tftpserverip} ]; then
	echo "Use 'setenv tftpserverip a.b.c.d' to set IP address."
	fi

	usb start; setenv autoload n; bootp;
	tftpboot ${tftpserverip}:
	bootm
	failed=
	/* Print a message when boot fails */
	echo CONFIG_SYS_BOARD boot failed - please check your image
	echo Load address is CONFIG_SYS_LOAD_ADDR

	If CONFIG_ENV_SOURCE_FILE is empty and the default filename is not present, then
	the old-style C environment is used instead. See below.

	Old-style C environment
	-----------------------

	Traditionally, the default environment is created in `include/env_default.h`,
	and can be augmented by various `CONFIG` defines. See that file for details. In
	particular you can define `CONFIG_EXTRA_ENV_SETTINGS` in your board file
	to add environment variables.

	Board maintainers are encouraged to migrate to the text-based environment as it
	is easier to maintain. The distro-board script still requires the old-style
	environment but work is underway to address this.


	List of environment variables
	-----------------------------

	Some device configuration options can be set using environment variables. In
	many cases the value in the default environment comes from a CONFIG option - see
	`include/env_default.h`) for this.

	This is most-likely not complete:

	baudrate
	Used to set the baudrate of the UART - it defaults to CONFIG_BAUDRATE (which
	defaults to 115200).

	bootdelay
	Delay before automatically running bootcmd. During this time the user
	can choose to enter the shell (or the boot menu if
	CONFIG_AUTOBOOT_MENU_SHOW=y):

	- 0 to autoboot with no delay, but you can stop it by key input.
	- -1 to disable autoboot.
	- -2 to autoboot with no delay and not check for abort

	The default value is defined by CONFIG_BOOTDELAY.
	The value of 'bootdelay' is overridden by the /config/bootdelay value in
	the device-tree if CONFIG_OF_CONTROL=y.

	bootcmd
	The command that is run if the user does not enter the shell during the
	boot delay.

	bootargs
	Command line arguments passed when booting an operating system or binary
	image

	bootfile
	Name of the image to load with TFTP

	bootm_low
	Memory range available for image processing in the bootm
	command can be restricted. This variable is given as
	a hexadecimal number and defines lowest address allowed
	for use by the bootm command. See also "bootm_size"
	environment variable. Address defined by "bootm_low" is
	also the base of the initial memory mapping for the Linux
	kernel -- see the description of CONFIG_SYS_BOOTMAPSZ and
	bootm_mapsize.

	bootm_mapsize
	Size of the initial memory mapping for the Linux kernel.
	This variable is given as a hexadecimal number and it
	defines the size of the memory region starting at base
	address bootm_low that is accessible by the Linux kernel
	during early boot. If unset, CONFIG_SYS_BOOTMAPSZ is used
	as the default value if it is defined, and bootm_size is
	used otherwise.

	bootm_size
	Memory range available for image processing in the bootm
	command can be restricted. This variable is given as
	a hexadecimal number and defines the size of the region
	allowed for use by the bootm command. See also "bootm_low"
	environment variable.

	bootstopkeysha256, bootdelaykey, bootstopkey
	See README.autoboot

	updatefile
	Location of the software update file on a TFTP server, used
	by the automatic software update feature. Please refer to
	documentation in doc/README.update for more details.

	autoload
	if set to "no" (any string beginning with 'n'),
	"bootp" and "dhcp" will just load perform a lookup of the
	configuration from the BOOTP server, but not try to
	load any image.

	autostart
	if set to "yes", an image loaded using the "bootp", "dhcp",
	"rarpboot", "tftpboot" or "diskboot" commands will
	be automatically started (by internally calling
	"bootm")

	If unset, or set to "1"/"yes"/"true" (case insensitive, just the first
	character is enough), a standalone image
	passed to the "bootm" command will be copied to the load address
	(and eventually uncompressed), but NOT be started.
	This can be used to load and uncompress arbitrary
	data.

	fdt_high
	if set this restricts the maximum address that the
	flattened device tree will be copied into upon boot.
	For example, if you have a system with 1 GB memory
	at physical address 0x10000000, while Linux kernel
	only recognizes the first 704 MB as low memory, you
	may need to set fdt_high as 0x3C000000 to have the
	device tree blob be copied to the maximum address
	of the 704 MB low memory, so that Linux kernel can
	access it during the boot procedure.

	If this is set to the special value 0xffffffff (32-bit machines) or
	0xffffffffffffffff (64-bit machines) then
	the fdt will not be copied at all on boot. For this
	to work it must reside in writable memory, have
	sufficient padding on the end of it for u-boot to
	add the information it needs into it, and the memory
	must be accessible by the kernel.

	fdtcontroladdr
	if set this is the address of the control flattened
	device tree used by U-Boot when CONFIG_OF_CONTROL is
	defined.

	initrd_high
	restrict positioning of initrd images:
	If this variable is not set, initrd images will be
	copied to the highest possible address in RAM; this
	is usually what you want since it allows for
	maximum initrd size. If for some reason you want to
	make sure that the initrd image is loaded below the
	CONFIG_SYS_BOOTMAPSZ limit, you can set this environment
	variable to a value of "no" or "off" or "0".
	Alternatively, you can set it to a maximum upper
	address to use (U-Boot will still check that it
	does not overwrite the U-Boot stack and data).

	For instance, when you have a system with 16 MB
	RAM, and want to reserve 4 MB from use by Linux,
	you can do this by adding "mem=12M" to the value of
	the "bootargs" variable. However, now you must make
	sure that the initrd image is placed in the first
	12 MB as well - this can be done with::

	setenv initrd_high 00c00000

	If you set initrd_high to 0xffffffff (32-bit machines) or
	0xffffffffffffffff (64-bit machines), this is an
	indication to U-Boot that all addresses are legal
	for the Linux kernel, including addresses in flash
	memory. In this case U-Boot will NOT COPY the
	ramdisk at all. This may be useful to reduce the
	boot time on your system, but requires that this
	feature is supported by your Linux kernel.

	ipaddr
	IP address; needed for tftpboot command

	loadaddr
	Default load address for commands like "bootp",
	"rarpboot", "tftpboot", "loadb" or "diskboot"

	loads_echo
	see CONFIG_LOADS_ECHO

	serverip
	TFTP server IP address; needed for tftpboot command

	bootretry
	see CONFIG_BOOT_RETRY_TIME

	bootdelaykey
	see CONFIG_AUTOBOOT_DELAY_STR

	bootstopkey
	see CONFIG_AUTOBOOT_STOP_STR

	ethprime
	controls which network interface is used first.

	ethact
	controls which interface is currently active.
	For example you can do the following::

	=> setenv ethact FEC
	=> ping 192.168.0.1 # traffic sent on FEC
	=> setenv ethact SCC
	=> ping 10.0.0.1 # traffic sent on SCC

	ethrotate
	When set to "no" U-Boot does not go through all
	available network interfaces.
	It just stays at the currently selected interface. When unset or set to
	anything other than "no", U-Boot does go through all
	available network interfaces.

	netretry
	When set to "no" each network operation will
	either succeed or fail without retrying.
	When set to "once" the network operation will
	fail when all the available network interfaces
	are tried once without success.
	Useful on scripts which control the retry operation
	themselves.

	silent_linux
	If set then Linux will be told to boot silently, by
	adding 'console=' to its command line. If "yes" it will be
	made silent. If "no" it will not be made silent. If
	unset, then it will be made silent if the U-Boot console
	is silent.

	tftpsrcp
	If this is set, the value is used for TFTP's
	UDP source port.

	tftpdstp
	If this is set, the value is used for TFTP's UDP
	destination port instead of the default port 69.

	tftpblocksize
	Block size to use for TFTP transfers; if not set,
	we use the TFTP server's default block size

	tftptimeout
	Retransmission timeout for TFTP packets (in milli-
	seconds, minimum value is 1000 = 1 second). Defines
	when a packet is considered to be lost so it has to
	be retransmitted. The default is 5000 = 5 seconds.
	Lowering this value may make downloads succeed
	faster in networks with high packet loss rates or
	with unreliable TFTP servers.

	tftptimeoutcountmax
	maximum count of TFTP timeouts (no
	unit, minimum value = 0). Defines how many timeouts
	can happen during a single file transfer before that
	transfer is aborted. The default is 10, and 0 means
	'no timeouts allowed'. Increasing this value may help
	downloads succeed with high packet loss rates, or with
	unreliable TFTP servers or client hardware.

	tftpwindowsize
	if this is set, the value is used for TFTP's
	window size as described by RFC 7440.
	This means the count of blocks we can receive before
	sending ack to server.

	vlan
	When set to a value < 4095 the traffic over
	Ethernet is encapsulated/received over 802.1q
	VLAN tagged frames.

	Note: This appears not to be used in U-Boot. See `README.VLAN`.

	bootpretryperiod
	Period during which BOOTP/DHCP sends retries.
	Unsigned value, in milliseconds. If not set, the period will
	be either the default (28000), or a value based on
	CONFIG_NET_RETRY_COUNT, if defined. This value has
	precedence over the value based on CONFIG_NET_RETRY_COUNT.

	memmatches
	Number of matches found by the last 'ms' command, in hex

	memaddr
	Address of the last match found by the 'ms' command, in hex,
	or 0 if none

	mempos
	Index position of the last match found by the 'ms' command,
	in units of the size (.b, .w, .l) of the search

	zbootbase
	(x86 only) Base address of the bzImage 'setup' block

	zbootaddr
	(x86 only) Address of the loaded bzImage, typically
	BZIMAGE_LOAD_ADDR which is 0x100000


	Image locations
	---------------

	The following image location variables contain the location of images
	used in booting. The "Image" column gives the role of the image and is
	not an environment variable name. The other columns are environment
	variable names. "File Name" gives the name of the file on a TFTP
	server, "RAM Address" gives the location in RAM the image will be
	loaded to, and "Flash Location" gives the image's address in NOR
	flash or offset in NAND flash.

	Note - these variables don't have to be defined for all boards, some
	boards currently use other variables for these purposes, and some
	boards use these variables for other purposes.

	Also note that most of these variables are just a commonly used set of variable
	names, used in some other variable definitions, but are not hard-coded anywhere
	in U-Boot code.

	================= ============== ================ ==============
	Image File Name RAM Address Flash Location
	================= ============== ================ ==============
	u-boot u-boot u-boot_addr_r u-boot_addr
	Linux kernel bootfile kernel_addr_r kernel_addr
	device tree blob fdtfile fdt_addr_r fdt_addr
	ramdisk ramdiskfile ramdisk_addr_r ramdisk_addr
	================= ============== ================ ==============


	Automatically updated variables
	-------------------------------

	The following environment variables may be used and automatically
	updated by the network boot commands ("bootp" and "rarpboot"),
	depending the information provided by your boot server:

	========= ===================================================
	Variable Notes
	========= ===================================================
	bootfile see above
	dnsip IP address of your Domain Name Server
	dnsip2 IP address of your secondary Domain Name Server
	gatewayip IP address of the Gateway (Router) to use
	hostname Target hostname
	ipaddr See above
	netmask Subnet Mask
	rootpath Pathname of the root filesystem on the NFS server
	serverip see above
	========= ===================================================


	Special environment variables
	-----------------------------

	There are two special Environment Variables:

	serial#
	contains hardware identification information such as type string and/or
	serial number
	ethaddr
	Ethernet address. If CONFIG_REGEX=y, also ethaddr (where is an integer).

	These variables can be set only once (usually during manufacturing of
	the board). U-Boot refuses to delete or overwrite these variables
	once they have been set, unless CONFIG_ENV_OVERWRITE is enabled in the board
	configuration.

	Also:

	ver
	Contains the U-Boot version string as printed
	with the "version" command. This variable is
	readonly (see CONFIG_VERSION_VARIABLE).

	Please note that changes to some configuration parameters may take
	only effect after the next boot (yes, that's just like Windows).


	External environment file
	-------------------------

	The `CONFIG_USE_DEFAULT_ENV_FILE` option provides a way to bypass the
	environment generation in U-Boot. If enabled, then `CONFIG_DEFAULT_ENV_FILE`
	provides the name of a file which is converted into the environment,
	completely bypassing the standard environment variables in `env_default.h`.

	The format is the same as accepted by the mkenvimage tool, with lines containing
	key=value pairs. Blank lines and lines beginning with # are ignored.

	Future work may unify this feature with the text-based environment, perhaps
	moving the contents of `env_default.h` to a text file.

	Implementation
	--------------

	See :doc:`../develop/environment` for internal development details.