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Stefano Babic3e1b3932013-02-23 00:53:25 +00001U-Boot Falcon Mode
2====================
3
4Introduction
5------------
6
7This document provides an overview of how to add support for Falcon Mode
8to a board.
9
10Falcon Mode is introduced to speed up the booting process, allowing
11to boot a Linux kernel (or whatever image) without a full blown U-Boot.
12
13Falcon Mode relies on the SPL framework. In fact, to make booting faster,
14U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot
15image. In most implementations, SPL is used to start U-Boot when booting from
16a mass storage, such as NAND or SD-Card. SPL has now support for other media,
17and can generally be seen as a way to start an image performing the minimum
18required initialization. SPL mainly initializes the RAM controller, and then
19copies U-Boot image into the memory.
20
21The Falcon Mode extends this way allowing to start the Linux kernel directly
22from SPL. A new command is added to U-Boot to prepare the parameters that SPL
23must pass to the kernel, using ATAGS or Device Tree.
24
25In normal mode, these parameters are generated each time before
26loading the kernel, passing to Linux the address in memory where
27the parameters can be read.
28With Falcon Mode, this snapshot can be saved into persistent storage and SPL is
29informed to load it before running the kernel.
30
31To boot the kernel, these steps under a Falcon-aware U-Boot are required:
32
331. Boot the board into U-Boot.
Tom Rinicfd6de92014-06-27 09:03:50 -040034After loading the desired legacy-format kernel image into memory (and DT as
35well, if used), use the "spl export" command to generate the kernel parameters
36area or the DT. U-Boot runs as when it boots the kernel, but stops before
37passing the control to the kernel.
Stefano Babic3e1b3932013-02-23 00:53:25 +000038
392. Save the prepared snapshot into persistent media.
40The address where to save it must be configured into board configuration
41file (CONFIG_CMD_SPL_NAND_OFS for NAND).
42
433. Boot the board into Falcon Mode. SPL will load the kernel and copy
44the parameters which are saved in the persistent area to the required address.
Tom Rinid118d762013-07-08 12:15:14 -040045If a valid uImage is not found at the defined location, U-Boot will be
46booted instead.
Stefano Babic3e1b3932013-02-23 00:53:25 +000047
48It is required to implement a custom mechanism to select if SPL loads U-Boot
49or another image.
50
51The value of a GPIO is a simple way to operate the selection, as well as
52reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.
53
54Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells
55SPL that U-Boot is not the only available image that SPL is able to start.
56
57Configuration
58----------------------------
59CONFIG_CMD_SPL Enable the "spl export" command.
60 The command "spl export" is then available in U-Boot
61 mode
62CONFIG_SYS_SPL_ARGS_ADDR Address in RAM where the parameters must be
63 copied by SPL.
64 In most cases, it is <start_of_ram> + 0x100
65
66CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored
67
68CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved.
69
Lukasz Majewski259bd3a2019-05-16 16:01:38 +020070CONFIG_CMD_SPL_NOR_OFS Offset in NOR where the parameters area was saved.
71
Wolfgang Denk0cf207e2021-09-27 17:42:39 +020072CONFIG_CMD_SPL_WRITE_SIZE Size of the parameters area to be copied
Stefano Babic3e1b3932013-02-23 00:53:25 +000073
74CONFIG_SPL_OS_BOOT Activate Falcon Mode.
75
76Function that a board must implement
77------------------------------------
78
79void spl_board_prepare_for_linux(void) : optional
80 Called from SPL before starting the kernel
81
82spl_start_uboot() : required
83 Returns "0" if SPL should start the kernel, "1" if U-Boot
84 must be started.
85
Tom Rini3523df02014-03-28 12:03:40 -040086Environment variables
87---------------------
88
89A board may chose to look at the environment for decisions about falcon
90mode. In this case the following variables may be supported:
91
Wolfgang Denk0cf207e2021-09-27 17:42:39 +020092boot_os : Set to yes/Yes/true/True/1 to enable booting to OS,
Tom Rini3523df02014-03-28 12:03:40 -040093 any other value to fall back to U-Boot (including
94 unset)
Tom Riniae1590e2014-03-28 12:03:42 -040095falcon_args_file : Filename to load as the 'args' portion of falcon mode
96 rather than the hard-coded value.
97falcon_image_file : Filename to load as the OS image portion of falcon
98 mode rather than the hard-coded value.
Stefano Babic3e1b3932013-02-23 00:53:25 +000099
100Using spl command
101-----------------
102
103spl - SPL configuration
104
105Usage:
106
107spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]
108
109img : "atags" or "fdt"
110kernel_addr : kernel is loaded as part of the boot process, but it is not started.
111 This is the address where a kernel image is stored.
112initrd_addr : Address of initial ramdisk
113 can be set to "-" if fdt_addr without initrd_addr is used
114fdt_addr : in case of fdt, the address of the device tree.
115
116The spl export command does not write to a storage media. The user is
117responsible to transfer the gathered information (assembled ATAGS list
118or prepared FDT) from temporary storage in RAM into persistant storage
119after each run of 'spl export'. Unfortunately the position of temporary
120storage can not be predicted nor provided at commandline, it depends
121highly on your system setup and your provided data (ATAGS or FDT).
122However at the end of an succesful 'spl export' run it will print the
Anatolij Gustschin767cb742017-08-17 21:01:48 +0200123RAM address of temporary storage. The RAM address of FDT will also be
124set in the environment variable 'fdtargsaddr', the new length of the
125prepared FDT will be set in the environment variable 'fdtargslen'.
126These environment variables can be used in scripts for writing updated
127FDT to persistent storage.
128
Stefano Babic3e1b3932013-02-23 00:53:25 +0000129Now the user have to save the generated BLOB from that printed address
130to the pre-defined address in persistent storage
131(CONFIG_CMD_SPL_NAND_OFS in case of NAND).
132The following example shows how to prepare the data for Falcon Mode on
133twister board with ATAGS BLOB.
134
135The "spl export" command is prepared to work with ATAGS and FDT. However,
136using FDT is at the moment untested. The ppc port (see a3m071 example
137later) prepares the fdt blob with the fdt command instead.
138
139
140Usage on the twister board:
141--------------------------------
142
143Using mtd names with the following (default) configuration
144for mtdparts:
145
146device nand0 <omap2-nand.0>, # parts = 9
147 #: name size offset mask_flags
148 0: MLO 0x00080000 0x00000000 0
149 1: u-boot 0x00100000 0x00080000 0
150 2: env1 0x00040000 0x00180000 0
151 3: env2 0x00040000 0x001c0000 0
152 4: kernel 0x00600000 0x00200000 0
153 5: bootparms 0x00040000 0x00800000 0
154 6: splashimg 0x00200000 0x00840000 0
155 7: mini 0x02800000 0x00a40000 0
156 8: rootfs 0x1cdc0000 0x03240000 0
157
158
159twister => nand read 82000000 kernel
160
161NAND read: device 0 offset 0x200000, size 0x600000
162 6291456 bytes read: OK
163
164Now the kernel is in RAM at address 0x82000000
165
166twister => spl export atags 0x82000000
167## Booting kernel from Legacy Image at 82000000 ...
168 Image Name: Linux-3.5.0-rc4-14089-gda0b7f4
169 Image Type: ARM Linux Kernel Image (uncompressed)
170 Data Size: 3654808 Bytes = 3.5 MiB
171 Load Address: 80008000
172 Entry Point: 80008000
173 Verifying Checksum ... OK
174 Loading Kernel Image ... OK
175OK
176cmdline subcommand not supported
177bdt subcommand not supported
178Argument image is now in RAM at: 0x80000100
179
180The result can be checked at address 0x80000100:
181
182twister => md 0x80000100
18380000100: 00000005 54410001 00000000 00000000 ......AT........
18480000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot
18580000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs
186
187The parameters generated with this step can be saved into NAND at the offset
1880x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
189
190nand erase.part bootparms
191nand write 0x80000100 bootparms 0x4000
192
193Now the parameters are stored into the NAND flash at the address
194CONFIG_CMD_SPL_NAND_OFS (=0x800000).
195
196Next time, the board can be started into Falcon Mode moving the
197setting the gpio (on twister gpio 55 is used) to kernel mode.
198
199The kernel is loaded directly by the SPL without passing through U-Boot.
200
201Example with FDT: a3m071 board
202-------------------------------
203
204To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
205prepard/patched first. U-Boot usually inserts some dynamic values into
206the DT binary (blob), e.g. autodetected memory size, MAC addresses,
207clocks speeds etc. To generate this patched DT blob, you can use
208the following command:
209
2101. Load fdt blob to SDRAM:
211=> tftp 1800000 a3m071/a3m071.dtb
212
2132. Set bootargs as desired for Linux booting (e.g. flash_mtd):
214=> run mtdargs addip2 addtty
215
2163. Use "fdt" commands to patch the DT blob:
217=> fdt addr 1800000
218=> fdt boardsetup
219=> fdt chosen
220
2214. Display patched DT blob (optional):
222=> fdt print
223
2245. Save fdt to NOR flash:
225=> erase fc060000 fc07ffff
226=> cp.b 1800000 fc060000 10000
227...
228
229
230Falcon Mode was presented at the RMLL 2012. Slides are available at:
231
232http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf