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U-Boot Falcon Mode
This document provides an overview of how to add support for Falcon Mode
to a board.
Falcon Mode is introduced to speed up the booting process, allowing
to boot a Linux kernel (or whatever image) without a full blown U-Boot.
Falcon Mode relies on the SPL framework. In fact, to make booting faster,
U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot
image. In most implementations, SPL is used to start U-Boot when booting from
a mass storage, such as NAND or SD-Card. SPL has now support for other media,
and can generally be seen as a way to start an image performing the minimum
required initialization. SPL mainly initializes the RAM controller, and then
copies U-Boot image into the memory.
The Falcon Mode extends this way allowing to start the Linux kernel directly
from SPL. A new command is added to U-Boot to prepare the parameters that SPL
must pass to the kernel, using ATAGS or Device Tree.
In normal mode, these parameters are generated each time before
loading the kernel, passing to Linux the address in memory where
the parameters can be read.
With Falcon Mode, this snapshot can be saved into persistent storage and SPL is
informed to load it before running the kernel.
To boot the kernel, these steps under a Falcon-aware U-Boot are required:
1. Boot the board into U-Boot.
After loading the desired legacy-format kernel image into memory (and DT as
well, if used), use the "spl export" command to generate the kernel parameters
area or the DT. U-Boot runs as when it boots the kernel, but stops before
passing the control to the kernel.
2. Save the prepared snapshot into persistent media.
The address where to save it must be configured into board configuration
3. Boot the board into Falcon Mode. SPL will load the kernel and copy
the parameters which are saved in the persistent area to the required address.
If a valid uImage is not found at the defined location, U-Boot will be
booted instead.
It is required to implement a custom mechanism to select if SPL loads U-Boot
or another image.
The value of a GPIO is a simple way to operate the selection, as well as
reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.
Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells
SPL that U-Boot is not the only available image that SPL is able to start.
CONFIG_CMD_SPL Enable the "spl export" command.
The command "spl export" is then available in U-Boot
CONFIG_SYS_SPL_ARGS_ADDR Address in RAM where the parameters must be
copied by SPL.
In most cases, it is <start_of_ram> + 0x100
CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored
CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved.
CONFIG_CMD_SPL_NOR_OFS Offset in NOR where the parameters area was saved.
CONFIG_CMD_SPL_WRITE_SIZE Size of the parameters area to be copied
CONFIG_SPL_OS_BOOT Activate Falcon Mode.
Function that a board must implement
void spl_board_prepare_for_linux(void) : optional
Called from SPL before starting the kernel
spl_start_uboot() : required
Returns "0" if SPL should start the kernel, "1" if U-Boot
must be started.
Environment variables
A board may chose to look at the environment for decisions about falcon
mode. In this case the following variables may be supported:
boot_os : Set to yes/Yes/true/True/1 to enable booting to OS,
any other value to fall back to U-Boot (including
falcon_args_file : Filename to load as the 'args' portion of falcon mode
rather than the hard-coded value.
falcon_image_file : Filename to load as the OS image portion of falcon
mode rather than the hard-coded value.
Using spl command
spl - SPL configuration
spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]
img : "atags" or "fdt"
kernel_addr : kernel is loaded as part of the boot process, but it is not started.
This is the address where a kernel image is stored.
initrd_addr : Address of initial ramdisk
can be set to "-" if fdt_addr without initrd_addr is used
fdt_addr : in case of fdt, the address of the device tree.
The spl export command does not write to a storage media. The user is
responsible to transfer the gathered information (assembled ATAGS list
or prepared FDT) from temporary storage in RAM into persistant storage
after each run of 'spl export'. Unfortunately the position of temporary
storage can not be predicted nor provided at commandline, it depends
highly on your system setup and your provided data (ATAGS or FDT).
However at the end of an succesful 'spl export' run it will print the
RAM address of temporary storage. The RAM address of FDT will also be
set in the environment variable 'fdtargsaddr', the new length of the
prepared FDT will be set in the environment variable 'fdtargslen'.
These environment variables can be used in scripts for writing updated
FDT to persistent storage.
Now the user have to save the generated BLOB from that printed address
to the pre-defined address in persistent storage
The following example shows how to prepare the data for Falcon Mode on
twister board with ATAGS BLOB.
The "spl export" command is prepared to work with ATAGS and FDT. However,
using FDT is at the moment untested. The ppc port (see a3m071 example
later) prepares the fdt blob with the fdt command instead.
Usage on the twister board:
Using mtd names with the following (default) configuration
for mtdparts:
device nand0 <omap2-nand.0>, # parts = 9
#: name size offset mask_flags
0: MLO 0x00080000 0x00000000 0
1: u-boot 0x00100000 0x00080000 0
2: env1 0x00040000 0x00180000 0
3: env2 0x00040000 0x001c0000 0
4: kernel 0x00600000 0x00200000 0
5: bootparms 0x00040000 0x00800000 0
6: splashimg 0x00200000 0x00840000 0
7: mini 0x02800000 0x00a40000 0
8: rootfs 0x1cdc0000 0x03240000 0
twister => nand read 82000000 kernel
NAND read: device 0 offset 0x200000, size 0x600000
6291456 bytes read: OK
Now the kernel is in RAM at address 0x82000000
twister => spl export atags 0x82000000
## Booting kernel from Legacy Image at 82000000 ...
Image Name: Linux-3.5.0-rc4-14089-gda0b7f4
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 3654808 Bytes = 3.5 MiB
Load Address: 80008000
Entry Point: 80008000
Verifying Checksum ... OK
Loading Kernel Image ... OK
cmdline subcommand not supported
bdt subcommand not supported
Argument image is now in RAM at: 0x80000100
The result can be checked at address 0x80000100:
twister => md 0x80000100
80000100: 00000005 54410001 00000000 00000000 ......AT........
80000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot
80000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs
The parameters generated with this step can be saved into NAND at the offset
0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
nand erase.part bootparms
nand write 0x80000100 bootparms 0x4000
Now the parameters are stored into the NAND flash at the address
Next time, the board can be started into Falcon Mode moving the
setting the gpio (on twister gpio 55 is used) to kernel mode.
The kernel is loaded directly by the SPL without passing through U-Boot.
Example with FDT: a3m071 board
To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
prepard/patched first. U-Boot usually inserts some dynamic values into
the DT binary (blob), e.g. autodetected memory size, MAC addresses,
clocks speeds etc. To generate this patched DT blob, you can use
the following command:
1. Load fdt blob to SDRAM:
=> tftp 1800000 a3m071/a3m071.dtb
2. Set bootargs as desired for Linux booting (e.g. flash_mtd):
=> run mtdargs addip2 addtty
3. Use "fdt" commands to patch the DT blob:
=> fdt addr 1800000
=> fdt boardsetup
=> fdt chosen
4. Display patched DT blob (optional):
=> fdt print
5. Save fdt to NOR flash:
=> erase fc060000 fc07ffff
=> cp.b 1800000 fc060000 10000
Falcon Mode was presented at the RMLL 2012. Slides are available at: