ae1590ed52
We add two new environment variables, falcon_args_file and falcon_image_file, which when set will override the compiled in default values for falcon mode. Signed-off-by: Tom Rini <trini@ti.com>
225 lines
8.0 KiB
Plaintext
225 lines
8.0 KiB
Plaintext
U-Boot Falcon Mode
|
|
====================
|
|
|
|
Introduction
|
|
------------
|
|
|
|
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.
|
|
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
|
|
file (CONFIG_CMD_SPL_NAND_OFS for NAND).
|
|
|
|
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.
|
|
|
|
Configuration
|
|
----------------------------
|
|
CONFIG_CMD_SPL Enable the "spl export" command.
|
|
The command "spl export" is then available in U-Boot
|
|
mode
|
|
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_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
|
|
unset)
|
|
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
|
|
|
|
Usage:
|
|
|
|
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.
|
|
Now the user have to save the generated BLOB from that printed address
|
|
to the pre-defined address in persistent storage
|
|
(CONFIG_CMD_SPL_NAND_OFS in case of NAND).
|
|
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
|
|
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
|
|
CONFIG_CMD_SPL_NAND_OFS (=0x800000).
|
|
|
|
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:
|
|
|
|
http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf
|