Dear Stefano Babic,
On 12.11.2012 11:59, Stefano Babic wrote:
Simple howto to add support to a board for booting the kernel from SPL ("Falcon" mode).
Signed-off-by: Stefano Babic sbabic@denx.de CC: Marek Vasut marex@denx.de CC: Otavio Salvador otavio@ossystems.com.br CC: Tom Rini trini@ti.com
doc/README.falcon | 124 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 124 insertions(+) create mode 100644 doc/README.falcon
diff --git a/doc/README.falcon b/doc/README.falcon new file mode 100644 index 0000000..d50b8c3 --- /dev/null +++ b/doc/README.falcon @@ -0,0 +1,124 @@ +U-Boot "Falcon" Mode +====================
+Introduction +------------
+This documents provides an overview 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 mostly implementations, SPL is used to start U-Boot when booting from
-----------------^ In most implementations?
+a mass storage, such as NAND or SD-Card. SPL has now support for other media, +and can be generalized seen as a way to start an image performing the minimum +required initialization. SPL initializes mainly the RAM controller, and after +that copies U-Boot image into the memory. The "Falcon" mode extends this way +allowing to start any kind of image, an in particular a Linux kernel, preparing
------------------------------------------^ and in particular? ------------------------------------------------------------------------^ to achieve that, to be able to boot linux, ... ? The 'preparing a snapshot...' part of this sentence sounds weird to me.
+a snapshot of the parameters (ATAG or DT) required by the kernel to boot.
+Falcon adds a command under U-Boot to reuse all code responsible to prepare +the interface with the kernel. In usual U-boot systems, 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, 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. Saves 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 area to the address required address.
--------------------------------^ first address is not necessary here
+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-Bootmode+CONFIG_SPL_OS_BOOT Activate Falcon mode.
A board should implement the following functions:+CONFIG_SPL_OS_BOOT_KEY Common name for GPIO used to select between U-Boot
and kernel image. Optional.+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
+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 starts the kernel, "1" if U-Bootmust be started.
In which way interact the CONFIG_SPL_OS_BOOT_KEY with the spl_start_uboot()? Is both required, can one use one or the other?
+Using spl command +-----------------
+twister => spl +spl - SPL configuration
+Usage: +spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr if <img> = fdt] - export a kernel parameter image
initrd_img can be set to "-" if fdt_addr without initrd img isused+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.
-------------------------------------------------------------^ persistently? This is the place in mass storage, right?
+init_addr : optional for atags - the address where the parameters area is generated into RAM
how about the initrd_addr mentioned above?
+fdt_addr : in case of fdt, the address of the device tree.
+Example (for the twister board):
+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 parameters generated with this step can be saved into NAND at the offset +0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
+Next time, the board can be started into "Falcon mode" moving the +CONFIG_SPL_OS_BOOT_KEY GPIO. +The kernel is loaded directly by the SPL without passing through U-Boot.
+Falcon mode was presented at the RMLL 2011. Slides are available at:
+http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf
Best regards
Andreas Bießmann