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 <sba...@denx.de>
> CC: Marek Vasut <ma...@denx.de>
> CC: Otavio Salvador <ota...@ossystems.com.br>
> CC: Tom Rini <tr...@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-Boot
> + mode
> +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-Boot
> + must 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
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