[U-Boot] [PATCH] Add README for the "Falcon" mode

[Stefano Babic]

Simple howto to add support to a board
for booting the kernel from SPL ("Falcon" mode).

Signed-off-by: Stefano Babic <sbabic at denx.de>
CC: Marek Vasut <marex at denx.de>
CC: Otavio Salvador <otavio at ossystems.com.br>
CC: Tom Rini <trini at 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
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+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
+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
+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.
+
+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.
+
+
+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.
+init_addr	: optional for atags - the address where the parameters area is generated into RAM
+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
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