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+U-Boot "Falcon" Mode
+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.
+CONFIG_CMD_SPL Enable the "spl export" command.
+ The command "spl export" is then available in U-Boot
+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
+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
+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
+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: