U-Boot Falcon Mode
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====================
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Introduction
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------------
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This document provides an overview of how to add support for Falcon Mode
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to a board.
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Falcon Mode is introduced to speed up the booting process, allowing
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to boot a Linux kernel (or whatever image) without a full blown U-Boot.
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Falcon Mode relies on the SPL framework. In fact, to make booting faster,
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U-Boot is split into two parts: the SPL (Secondary Program Loader) and U-Boot
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image. In most implementations, SPL is used to start U-Boot when booting from
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a mass storage, such as NAND or SD-Card. SPL has now support for other media,
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and can generally be seen as a way to start an image performing the minimum
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required initialization. SPL mainly initializes the RAM controller, and then
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copies U-Boot image into the memory.
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The Falcon Mode extends this way allowing to start the Linux kernel directly
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from SPL. A new command is added to U-Boot to prepare the parameters that SPL
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must pass to the kernel, using ATAGS or Device Tree.
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In normal mode, these parameters are generated each time before
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loading the kernel, passing to Linux the address in memory where
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the parameters can be read.
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With Falcon Mode, this snapshot can be saved into persistent storage and SPL is
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informed to load it before running the kernel.
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To boot the kernel, these steps under a Falcon-aware U-Boot are required:
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1. Boot the board into U-Boot.
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After loading the desired legacy-format kernel image into memory (and DT as
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well, if used), use the "spl export" command to generate the kernel parameters
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area or the DT. U-Boot runs as when it boots the kernel, but stops before
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passing the control to the kernel.
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2. Save the prepared snapshot into persistent media.
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The address where to save it must be configured into board configuration
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file (CONFIG_CMD_SPL_NAND_OFS for NAND).
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3. Boot the board into Falcon Mode. SPL will load the kernel and copy
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the parameters which are saved in the persistent area to the required address.
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If a valid uImage is not found at the defined location, U-Boot will be
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booted instead.
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It is required to implement a custom mechanism to select if SPL loads U-Boot
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or another image.
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The value of a GPIO is a simple way to operate the selection, as well as
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reading a character from the SPL console if CONFIG_SPL_CONSOLE is set.
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Falcon Mode is generally activated by setting CONFIG_SPL_OS_BOOT. This tells
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SPL that U-Boot is not the only available image that SPL is able to start.
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Configuration
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----------------------------
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CONFIG_CMD_SPL Enable the "spl export" command.
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The command "spl export" is then available in U-Boot
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mode
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CONFIG_SYS_SPL_ARGS_ADDR Address in RAM where the parameters must be
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copied by SPL.
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In most cases, it is <start_of_ram> + 0x100
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CONFIG_SYS_NAND_SPL_KERNEL_OFFS Offset in NAND where the kernel is stored
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CONFIG_CMD_SPL_NAND_OFS Offset in NAND where the parameters area was saved.
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CONFIG_CMD_SPL_WRITE_SIZE Size of the parameters area to be copied
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CONFIG_SPL_OS_BOOT Activate Falcon Mode.
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Function that a board must implement
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------------------------------------
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void spl_board_prepare_for_linux(void) : optional
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Called from SPL before starting the kernel
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spl_start_uboot() : required
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Returns "0" if SPL should start the kernel, "1" if U-Boot
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must be started.
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Environment variables
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---------------------
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A board may chose to look at the environment for decisions about falcon
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mode. In this case the following variables may be supported:
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boot_os : Set to yes/Yes/true/True/1 to enable booting to OS,
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any other value to fall back to U-Boot (including
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unset)
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falcon_args_file : Filename to load as the 'args' portion of falcon mode
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rather than the hard-coded value.
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falcon_image_file : Filename to load as the OS image portion of falcon
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mode rather than the hard-coded value.
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Using spl command
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-----------------
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spl - SPL configuration
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Usage:
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spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]
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img : "atags" or "fdt"
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kernel_addr : kernel is loaded as part of the boot process, but it is not started.
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This is the address where a kernel image is stored.
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initrd_addr : Address of initial ramdisk
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can be set to "-" if fdt_addr without initrd_addr is used
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fdt_addr : in case of fdt, the address of the device tree.
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The spl export command does not write to a storage media. The user is
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responsible to transfer the gathered information (assembled ATAGS list
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or prepared FDT) from temporary storage in RAM into persistant storage
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after each run of 'spl export'. Unfortunately the position of temporary
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storage can not be predicted nor provided at commandline, it depends
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highly on your system setup and your provided data (ATAGS or FDT).
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However at the end of an succesful 'spl export' run it will print the
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RAM address of temporary storage. The RAM address of FDT will also be
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set in the environment variable 'fdtargsaddr', the new length of the
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prepared FDT will be set in the environment variable 'fdtargslen'.
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These environment variables can be used in scripts for writing updated
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FDT to persistent storage.
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Now the user have to save the generated BLOB from that printed address
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to the pre-defined address in persistent storage
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(CONFIG_CMD_SPL_NAND_OFS in case of NAND).
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The following example shows how to prepare the data for Falcon Mode on
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twister board with ATAGS BLOB.
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The "spl export" command is prepared to work with ATAGS and FDT. However,
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using FDT is at the moment untested. The ppc port (see a3m071 example
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later) prepares the fdt blob with the fdt command instead.
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Usage on the twister board:
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--------------------------------
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Using mtd names with the following (default) configuration
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for mtdparts:
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device nand0 <omap2-nand.0>, # parts = 9
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#: name size offset mask_flags
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0: MLO 0x00080000 0x00000000 0
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1: u-boot 0x00100000 0x00080000 0
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2: env1 0x00040000 0x00180000 0
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3: env2 0x00040000 0x001c0000 0
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4: kernel 0x00600000 0x00200000 0
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5: bootparms 0x00040000 0x00800000 0
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6: splashimg 0x00200000 0x00840000 0
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7: mini 0x02800000 0x00a40000 0
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8: rootfs 0x1cdc0000 0x03240000 0
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twister => nand read 82000000 kernel
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NAND read: device 0 offset 0x200000, size 0x600000
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6291456 bytes read: OK
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Now the kernel is in RAM at address 0x82000000
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twister => spl export atags 0x82000000
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## Booting kernel from Legacy Image at 82000000 ...
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Image Name: Linux-3.5.0-rc4-14089-gda0b7f4
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Image Type: ARM Linux Kernel Image (uncompressed)
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Data Size: 3654808 Bytes = 3.5 MiB
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Load Address: 80008000
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Entry Point: 80008000
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Verifying Checksum ... OK
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Loading Kernel Image ... OK
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OK
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cmdline subcommand not supported
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bdt subcommand not supported
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Argument image is now in RAM at: 0x80000100
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The result can be checked at address 0x80000100:
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twister => md 0x80000100
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80000100: 00000005 54410001 00000000 00000000 ......AT........
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80000110: 00000000 00000067 54410009 746f6f72 ....g.....ATroot
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80000120: 65642f3d 666e2f76 77722073 73666e20 =/dev/nfs rw nfs
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The parameters generated with this step can be saved into NAND at the offset
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0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
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nand erase.part bootparms
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nand write 0x80000100 bootparms 0x4000
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Now the parameters are stored into the NAND flash at the address
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CONFIG_CMD_SPL_NAND_OFS (=0x800000).
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Next time, the board can be started into Falcon Mode moving the
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setting the gpio (on twister gpio 55 is used) to kernel mode.
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The kernel is loaded directly by the SPL without passing through U-Boot.
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Example with FDT: a3m071 board
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-------------------------------
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To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
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prepard/patched first. U-Boot usually inserts some dynamic values into
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the DT binary (blob), e.g. autodetected memory size, MAC addresses,
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clocks speeds etc. To generate this patched DT blob, you can use
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the following command:
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1. Load fdt blob to SDRAM:
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=> tftp 1800000 a3m071/a3m071.dtb
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2. Set bootargs as desired for Linux booting (e.g. flash_mtd):
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=> run mtdargs addip2 addtty
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3. Use "fdt" commands to patch the DT blob:
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=> fdt addr 1800000
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=> fdt boardsetup
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=> fdt chosen
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4. Display patched DT blob (optional):
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=> fdt print
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5. Save fdt to NOR flash:
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=> erase fc060000 fc07ffff
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=> cp.b 1800000 fc060000 10000
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...
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Falcon Mode was presented at the RMLL 2012. Slides are available at:
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http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf
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