U-Boot Falcon Mode
  ====================
  
  Introduction
  ------------
  
  This document provides an overview of 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 most 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 generally be seen as a way to start an image performing the minimum
  required initialization. SPL mainly initializes the RAM controller, and then
  copies U-Boot image into the memory.
  
  The Falcon Mode extends this way allowing to start the Linux kernel directly
  from SPL. A new command is added to U-Boot to prepare the parameters that SPL
  must pass to the kernel, using ATAGS or Device Tree.
  
  In normal mode, 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 Mode, 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.

  After loading the desired legacy-format kernel image into memory (and DT as
  well, if used), 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. Save 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 which are saved in the persistent area to the required address.

  If a valid uImage is not found at the defined location, U-Boot will be
  booted instead.

  
  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_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
  
  CONFIG_SPL_OS_BOOT	Activate Falcon Mode.
  
  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 should start the kernel, "1" if U-Boot
  		must be started.

  Environment variables
  ---------------------
  
  A board may chose to look at the environment for decisions about falcon
  mode.  In this case the following variables may be supported:
  
  boot_os : 		Set to yes/Yes/true/True/1 to enable booting to OS,
  			any other value to fall back to U-Boot (including
  			unset)

  falcon_args_file :	Filename to load as the 'args' portion of falcon mode
  			rather than the hard-coded value.
  falcon_image_file :	Filename to load as the OS image portion of falcon
  			mode rather than the hard-coded value.

  
  Using spl command
  -----------------
  
  spl - SPL configuration
  
  Usage:
  
  spl export <img=atags|fdt> [kernel_addr] [initrd_addr] [fdt_addr ]
  
  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.
  initrd_addr	: Address of initial ramdisk
  		  can be set to "-" if fdt_addr without initrd_addr is used
  fdt_addr	: in case of fdt, the address of the device tree.
  
  The spl export command does not write to a storage media. The user is
  responsible to transfer the gathered information (assembled ATAGS list
  or prepared FDT) from temporary storage in RAM into persistant storage
  after each run of 'spl export'. Unfortunately the position of temporary
  storage can not be predicted nor provided at commandline, it depends
  highly on your system setup and your provided data (ATAGS or FDT).
  However at the end of an succesful 'spl export' run it will print the
  RAM address of temporary storage.
  Now the user have to save the generated BLOB from that printed address
  to the pre-defined address in persistent storage
  (CONFIG_CMD_SPL_NAND_OFS in case of NAND).
  The following example shows how to prepare the data for Falcon Mode on
  twister board with ATAGS BLOB.
  
  The "spl export" command is prepared to work with ATAGS and FDT. However,
  using FDT is at the moment untested. The ppc port (see a3m071 example
  later) prepares the fdt blob with the fdt command instead.
  
  
  Usage on the twister board:
  --------------------------------
  
  Using mtd names with the following (default) configuration
  for mtdparts:
  
  device nand0 <omap2-nand.0>, # parts = 9
   #: name		size		offset		mask_flags
   0: MLO                 0x00080000      0x00000000      0
   1: u-boot              0x00100000      0x00080000      0
   2: env1                0x00040000      0x00180000      0
   3: env2                0x00040000      0x001c0000      0
   4: kernel              0x00600000      0x00200000      0
   5: bootparms           0x00040000      0x00800000      0
   6: splashimg           0x00200000      0x00840000      0
   7: mini                0x02800000      0x00a40000      0
   8: rootfs              0x1cdc0000      0x03240000      0
  
  
  twister => nand read 82000000 kernel
  
  NAND read: device 0 offset 0x200000, size 0x600000
   6291456 bytes read: OK
  
  Now the kernel is in RAM at address 0x82000000
  
  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 result can be checked at address 0x80000100:
  
  twister => md 0x80000100
  80000100: 00000005 54410001 00000000 00000000    ......AT........
  80000110: 00000000 00000067 54410009 746f6f72    ....g.....ATroot
  80000120: 65642f3d 666e2f76 77722073 73666e20    =/dev/nfs rw nfs
  
  The parameters generated with this step can be saved into NAND at the offset
  0x800000 (value for twister for CONFIG_CMD_SPL_NAND_OFS)
  
  nand erase.part bootparms
  nand write 0x80000100 bootparms 0x4000
  
  Now the parameters are stored into the NAND flash at the address
  CONFIG_CMD_SPL_NAND_OFS (=0x800000).
  
  Next time, the board can be started into Falcon Mode moving the
  setting the gpio (on twister gpio 55 is used) to kernel mode.
  
  The kernel is loaded directly by the SPL without passing through U-Boot.
  
  Example with FDT: a3m071 board
  -------------------------------
  
  To boot the Linux kernel from the SPL, the DT blob (fdt) needs to get
  prepard/patched first. U-Boot usually inserts some dynamic values into
  the DT binary (blob), e.g. autodetected memory size, MAC addresses,
  clocks speeds etc. To generate this patched DT blob, you can use
  the following command:
  
  1. Load fdt blob to SDRAM:
  => tftp 1800000 a3m071/a3m071.dtb
  
  2. Set bootargs as desired for Linux booting (e.g. flash_mtd):
  => run mtdargs addip2 addtty
  
  3. Use "fdt" commands to patch the DT blob:
  => fdt addr 1800000
  => fdt boardsetup
  => fdt chosen
  
  4. Display patched DT blob (optional):
  => fdt print
  
  5. Save fdt to NOR flash:
  => erase fc060000 fc07ffff
  => cp.b 1800000 fc060000 10000
  ...
  
  
  Falcon Mode was presented at the RMLL 2012. Slides are available at:
  
  http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf