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

Signed-off-by: Stefano Babic <sba...@denx.de>
---
Changes in v5:
- several fixes for the language, rephrasing some unclear parts (Vikram 
Narayanan)

Changes in v4:
- fix capitalization, styling, in spl help (Andreas Biessmann)
- move CONFIG_SPL_OS_BOOT before function in doc (Andreas Biessmann)

Changes in v3:
- parameter initrd_addr was removed in V2 (Andreas Biessmann)
- added patch to fix help usage for spl export (Andreas Biessmann)
- Added empty lines (Otavio Salvador)
- add a more exhaustive description explaining that
  spl export does not save into media (Lukasz Majewski).

Changes in v2:
- spelling, language fixes (Andreas Biessman)
- rewrite some unclear sentences
- drop CONFIG_SPL_OS_BOOT_KEY
- make example with twister more exhaustive

 doc/README.falcon |  169 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 169 insertions(+)
 create mode 100644 doc/README.falcon

diff --git a/doc/README.falcon b/doc/README.falcon
new file mode 100644
index 0000000..72fe04a
--- /dev/null
+++ b/doc/README.falcon
@@ -0,0 +1,169 @@
+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 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 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.
+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 area to the 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_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 starts the kernel, "1" if U-Boot
+               must be started.
+
+
+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 img is used
+fdt_addr       : in case of fdt, the address of the device tree.
+
+The spl puts its result at a self gained position. The position is defined at 
compile
+time or when generating the uImage but not at command line for 'spl export'
+(see spl_export(): gd->bd->bi_boot_params vs. images.ft_addr).
+
+spl export' does not write directly to a storage media. This command is 
intended to save
+the prepared information in RAM.
+The user is responsible to save the data into the required media, as described
+on the following example, at the offset set at compile time
+(CONFIG_CMD_SPL_NAND_OFS in case of NAND).
+
+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.
+
+Falcon Mode was presented at the RMLL 2012. Slides are available at:
+
+http://schedule2012.rmll.info/IMG/pdf/LSM2012_UbootFalconMode_Babic.pdf
-- 
1.7.9.5

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