Add README for the "Falcon" mode

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

Signed-off-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>

doc/README.falcon

+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.
+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.
+
+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.
+
+
+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