imx: ventana: update README with Falcon mode documentation

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

board/gateworks/gw_ventana/README

@@ -149,3 +149,172 @@ This information is taken from:
 
 More details about the i.MX6 BOOT ROM can be found in the IMX6 reference manual.
 
+4. Falcon Mode
+------------------------------
+
+The Gateworks Ventana board config enables Falcon mode (CONFIG_SPL_OS_BOOT)
+which allows the SPL to boot directly to an OS instead of to U-Boot
+(u-boot.img) thus acheiving a faster overall boot time. The time savings
+depends on your boot medium (ie NAND Flash vs micro-SD) and size/storage
+of the OS. The time savings can be anywhere from 2 seconds (256MB NAND Flash
+with ~1MB kernel) to 6 seconds or more (2GB NAND Flash with ~6 kernel)
+
+The Gateworks Ventana board supports Falcon mode for the following boot
+medium:
+ - NAND flash
+ - micro-SD
+
+For all boot mediums, raw mode is used. While support of more complex storage
+such as files on top of FAT/EXT filesystem is possible but not practical
+as the size of the SPL is fairly limitted (to 64KB based on the smallest
+size of available IMX6 iRAM) as well as the fact that this would increase
+OS load time which defeats the purpose of Falcon mode in the first place.
+
+The SPL decides to boot either U-Boot (u-boot.img) or the OS (args + kernel)
+based on the return value of the spl_start_uboot() function. While often
+this can simply be the state of a GPIO based pushbutton or DIP switch, for
+Gateworks Ventana, we use the U-Boot environment 'boot_os' variable which if
+set to '1' will choose to boot the OS rather than U-Boot. While the choice
+of adding env support to the SPL adds a little bit of time to the boot
+process as well as (significant really) SPL code space this was deemed most
+flexible as within the large variety of Gateworks Ventana boards not all of
+them have a user pushbutton and that pushbutton may be configured as a hard
+reset per user configuration.
+
+To use Falcon mode it is required that you first 'prepare' the 'args' data
+that is stored on your boot medium along with the kernel (which can be any
+OS or bare-metal application). In the case of the Linux kernel the 'args'
+is the flatenned device-tree which normally gets altered prior to booting linux
+by U-Boot's 'bootm' command. To achieve this for SPL we use the
+'spl export fdt' command in U-Boot after loading the kernel and dtb which
+will go through the same process of modifying the device-tree for the board
+being executed on but not jump to the kernel. This allows you to save the
+args data to the location the SPL expects it and then enable Falcon mode.
+
+It is important to realize that there are certain values in the dtb that
+are board model specific (IMX6Q vs IMX6DL for example) and board specific
+(board serial number, MAC addrs) so you do not want to use the 'args'
+data prepared from one board on another board.
+
+4.1. Falcon Mode on NAND flash
+------------------------------
+To prepare a Gateworks Ventana board that boots from NAND flash for Falcon
+mode you must program your flash such that the 'args' and 'kernel' are
+located where defined at compile time by the following:
+   CONFIG_CMD_SPL_NAND_OFS         17MB - offset of 'args'
+   CONFIG_SYS_NAND_SPL_KERNEL_OFFS 18MB - offset of 'kernel'
+
+The location offsets defined above are defaults chosen by Gateworks and are
+flexible if you want to re-define them.
+
+The following steps executed in U-Boot will configure Falcon mode for NAND
+using rootfs (ubi), kernel (uImage), and dtb from the network:
+
+ # change mtd partitions to the above mapping
+ Ventana > setenv mtdparts 'mtdparts=nand:14m(spl),2m(uboot),1m(env),1m(args),10m(kernel),-(rootfs)'
+
+ # flash rootfs (at 28MB)
+ Ventana > tftp ${loadaddr} rootfs_${flash_layout}.ubi && \
+   nand erase.part rootfs && nand write ${loadaddr} rootfs ${filesize}
+
+ # load the device-tree
+ Ventana > tftp ${fdt_addr} ventana/${fdt_file2}
+
+ # load the kernel
+ Ventana > tftp ${loadaddr} ventana/uImage
+
+ # flash kernel (at 18MB)
+ Ventana > nand erase.part kernel && nand write ${loadaddr} kernel ${filesize}
+
+ # set kernel args for the console and rootfs (used by spl export)
+ Ventana > setenv bootargs 'console=ttymxc1,115200 root=ubi0:rootfs ubi.mtd=5 rootfstype=ubifs quiet'
+
+ # create args based on env, board, EEPROM, and dtb
+ Ventana > spl export fdt ${loadaddr} - ${fdt_addr}
+
+ # flash args (at 17MB)
+ Ventana > nand erase.part args && nand write 18000000 args 100000
+
+ # set boot_os env var to enable booting to Linux
+ Ventana > setenv boot_os 1 && saveenv
+
+Be sure to adjust 'bootargs' above to your OS needs (this will be different
+for various distros such as OpenWrt, Yocto, Android, etc). You can use the
+value obtained from 'cat /proc/cmdline' when booted to Linux.
+
+This information is taken from:
+  http://trac.gateworks.com/wiki/ventana/bootloader/falcon-mode#nand
+
+
+4.2. Falcon Mode on micro-SD card
+---------------------------------
+
+To prepare a Gateworks Ventana board with a primary boot device of micro-SD
+you first need to make sure you build U-Boot with CONFIG_ENV_IS_IN_MMC
+instead of CONFIG_ENV_IS_IN_NAND.
+
+For micro-SD based Falcon mode you must program your micro-SD such that
+the 'args' and 'kernel' are located where defined at compile time
+by the following:
+   CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTOR 0x800 (1MB) - offset of 'args'
+   CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR 0x1000 (2MB) - offset of 'kernel'
+
+The location offsets defined above are defaults chosen by Gateworks and are
+flexible if you want to re-define them.
+
+First you must prepare a micro-SD such that the SPL can be loaded by the
+IMX6 BOOT ROM (fixed offset of 1KB), and U-Boot can be loaded by the SPL
+(fixed offset of 69KB defined by CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR).
+
+The following shell commands are executed on a Linux host (adjust DEV to the
+block storage device of your micro-SD):
+
+ DEV=/dev/sdc
+ # zero out 1MB of device
+ sudo dd if=/dev/zero of=$DEV count=1 bs=1M oflag=sync status=none && sync
+ # copy SPL to 1KB offset
+ sudo dd if=SPL of=$DEV bs=1K seek=1 oflag=sync status=none && sync
+ # copy U-Boot to 69KB offset
+ sudo dd if=u-boot.img of=$DEV bs=1K seek=69 oflag=sync status=none && sync
+ # create a partition table with a single rootfs partition starting at 10MB
+ printf "10,,L\n" | sudo sfdisk --in-order --no-reread -L -uM $DEV && sync
+ # format partition
+ sudo mkfs.ext4 -L root ${DEV}1
+ # mount the partition
+ sudo udisks --mount ${DEV}1
+ # extract filesystem
+ sudo tar xvf rootfs.tar.gz -C /media/root
+ # flush and unmount
+ sync && sudo umount /media/root
+
+Now that your micro-SD partitioning has been adjusted to leave room for the
+raw 'args' and 'kernel' data boot the board with the prepared micro-SD, break
+out in U-Boot and use the following to enable Falcon mode:
+
+ # load device-tree from rootfs
+ Ventana > ext2load mmc 0:1 ${fdt_addr} boot/${fdt_file2}
+
+ # load kernel from rootfs
+ Ventana > ext2load mmc 0:1 ${loadaddr} boot/uImage
+
+ # write kernel at 2MB offset
+ Ventana > mmc write ${loadaddr} 0x1000 0x4000
+
+ # setup kernel bootargs
+ Ventana > setenv bootargs 'console=ttymxc1,115200 root=/dev/mmcblk0p1 rootfstype=ext4 rootwait rw'
+
+ # prepare args
+ Ventana > spl export fdt ${loadaddr} - ${fdt_addr}
+
+ # write args 1MB data (0x800 sectors) to 1MB offset (0x800 sectors)
+ Ventana > mmc write 18000000 0x800 0x800
+
+ # set boot_os to enable falcon mode
+ Ventana > setenv boot_os 1 && saveenv
+
+Be sure to adjust 'bootargs' above to your OS needs (this will be different
+for various distros such as OpenWrt, Yocto, Android, etc). You can use the
+value obtained from 'cat /proc/cmdline' when booted to Linux.
+
+This information is taken from:
+  http://trac.gateworks.com/wiki/ventana/bootloader/falcon-mode#microsd