Alexander Pockes authored Dec 19, 2016 and Michael Schanz committed Mar 15, 2017

Based on the results of Alexander Schuller's Bachelor Thesis (i.mx6 quickboot)

Overview
--------
A standard u-boot build consists of two image files: uboot.img and SPL.
The bootrom is loading the SPL image which performs some basic/initial
configuraion tasks. Afterwards, SPL loads the uboot.img which loads device-tree
and kernel image files.
Falcon mode means, enabling SPL to load/execute the kernel image directly.
This accelerates boot time but requires special u-boot configuration as well as
special (u)SD-card/EMMC setup.

congatec's falcon mode implementation enables SPL to...
-> load the kernel image directly from an arbitrary MMC device (uSD, SD, EMMC)
-> select the boot target (uboot.img/kernel) dependent on GPIO-level or
   environment-settings
-> load uboot.img stored at arbitrary MMC device by definition of
   <CONFIG_CGT_FALCON_MODE> and undefining <CONFIG_CGT_FALCON_MODE_OS_BOOT>.
   Please note, the environment has to be stored at SPI-flash.

Notes
-----
-> MMC device must be specified by definition of <CONFIG_CGT_FALCON_MODE_USD>,
   <CONFIG_CGT_FALCON_MODE_SD> or <CONFIG_CGT_FALCON_MODE_EMMC>
-> Boot target selection via GPIO...
    - is enabled by definition of <CONFIG_CGT_FALCON_MODE_IMG_SEL_GPIO>
    - is customizable via <BOOT_MODE_BTN>
-> Boot target selection via environment variable...
    - is enabled by definition of <CONFIG_CGT_FALCON_MODE_IMG_SEL_ENV>
    - can be combined with selection via GPIO

Standard behaviour
------------------
Preconditions:
    -> Proper MMC setup
        - argument file stored at sector 0x800
        - kernel image (uImage) stored at sector 0x1000
        - ext3 root filesystem starting at sector 0x8000 (32768)
    -> Falcon mode enabled <SPL> and <uboot.img> are stored at SPI-flash

a) Boot target selection via GPIO ENABLED:
    SPL tries to load kernel/uboot.img image from MMC device, if there is no
    boot image found, SPL loads uboot.img from SPI-flash.
    Press <BOOT_MODE_BTN> in order to force execution of uboot.img.

b) Boot target selection via environment ENABLED:
    (I) <boot_os> is NOT set to <1>
        SPL tries to load uboot.img from MMC, if there is no uboot.img
        found, SPL loads uboot.img from SPI-flash.
        In order to load the kernel directly, the environtment variable
        <boot_os> must be set to <1>
    (II) <boot_os> is set to <1>
        SPL tries to load kernel image from MMC, if there is no kernel image
        found, SPL loads uboot.img from SPI-flash.

c) Both boot target selections are ENABLED:
    (I) <boot_os> is NOT set to <1>
        SPL tries to load uboot.img from MMC, if there is no uboot.img found,
        SPL loads uboot.img from SPI-flash.
    (II) <boot_os> is set to <1>
        SPL tries to load kernelimage from MMC, if there is no boot
        image found, SPL loads uboot.img from SPI-flash.
        Press <BOOT_MODE_BTN> in order to force execution of uboot.img.

d) Both boot target selections are DISABLED:
    SPL tries to load kernel/uboot.img image from MMC device, if there is no
    boot image found, SPL loads uboot.img from SPI-flash.

Important changes at cgtqmx6eval specific code:
-----------------------------------------------
- boot order changed: MMC (1), SPI (2) by overloading board_boot_order()
- SPL: selection of image type to boot (uboot.img/kernel) via GPIO or/and
  environment variable <boot_os> by overloading spl_start_uboot()

Minor changes at common/imx6-related u-boot code required:
----------------------------------------------------------
- arch/arm/imx-common/spl.c: introducing spl_boot_mode_mmc()
    spl_boot_mode() queries OTP register in order to determine the (boot)
    device, the kernel has to be loaded. The underlying assumption is, that
    SPL and u-boot.img/kernel image are always located at the same device.
    Because of the limited size of the SPI-flash (4 MB), the kernel image
    can't be stored at SPI-flash in most cases. spl_boot_mode_mmc()
    determines the boot device - device, where uboot.img/kernel image is
    stored - by evaluating the variable boot_device, refering to the
    specified boot priority list (spl_boot_list[]).
- include/spl.h: spl_boot_mode_mmc() prototype added
- common/spl/spl_mmc.c:
    calling spl_boot_mode_mmc() instead of spl_boot_mode() dependent on
    <CONFIG_CGT_FALCON_MODE>