Generic SPL framework
  =====================
  
  Overview
  --------
  
  To unify all existing implementations for a secondary program loader (SPL)
  and to allow simply adding of new implementations this generic SPL framework
  has been created. With this framework almost all source files for a board
  can be reused. No code duplication or symlinking is necessary anymore.
  
  
  How it works
  ------------

  The object files for SPL are built separately and placed in the "spl" directory.

  The final binaries which are generated are u-boot-spl, u-boot-spl.bin and
  u-boot-spl.map.

  A config option named CONFIG_SPL_BUILD is enabled by Kconfig for SPL.

  Source files can therefore be compiled for SPL with different settings.

  
  For example:
  
  ifeq ($(CONFIG_SPL_BUILD),y)

  obj-y += board_spl.o

  else

  obj-y += board.o

  endif

  obj-$(CONFIG_SPL_BUILD) += foo.o

  
  #ifdef CONFIG_SPL_BUILD

  	foo();

  #endif

  The building of SPL images can be enabled by CONFIG_SPL option in Kconfig.

  Because SPL images normally have a different text base, one has to be
  configured by defining CONFIG_SPL_TEXT_BASE. The linker script has to be

  defined with CONFIG_SPL_LDSCRIPT.
  
  To support generic U-Boot libraries and drivers in the SPL binary one can
  optionally define CONFIG_SPL_XXX_SUPPORT. Currently following options
  are supported:
  
  CONFIG_SPL_LIBCOMMON_SUPPORT (common/libcommon.o)
  CONFIG_SPL_LIBDISK_SUPPORT (disk/libdisk.o)
  CONFIG_SPL_I2C_SUPPORT (drivers/i2c/libi2c.o)
  CONFIG_SPL_GPIO_SUPPORT (drivers/gpio/libgpio.o)
  CONFIG_SPL_MMC_SUPPORT (drivers/mmc/libmmc.o)
  CONFIG_SPL_SERIAL_SUPPORT (drivers/serial/libserial.o)
  CONFIG_SPL_SPI_FLASH_SUPPORT (drivers/mtd/spi/libspi_flash.o)
  CONFIG_SPL_SPI_SUPPORT (drivers/spi/libspi.o)

  CONFIG_SPL_FS_FAT (fs/fat/libfat.o)

  CONFIG_SPL_FS_EXT4

  CONFIG_SPL_LIBGENERIC_SUPPORT (lib/libgeneric.o)

  CONFIG_SPL_POWER_SUPPORT (drivers/power/libpower.o)

  CONFIG_SPL_NAND_SUPPORT (drivers/mtd/nand/raw/libnand.o)

  CONFIG_SPL_DRIVERS_MISC_SUPPORT (drivers/misc)

  CONFIG_SPL_DMA (drivers/dma/libdma.o)

  CONFIG_SPL_POST_MEM_SUPPORT (post/drivers/memory.o)

  CONFIG_SPL_NAND_LOAD (drivers/mtd/nand/raw/nand_spl_load.o)

  CONFIG_SPL_SPI_LOAD (drivers/mtd/spi/spi_spl_load.o)

  CONFIG_SPL_RAM_DEVICE (common/spl/spl.c)

  CONFIG_SPL_WATCHDOG_SUPPORT (drivers/watchdog/libwatchdog.o)

  Device tree
  -----------
  The U-Boot device tree is filtered by the fdtgrep tools during the build
  process to generate a much smaller device tree used in SPL (spl/u-boot-spl.dtb)
  with:
  - the mandatory nodes (/alias, /chosen, /config)
  - the nodes with one pre-relocation property:
    'u-boot,dm-pre-reloc' or 'u-boot,dm-spl'

  fdtgrep is also used to remove:

  - the properties defined in CONFIG_OF_SPL_REMOVE_PROPS
  - all the pre-relocation properties
    ('u-boot,dm-pre-reloc', 'u-boot,dm-spl' and 'u-boot,dm-tpl')
  
  All the nodes remaining in the SPL devicetree are bound
  (see driver-model/README.txt).

  
  Debugging
  ---------
  
  When building SPL with DEBUG set you may also need to set CONFIG_PANIC_HANG
  as in most cases do_reset is not defined within SPL.

  
  
  Estimating stack usage
  ----------------------
  
  With gcc 4.6 (and later) and the use of GNU cflow it is possible to estimate
  stack usage at various points in run sequence of SPL.  The -fstack-usage option
  to gcc will produce '.su' files (such as arch/arm/cpu/armv7/syslib.su) that
  will give stack usage information and cflow can construct program flow.
  
  Must have gcc 4.6 or later, which supports -fstack-usage
  
  1) Build normally
  2) Perform the following shell command to generate a list of C files used in
  SPL:
  $ find spl -name '*.su' | sed -e 's:^spl/::' -e 's:[.]su$:.c:' > used-spl.list
  3) Execute cflow:
  $ cflow --main=board_init_r `cat used-spl.list` 2>&1 | $PAGER
  
  cflow will spit out a number of warnings as it does not parse
  the config files and picks functions based on #ifdef.  Parsing the '.i'
  files instead introduces another set of headaches.  These warnings are
  not usually important to understanding the flow, however.