u-boot/doc/README.SPL

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
------------

There is a new directory TOPDIR/spl which contains only a Makefile.
The object files are built separately for SPL and placed in this directory.
The final binaries which are generated are u-boot-spl, u-boot-spl.bin and
u-boot-spl.map.

During the SPL build a variable named CONFIG_SPL_BUILD is exported
in the make environment and also appended to CPPFLAGS with -DCONFIG_SPL_BUILD.
Source files can therefore be compiled for SPL with different settings.
ARM-based boards have previously used the option CONFIG_PRELOADER for it.

For example:

ifeq ($(CONFIG_SPL_BUILD),y)
COBJS-y += board_spl.o
else
COBJS-y += board.o
endif

COBJS-$(CONFIG_SPL_BUILD) += foo.o

#ifdef CONFIG_SPL_BUILD
	foo();
#endif


The building of SPL images can be with:

#define CONFIG_SPL

Because SPL images normally have a different text base, one have to be
configured by defining CONFIG_SPL_TEXT_BASE. The linker script have 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_FAT_SUPPORT (fs/fat/libfat.o)
CONFIG_SPL_LIBGENERIC_SUPPORT (lib/libgeneric.o)