1.uboot SPL架构简介
在uboot-2011之后的版本中多了一个叫SPL的架构，这个架构有什么用呢？在uboot-2011的/doc/README.spl文件有简单的介绍：
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.
为了统一所有现有实现第二段的程序加载程序(SPL)并允许简单地添加新的实现这个，一个通用SPL框架已创建。用这个框架板的几乎所有的源文件可以重用。没有代码重复或符号链接是必要的了。
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)
移植过uboot的都知道，uboot的启动其实是分为BL0,BL1,BL2三个阶段的，即：ROM->SPL->uboot.img.而这个SPL架构将可以编译产生一个uboot-spl.bin。即BL1的代码。也就是说SPL结构其实做的工作就是uboot的BL1阶段的工作。
2.SPL结构是如何工作的
cd 进入spl目录，从编译文件看这个架构都包括了什么文件：
我们先来看该目录Makefile：
当CONFIG_SPL_BUILD := y driver下设备驱动和arch/arm与cup相关的东东将被编译。
CONFIG_SPL_BUILD := y
export CONFIG_SPL_BUILD
include $(TOPDIR)/config.mk
# We want the final binaries in this directory
obj := $(OBJTREE)/spl/
HAVE_VENDOR_COMMON_LIB := $(shell [ -f $(SRCTREE)/board/$(VENDOR)/common/Makefile ] && echo y || echo n)
START := $(CPUDIR)/start.o
LIBS-y += arch/$(ARCH)/lib/lib$(ARCH).o
LIBS-y += $(CPUDIR)/lib$(CPU).o
ifdef SOC
LIBS-y += $(CPUDIR)/$(SOC)/lib$(SOC).o
endif
LIBS-y += board/$(BOARDDIR)/lib$(BOARD).o
LIBS-$(HAVE_VENDOR_COMMON_LIB) += board/$(VENDOR)/common/lib$(VENDOR).o
LIBS-$(CONFIG_SPL_LIBCOMMON_SUPPORT) += common/libcommon.o
LIBS-$(CONFIG_SPL_LIBDISK_SUPPORT) += disk/libdisk.o
LIBS-$(CONFIG_SPL_I2C_SUPPORT) += drivers/i2c/libi2c.o
LIBS-$(CONFIG_SPL_GPIO_SUPPORT) += drivers/gpio/libgpio.o
LIBS-$(CONFIG_SPL_MMC_SUPPORT) += drivers/mmc/libmmc.o
LIBS-$(CONFIG_SPL_SERIAL_SUPPORT) += drivers/serial/libserial.o
LIBS-$(CONFIG_SPL_SPI_FLASH_SUPPORT) += drivers/mtd/spi/libspi_flash.o
LIBS-$(CONFIG_SPL_SPI_SUPPORT) += drivers/spi/libspi.o
LIBS-$(CONFIG_SPL_FAT_SUPPORT) += fs/fat/libfat.o
LIBS-$(CONFIG_SPL_LIBGENERIC_SUPPORT) += lib/libgeneric.o
接下来是连接到一个脚本，u-boot-spl.lds
# Linker Script
ifdef CONFIG_SPL_LDSCRIPT
# need to strip off double quotes
LDSCRIPT := $(addprefix $(SRCTREE)/,$(subst ",,$(CONFIG_SPL_LDSCRIPT)))
endif
ifeq ($(wildcard $(LDSCRIPT)),)
LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot-spl.lds
endif
ifeq ($(wildcard $(LDSCRIPT)),)
LDSCRIPT := $(TOPDIR)/$(CPUDIR)/u-boot-spl.lds
endif
ifeq ($(wildcard $(LDSCRIPT)),)
$(error could not find linker script)
endif
在arm/cpu/armv7目录下找到u-boot-spl.lds这个连接脚本
MEMORY { .sram : ORIGIN = CONFIG_SPL_TEXT_BASE, LENGTH = CONFIG_SPL_MAX_SIZE }
MEMORY { .sdram : ORIGIN = CONFIG_SPL_BSS_START_ADDR, LENGTH = CONFIG_SPL_BSS_MAX_SIZE }
OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
.text :
{
__start = .;
arch/arm/cpu/armv7/start.o (.text)
*(.text*)
} >.sram
. = ALIGN(4);
.rodata : { *(SORT_BY_ALIGNMENT(.rodata*)) } >.sram
. = ALIGN(4);
.data : { *(SORT_BY_ALIGNMENT(.data*)) } >.sram
. = ALIGN(4);
__image_copy_end = .;
_end = .;
.bss :
{
. = ALIGN(4);
__bss_start = .;
*(.bss*)
. = ALIGN(4);
__bss_end__ = .;
} >.sdram
}
其实这个连接脚本最后连接到还是arch/arm/cpu/armv7/start.S,作为他的入口。
(reset) <arch/arm/cpu/armv7/start.S> (b lowlevel_init: arch/arm/cpu/armv7/lowlevel_init.S) (b _main) –> <arch/arm/lib/crt0.S> (bl board_init_f) –> <arch/arm/lib/spl.c> (board_init_r) –> <common/spl/spl.c> (jump_to_image_no_args去启动u-boot) 到此SPL的生命周期结束。
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