Beaglebone Black——嵌入式linux系统u-boot编译
首先是U-boot源码的来源，由于使用的是TI的BBB板子，所以TI自然也提供了自己定制的u-boot源码，该源码就在TI-SDK包中有，另外该安装包中的u-boot版本是2013.10版本，内核版本为3.12.10，除了源码包还自带工具链。
TI-SDK最新版下载链接：http://software-dl.ti.com/sitara_linux/esd/AM335xSDK/latest/index_FDS.html
当然，还有另外一个德国佬官方的u-boot源码，网站首页：http://www.denx.de/en/News/WebHome和git代码库：git://www.denx.de/git/u-boot.git。
以官方u-boot为例进行u-boot的编译，进入源码根目录可以看到内容如下，
zhouyu@zhouyu-virtual-machine:~$ cd u-boot/
zhouyu@zhouyu-virtual-machine:~/u-boot$ ls
api configs dts Kconfig Makefile snapshot.commit
arch CREDITS examples lib net spl
board disk fs Licenses post test
common doc include MAINTAINERS README tools
config.mk drivers Kbuild MAKEALL scripts
其中打开根目录下的Makefile文件，可以查看到本源码的版本，如下图：
step1:在进行任何操作之前，先清除源码树，
zhouyu@zhouyu-virtual-machine:~/u-boot$ make distclean
CLEAN scripts/basic
CLEAN scripts/kconfig
CLEAN include/config include/generated spl
CLEAN .config include/autoconf.mk include/autoconf.mk.dep include/config.h
zhouyu@zhouyu-virtual-machine:~/u-boot$
step2：编译之前的配置工作
zhouyu@zhouyu-virtual-machine:~/u-boot$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -j4 O=am335x_beaglebone_black am335x_boneblack_defconfig
GEN /home/zhouyu/u-boot/am335x_beaglebone_black/Makefile
HOSTCC scripts/basic/fixdep
HOSTCC scripts/kconfig/conf.o
SHIPPED scripts/kconfig/zconf.tab.c
SHIPPED scripts/kconfig/zconf.lex.c
SHIPPED scripts/kconfig/zconf.hash.c
HOSTCC scripts/kconfig/zconf.tab.o
HOSTLD scripts/kconfig/conf
#
# configuration written to .config
#
#
# configuration written to spl/.config
#
zhouyu@zhouyu-virtual-machine:~/u-boot$
make之后的第一个参数向Makefile指明了编译的目标体系结构，第二个参数向Makefile指明了使用哪个编译工具链，第四个参数告诉make程序，这个编译任务要分配4个线程来并行进行，以加快编译速度，通常这个数依据自己的处理器的核数来选择，一般选择2倍核数，第五个参数告诉Makefile编译时产生的中间文件以及最后的结果都存放在源码树根目录下的am335x_beaglebone_black文件夹中，这对于使用同一个源码树来编译多个不同配置的u-boot时是一个很好的选择。最后一个参数向Makefile指明了要使用哪个文件来进行具体的配置，最后所有的配置都将写进源码树根目录下面的.config文件中。
而至于具体后面应该跟一个什么样的配置参数，可以从下图中寻找答案
在源码树的configs文件夹中可以看到很多配置文件，我选择的是第一个红线框的配置文件。
step3：正式开始编译工作
zhouyu@zhouyu-virtual-machine:~/u-boot$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -j4 O=am335x_beaglebone_black all
GEN /home/zhouyu/u-boot/am335x_beaglebone_black/Makefile
scripts/kconfig/conf --silentoldconfig Kconfig
scripts/kconfig/conf --silentoldconfig Kconfig
CHK include/config.h
UPD include/config.h
GEN include/autoconf.mk
GEN include/autoconf.mk.dep
GEN spl/include/autoconf.mk
CHK include/config/uboot.release
CHK include/generated/timestamp_autogenerated.h
GEN /home/zhouyu/u-boot/am335x_beaglebone_black/Makefile
UPD include/generated/timestamp_autogenerated.h
UPD include/config/uboot.release
Using /home/zhouyu/u-boot as source for U-Boot
CHK include/generated/version_autogenerated.h
UPD include/generated/version_autogenerated.h
CC lib/asm-offsets.s
CC arch/arm/lib/asm-offsets.s
GEN include/generated/asm-offsets.h
GEN include/generated/generic-asm-offsets.h
LDS u-boot.lds
HOSTCC tools/gen_eth_addr
HOSTCC tools/img2srec
HOSTCC tools/aisimage.o
HOSTCC tools/atmelimage.o
WRAP tools/common/bootm.c
WRAP tools/common/image-fit.c
WRAP tools/common/image.c
HOSTCC tools/default_image.o
HOSTCC tools/dumpimage.o
HOSTCC tools/fit_common.o
HOSTCC tools/fit_image.o
HOSTCC tools/gpimage-common.o
HOSTCC tools/gpimage.o
HOSTCC tools/image-host.o
HOSTCC tools/imagetool.o
HOSTCC tools/imximage.o
HOSTCC tools/kwbimage.o
WRAP tools/lib/crc32.c
WRAP tools/lib/fdtdec.c
WRAP tools/lib/fdtdec_common.c
WRAP tools/lib/libfdt/fdt.c
WRAP tools/lib/libfdt/fdt_ro.c
WRAP tools/lib/libfdt/fdt_rw.c
WRAP tools/lib/libfdt/fdt_strerror.c
WRAP tools/lib/libfdt/fdt_wip.c
WRAP tools/lib/md5.c
WRAP tools/lib/sha1.c
WRAP tools/lib/sha256.c
HOSTCC tools/mkenvimage.o
HOSTCC tools/mkimage.o
HOSTCC tools/mxsimage.o
HOSTCC tools/omapimage.o
HOSTCC tools/os_support.o
HOSTCC tools/pbl_crc32.o
HOSTCC tools/pblimage.o
HOSTCC tools/ublimage.o
HOSTCC tools/proftool
HOSTCC tools/common/bootm.o
HOSTCC tools/common/image-fit.o
HOSTCC tools/common/image.o
HOSTCC tools/lib/crc32.o
HOSTCC tools/lib/fdtdec.o
HOSTCC tools/lib/fdtdec_common.o
HOSTCC tools/lib/libfdt/fdt.o
HOSTCC tools/lib/libfdt/fdt_ro.o
HOSTCC tools/lib/libfdt/fdt_rw.o
HOSTCC tools/lib/libfdt/fdt_strerror.o
HOSTCC tools/lib/libfdt/fdt_wip.o
HOSTCC tools/lib/md5.o
HOSTCC tools/lib/sha1.o
HOSTCC tools/lib/sha256.o
HOSTLD tools/mkenvimage
HOSTLD tools/dumpimage
HOSTLD tools/mkimage
LD arch/arm/cpu/built-in.o
CC arch/arm/cpu/armv7/cache_v7.o
CC arch/arm/cpu/armv7/am33xx/clock_am33xx.o
CC arch/arm/cpu/armv7/am33xx/clock.o
AS arch/arm/lib/vectors.o
AS arch/arm/lib/crt0.o
AS arch/arm/lib/relocate.o
CC arch/arm/lib/bootm-fdt.o
CC arch/arm/lib/bootm.o
CC arch/arm/cpu/armv7/am33xx/sys_info.o
CC arch/arm/cpu/armv7/cpu.o
CC arch/arm/cpu/armv7/am33xx/ddr.o
CC arch/arm/cpu/armv7/syslib.o
CC arch/arm/lib/sections.o
CC arch/arm/lib/interrupts.o
CC board/ti/am335x/board.o
CC arch/arm/cpu/armv7/am33xx/emif4.o
CC arch/arm/cpu/armv7/omap-common/reset.o
CC arch/arm/cpu/armv7/omap-common/timer.o
CC arch/arm/cpu/armv7/am33xx/board.o
CC arch/arm/lib/reset.o
LD board/ti/am335x/built-in.o
CC arch/arm/cpu/armv7/omap-common/utils.o
CC arch/arm/lib/cache.o
CC arch/arm/cpu/armv7/am33xx/mux.o
CC arch/arm/lib/cache-cp15.o
CC arch/arm/cpu/armv7/omap-common/omap-cache.o
LD arch/arm/cpu/armv7/am33xx/built-in.o
CC common/main.o
CC disk/part.o
AR arch/arm/lib/lib.a
CC arch/arm/cpu/armv7/omap-common/boot-common.o
CC arch/arm/lib/eabi_compat.o
CC common/command.o
LD arch/arm/lib/built-in.o
AS arch/arm/cpu/armv7/omap-common/lowlevel_init.o
CC arch/arm/cpu/armv7/omap-common/mem-common.o
LD drivers/block/built-in.o
CC disk/part_dos.o
CC drivers/bootcount/bootcount.o
LD arch/arm/cpu/armv7/omap-common/built-in.o
AS arch/arm/cpu/armv7/start.o
LD arch/arm/cpu/armv7/built-in.o
CC drivers/bootcount/bootcount_davinci.o
CC common/exports.o
LD drivers/dma/built-in.o
CC common/hash.o
CC disk/part_efi.o
LD drivers/bootcount/built-in.o
LD drivers/crypto/built-in.o
CC drivers/gpio/omap_gpio.o
CC drivers/dfu/dfu.o
CC common/cli_hush.o
LD drivers/gpio/built-in.o
CC drivers/i2c/i2c_core.o
CC drivers/i2c/omap24xx_i2c.o
CC drivers/dfu/dfu_mmc.o
LD disk/built-in.o
LD drivers/hwmon/built-in.o
LD drivers/memory/built-in.o
LD drivers/misc/built-in.o
CC drivers/pcmcia/tqm8xx_pcmcia.o
CC drivers/dfu/dfu_ram.o
LD drivers/i2c/built-in.o
CC drivers/rtc/date.o
CC common/cli_simple.o
LD drivers/pcmcia/built-in.o
CC common/cli.o
LD drivers/dfu/built-in.o
CC common/cli_readline.o
LD drivers/rtc/built-in.o
LD drivers/sound/built-in.o
LD drivers/tpm/built-in.o
LD drivers/twserial/built-in.o
LD drivers/video/built-in.o
CC drivers/watchdog/omap_wdt.o
CC common/s_record.o
CC common/xyzModem.o
CC common/cmd_disk.o
CC common/autoboot.o
LD drivers/watchdog/built-in.o
LD drivers/built-in.o
CC common/board_f.o
CC common/board_r.o
CC drivers/input/input.o
CC common/cmd_boot.o
CC common/cmd_bootm.o
CC common/bootm.o
LD drivers/input/built-in.o
LD drivers/mtd/built-in.o
CC drivers/mmc/mmc.o
CC common/bootm_os.o
CC drivers/mmc/omap_hsmmc.o
CC common/cmd_help.o
CC common/cmd_version.o
CC common/env_attr.o
CC common/env_callback.o
CC common/env_flags.o
CC common/env_mmc.o
CC common/cmd_source.o
CC drivers/mmc/mmc_write.o
CC common/cmd_bdinfo.o
CC common/cmd_console.o
LD drivers/mtd/onenand/built-in.o
CC common/cmd_echo.o
LD drivers/mmc/built-in.o
CC drivers/mtd/spi/sf.o
CC drivers/net/cpsw.o
CC common/cmd_eeprom.o
CC drivers/mtd/spi/sf_params.o
CC drivers/net/phy/phy.o
CC drivers/mtd/spi/sf_probe.o
CC common/cmd_exit.o
CC drivers/net/phy/smsc.o
LD drivers/net/built-in.o
LD drivers/pci/built-in.o
CC common/cmd_ext4.o
CC drivers/mtd/spi/sf_ops.o
LD drivers/power/built-in.o
LD drivers/power/battery/built-in.o
LD drivers/power/fuel_gauge/built-in.o
LD drivers/net/phy/built-in.o
LD drivers/power/mfd/built-in.o
CC common/cmd_ext2.o
CC drivers/serial/serial.o
CC drivers/power/pmic/pmic_tps65217.o
CC common/cmd_fat.o
LD drivers/mtd/spi/built-in.o
CC drivers/spi/spi.o
CC drivers/power/pmic/pmic_tps65910.o
CC common/cmd_fdt.o
CC drivers/serial/ns16550.o
CC drivers/spi/omap3_spi.o
LD drivers/power/pmic/built-in.o
LD drivers/usb/eth/built-in.o
CC common/fdt_support.o
CC drivers/serial/serial_ns16550.o
LD drivers/serial/built-in.o
CC drivers/usb/gadget/epautoconf.o
CC common/cmd_fs.o
LD drivers/spi/built-in.o
CC common/cmd_gpio.o
CC drivers/usb/gadget/config.o
CC common/cmd_i2c.o
CC common/cmd_itest.o
CC drivers/usb/gadget/usbstring.o
LD drivers/usb/host/built-in.o
CC common/cmd_load.o
CC drivers/usb/gadget/g_dnl.o
CC common/cmd_mem.o
LD drivers/usb/musb/built-in.o
LD drivers/usb/phy/built-in.o
CC drivers/usb/musb-new/musb_gadget.o
LD drivers/usb/ulpi/built-in.o
CC fs/fs.o
CC common/miiphyutil.o
CC fs/ext4/ext4fs.o
CC drivers/usb/gadget/f_dfu.o
CC fs/ext4/ext4_common.o
CC drivers/usb/gadget/ether.o
CC drivers/usb/musb-new/musb_gadget_ep0.o
CC common/cmd_mii.o
CC drivers/usb/musb-new/musb_core.o
CC common/cmd_mdio.o
CC fs/ext4/dev.o
CC common/cmd_misc.o
CC drivers/usb/gadget/rndis.o
LD fs/ext4/built-in.o
CC fs/fat/fat_write.o
CC drivers/usb/musb-new/musb_uboot.o
CC common/cmd_mmc.o
CC drivers/usb/musb-new/musb_host.o
LD drivers/usb/gadget/built-in.o
CC common/cmd_net.o
CC lib/lzo/lzo1x_decompress.o
CC common/cmd_part.o
CC common/cmd_pcmcia.o
LD lib/lzo/built-in.o
CC lib/zlib/zlib.o
CC common/cmd_sf.o
CC drivers/usb/musb-new/musb_dsps.o
LD drivers/usb/musb-new/built-in.o
CC common/cmd_spi.o
CC common/cmd_test.o
CC fs/fat/file.o
CC lib/crc7.o
CC common/cmd_usb.o
CC common/usb.o
LD fs/fat/built-in.o
LD fs/built-in.o
LD lib/zlib/built-in.o
CC lib/crc8.o
CC lib/libfdt/fdt.o
CC lib/crc16.o
CC lib/fdtdec_common.o
CC lib/libfdt/fdt_ro.o
CC common/usb_hub.o
CC lib/gunzip.o
CC common/usb_storage.o
CC lib/initcall.o
CC common/cmd_ximg.o
CC lib/lmb.o
CC lib/libfdt/fdt_rw.o
CC common/cmd_spl.o
CC lib/ldiv.o
CC lib/md5.o
CC net/arp.o
CC common/flash.o
CC lib/libfdt/fdt_strerror.o
CC lib/libfdt/fdt_sw.o
CC common/splash.o
CC net/bootp.o
CC lib/net_utils.o
CC common/cmd_dfu.o
CC lib/libfdt/fdt_wip.o
CC lib/qsort.o
CC net/eth.o
CC common/cmd_gpt.o
CC lib/sha1.o
CC lib/libfdt/fdt_empty_tree.o
CC lib/libfdt/fdt_addresses.o
LD lib/libfdt/built-in.o
CC lib/sha256.o
CC net/net.o
CC common/cmd_nvedit.o
CC lib/strmhz.o
CC lib/hashtable.o
CC common/env_common.o
CC net/nfs.o
CC net/ping.o
CC common/console.o
CC lib/errno.o
CC lib/display_options.o
CC net/tftp.o
CC lib/ctype.o
CC lib/crc32.o
CC lib/div64.o
CC common/dlmalloc.o
LD net/built-in.o
CC lib/hang.o
LD test/built-in.o
CC lib/linux_string.o
LD test/dm/built-in.o
CC lib/string.o
CC common/image.o
CC common/image-fdt.o
CC lib/time.o
AS spl/arch/arm/lib/vectors.o
AS spl/arch/arm/lib/crt0.o
CC spl/arch/arm/lib/spl.o
CC common/image-fit.o
CC spl/arch/arm/lib/sections.o
CC lib/uuid.o
CC spl/arch/arm/lib/interrupts.o
CC common/memsize.o
CC spl/arch/arm/lib/reset.o
CC lib/vsprintf.o
CC common/stdio.o
CC spl/arch/arm/lib/cache.o
CC spl/arch/arm/lib/cache-cp15.o
AR spl/arch/arm/lib/lib.a
CC spl/arch/arm/lib/eabi_compat.o
LD spl/arch/arm/lib/built-in.o
CC spl/arch/arm/cpu/armv7/cache_v7.o
CC spl/arch/arm/cpu/armv7/am33xx/clock_am33xx.o
LD common/built-in.o
CC spl/arch/arm/cpu/armv7/am33xx/clock.o
CC spl/arch/arm/cpu/armv7/cpu.o
CC spl/arch/arm/cpu/armv7/syslib.o
LD lib/built-in.o
AS spl/arch/arm/cpu/armv7/lowlevel_init.o
AS spl/arch/arm/cpu/armv7/start.o
CC spl/arch/arm/cpu/armv7/am33xx/sys_info.o
CC spl/arch/arm/cpu/armv7/omap-common/reset.o
CC spl/arch/arm/cpu/armv7/am33xx/ddr.o
CC examples/standalone/stubs.o
CC spl/arch/arm/cpu/armv7/omap-common/timer.o
CC spl/arch/arm/cpu/armv7/am33xx/emif4.o
CC spl/arch/arm/cpu/armv7/omap-common/utils.o
CC examples/standalone/hello_world.o
CC spl/board/ti/am335x/mux.o
CC spl/arch/arm/cpu/armv7/am33xx/board.o
CC spl/arch/arm/cpu/armv7/omap-common/omap-cache.o
CC spl/arch/arm/cpu/armv7/am33xx/mux.o
LD examples/standalone/libstubs.o
CC spl/board/ti/am335x/board.o
LD examples/standalone/hello_world
CC spl/arch/arm/cpu/armv7/omap-common/boot-common.o
LD spl/arch/arm/cpu/armv7/am33xx/built-in.o
AS spl/arch/arm/cpu/armv7/omap-common/lowlevel_init.o
CC spl/arch/arm/cpu/armv7/omap-common/mem-common.o
OBJCOPY examples/standalone/hello_world.srec
LD spl/board/ti/am335x/built-in.o
CC spl/common/spl/spl.o
OBJCOPY examples/standalone/hello_world.bin
LD spl/arch/arm/cpu/armv7/omap-common/built-in.o
CC spl/common/spl/spl_ymodem.o
LD spl/arch/arm/cpu/armv7/built-in.o
CC spl/common/spl/spl_net.o
CC spl/common/spl/spl_mmc.o
CC spl/common/spl/spl_fat.o
CC spl/disk/part.o
CC spl/disk/part_dos.o
CC spl/common/xyzModem.o
LD spl/common/spl/built-in.o
CC spl/drivers/i2c/i2c_core.o
CC spl/common/miiphyutil.o
CC spl/drivers/i2c/omap24xx_i2c.o
CC spl/disk/part_efi.o
CC spl/common/env_attr.o
CC spl/common/env_flags.o
CC spl/common/env_callback.o
LD spl/drivers/i2c/built-in.o
CC spl/drivers/gpio/omap_gpio.o
CC spl/common/env_nowhere.o
CC spl/common/cmd_nvedit.o
LD spl/disk/built-in.o
CC spl/drivers/mmc/mmc.o
LD spl/drivers/gpio/built-in.o
CC spl/drivers/mmc/omap_hsmmc.o
CC spl/common/env_common.o
CC spl/drivers/serial/serial.o
CC spl/drivers/serial/ns16550.o
CC spl/common/console.o
CC spl/drivers/serial/serial_ns16550.o
LD spl/drivers/mmc/built-in.o
CC spl/fs/fat/fat_write.o
CC spl/lib/crc16.o
LD spl/drivers/serial/built-in.o
CC spl/lib/net_utils.o
CC spl/lib/hashtable.o
LD spl/drivers/power/built-in.o
CC spl/common/dlmalloc.o
CC spl/lib/errno.o
CC spl/lib/display_options.o
CC spl/common/image.o
CC spl/lib/crc32.o
CC spl/common/image-fdt.o
CC spl/lib/ctype.o
CC spl/lib/div64.o
CC spl/lib/hang.o
CC spl/common/memsize.o
CC spl/common/stdio.o
CC spl/lib/linux_string.o
CC spl/lib/string.o
LD u-boot
LD spl/common/built-in.o
CC spl/drivers/power/pmic/pmic_tps65217.o
LD spl/fs/fat/built-in.o
LD spl/fs/built-in.o
CC spl/drivers/power/pmic/pmic_tps65910.o
CC spl/lib/time.o
LD spl/drivers/power/pmic/built-in.o
CC spl/net/arp.o
CC spl/net/bootp.o
CC spl/lib/uuid.o
CC spl/net/eth.o
CC spl/drivers/watchdog/omap_wdt.o
CC spl/lib/vsprintf.o
LD spl/drivers/watchdog/built-in.o
CC spl/drivers/usb/musb-new/musb_gadget.o
LD spl/arch/arm/cpu/built-in.o
CC spl/drivers/usb/musb-new/musb_gadget_ep0.o
CC spl/net/net.o
LD spl/lib/built-in.o
LDS spl/u-boot-spl.lds
CC spl/drivers/usb/musb-new/musb_core.o
CC spl/drivers/usb/musb-new/musb_uboot.o
CC spl/net/tftp.o
CC spl/drivers/usb/musb-new/musb_host.o
CC spl/drivers/usb/musb-new/musb_dsps.o
LD spl/net/built-in.o
OBJCOPY u-boot.bin
OBJCOPY u-boot.srec
LD spl/drivers/usb/musb-new/built-in.o
LD spl/u-boot-spl
OBJCOPY spl/u-boot-spl.bin
MKIMAGE MLO
MKIMAGE u-boot.img
编译工作至此完成。得到的结果如下图
图中所示文件和文件夹都在O=am335x_beaglebone_black所指定的文件夹中。
u-boot——ELF格式的可执行文件，由于里面含有调试程序所选的信息，所以该格式的可执行文件体积最大。
u-boot.bin——可以直接写入Flash的二进制可执行文件，直接被运行。
u-boot.srec——u-boot映像的S-Record格式
u-boot.map——这个文件包含了u-boot可执行文件的映像符号表
MLO——这个文件也是一个引导加载程序，用于加载u-boot.img到SDRAM。
u-boot-spl.bin——这个文件实际上就是纯粹的SPL程序，和上图的MLO文件相比，MLO=头部信息+u-boot-spl.bin。
具体使用那个文件根据自己的需要选择。
从SD卡启动时，应该把MLO、u-boot.img文件放到boot分区。
从串口启动时，应该传送u-boot-spl.bin、u-boot.img文件。