u-boot 学习系列 1

u-boot这个东西从自我N年前使用到现在，变化好多，今天开始重新研究下，本系列的研究都是基于BeagleBoneBlack（bbb）开发板和 u-boot v201801版本的。

SPL介绍

在源代码中 doc/README.SPL 中说得比较明白，我这里再归纳下。

现在很多处理器都内置一个BOOT ROM，执行部分初始化，并可从各种外设和存储器中加载程序并执行，BOOT ROM中固化的程序被称为一级程序加载器，被它加载的程序就称为二级程序加载器（secondary program loader，即SPL）。其实u-boot本身就可以作为二级程序加载器，但不幸的是一般BOOT ROM之后，主存储器都是没有初始化的，例如BBB只有109K的内置RAM可用，这就限制了程序的大小，全功能的u-boot不能运行在这么小的RAM上运行。于是精简的u-boot，u-boot-spl就此问世。

SPL精简代码的方式

在 Kconfig 文件中有一个 CONFIG_SPL 的选项，使能后，就会定义 CONFIG_SPL_BUILD

Makefile层面使用 CONFIG_SPL_BUILD 选择不同的文件或文件夹进行编译，例如

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

obj-$(CONFIG_SPL_BUILD) += foo.o

而C代码层面，则使用 CONFIG_SPL_BUILD 宏来进行条件编译，例如

#ifdef CONFIG_SPL_BUILD
    foo();
#endif

因为C代码中有条件编译选项，所以SPL和主u-boot代码，不能复用.o文件，所有文件必须全部重新编译。下面我们就来编译试试看（arm-linux-gnueabihf-gcc请到linaro官方网站下载，然后放到PATH中即可）

make distclean
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- O=bbb-build am335x_boneblack_defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- O=bbb-build

为了说明下问题，我把打印的输出放到这里

  1   CHK     include/config/uboot.release
  2   Using .. as source for U-Boot
  3   GEN     ./Makefile
  4   CHK     include/generated/version_autogenerated.h
  5   CHK     include/generated/timestamp_autogenerated.h
  6   UPD     include/generated/timestamp_autogenerated.h
  7   HOSTCC  scripts/basic/fixdep
  8   CC      lib/asm-offsets.s
  9   CHK     include/generated/generic-asm-offsets.h
 10   CC      arch/arm/lib/asm-offsets.s
 11   CHK     include/generated/asm-offsets.h
 12   HOSTCC  tools/gen_eth_addr
 13   HOSTCC  tools/gen_ethaddr_crc.o
 14   WRAP    tools/lib/crc8.c
 15   HOSTCC  tools/lib/crc8.o
 16   HOSTLD  tools/gen_ethaddr_crc
 17   HOSTCC  tools/img2srec
 18   HOSTCC  tools/mkenvimage.o
 19   HOSTCC  tools/os_support.o
 20   WRAP    tools/lib/crc32.c
 21   HOSTCC  tools/lib/crc32.o
 22   HOSTLD  tools/mkenvimage
 23   HOSTCC  tools/aisimage.o
 24   HOSTCC  tools/atmelimage.o
 25   WRAP    tools/common/bootm.c
 26   HOSTCC  tools/common/bootm.o
 27   HOSTCC  tools/default_image.o
 28   WRAP    tools/lib/fdtdec_common.c
 29   HOSTCC  tools/lib/fdtdec_common.o
 30   WRAP    tools/lib/fdtdec.c
 31   HOSTCC  tools/lib/fdtdec.o
 32   HOSTCC  tools/fit_common.o
 33   HOSTCC  tools/fit_image.o
 34   WRAP    tools/common/image-fit.c
 35   HOSTCC  tools/common/image-fit.o
 36   HOSTCC  tools/image-host.o
 37   WRAP    tools/common/image.c
 38   HOSTCC  tools/common/image.o
 39   HOSTCC  tools/imagetool.o
 40   HOSTCC  tools/imximage.o
 41   HOSTCC  tools/kwbimage.o
 42   WRAP    tools/lib/md5.c
 43   HOSTCC  tools/lib/md5.o
 44   HOSTCC  tools/lpc32xximage.o
 45   HOSTCC  tools/mxsimage.o
 46   HOSTCC  tools/omapimage.o
 47   HOSTCC  tools/pblimage.o
 48   HOSTCC  tools/pbl_crc32.o
 49   HOSTCC  tools/vybridimage.o
 50   WRAP    tools/lib/rc4.c
 51   HOSTCC  tools/lib/rc4.o
 52   HOSTCC  tools/rkcommon.o
 53   HOSTCC  tools/rkimage.o
 54   HOSTCC  tools/rksd.o
 55   HOSTCC  tools/rkspi.o
 56   HOSTCC  tools/socfpgaimage.o
 57   WRAP    tools/lib/sha1.c
 58   HOSTCC  tools/lib/sha1.o
 59   WRAP    tools/lib/sha256.c
 60   HOSTCC  tools/lib/sha256.o
 61   WRAP    tools/common/hash.c
 62   HOSTCC  tools/common/hash.o
 63   HOSTCC  tools/ublimage.o
 64   HOSTCC  tools/zynqimage.o
 65   HOSTCC  tools/zynqmpimage.o
 66   HOSTCC  tools/libfdt/fdt.o
 67   HOSTCC  tools/libfdt/fdt_wip.o
 68   HOSTCC  tools/libfdt/fdt_sw.o
 69   HOSTCC  tools/libfdt/fdt_strerror.o
 70   HOSTCC  tools/libfdt/fdt_empty_tree.o
 71   HOSTCC  tools/libfdt/fdt_addresses.o
 72   HOSTCC  tools/libfdt/fdt_overlay.o
 73   WRAP    tools/lib/libfdt/fdt_ro.c
 74   HOSTCC  tools/lib/libfdt/fdt_ro.o
 75   WRAP    tools/lib/libfdt/fdt_rw.c
 76   HOSTCC  tools/lib/libfdt/fdt_rw.o
 77   WRAP    tools/lib/libfdt/fdt_region.c
 78   HOSTCC  tools/lib/libfdt/fdt_region.o
 79   HOSTCC  tools/gpimage.o
 80   HOSTCC  tools/gpimage-common.o
 81   HOSTCC  tools/dumpimage.o
 82   HOSTLD  tools/dumpimage
 83   HOSTCC  tools/mkimage.o
 84   HOSTLD  tools/mkimage
 85   HOSTCC  tools/proftool
 86   HOSTCC  tools/fdtgrep.o
 87   HOSTLD  tools/fdtgrep
 88   LD      arch/arm/cpu/built-in.o
 89   CC      arch/arm/cpu/armv7/cache_v7.o
 90   AS      arch/arm/cpu/armv7/cache_v7_asm.o
 91   CC      arch/arm/cpu/armv7/cpu.o
 92   CC      arch/arm/cpu/armv7/cp15.o
 93   CC      arch/arm/cpu/armv7/syslib.o
 94   LD      arch/arm/cpu/armv7/built-in.o
 95   AS      arch/arm/cpu/armv7/start.o
 96   AS      arch/arm/lib/vectors.o
 97   AS      arch/arm/lib/crt0.o
 98   AS      arch/arm/lib/setjmp.o
 99   AS      arch/arm/lib/relocate.o
100   CC      arch/arm/lib/bootm-fdt.o
101   CC      arch/arm/lib/bootm.o
102   CC      arch/arm/lib/zimage.o
103   AS      arch/arm/lib/memset.o
104   AS      arch/arm/lib/memcpy.o
105   CC      arch/arm/lib/sections.o
106   CC      arch/arm/lib/stack.o
107   CC      arch/arm/lib/interrupts.o
108   CC      arch/arm/lib/reset.o
109   CC      arch/arm/lib/cache.o
110   CC      arch/arm/lib/cache-cp15.o
111   CC      arch/arm/lib/psci-dt.o
112   LD      arch/arm/lib/built-in.o
113   AS      arch/arm/lib/ashldi3.o
114   AS      arch/arm/lib/ashrdi3.o
115   CC      arch/arm/lib/div0.o
116   AS      arch/arm/lib/div64.o
117   AS      arch/arm/lib/lib1funcs.o
118   AS      arch/arm/lib/lshrdi3.o
119   AS      arch/arm/lib/muldi3.o
120   AS      arch/arm/lib/uldivmod.o
121   AR      arch/arm/lib/lib.a
122   CC      arch/arm/lib/eabi_compat.o
123   AS      arch/arm/lib/crt0_arm_efi.o
124   CC      arch/arm/lib/reloc_arm_efi.o
125   CC      arch/arm/mach-omap2/am33xx/clock_am33xx.o
126   CC      arch/arm/mach-omap2/am33xx/clock.o
127   CC      arch/arm/mach-omap2/am33xx/sys_info.o
128   CC      arch/arm/mach-omap2/am33xx/ddr.o
129   CC      arch/arm/mach-omap2/am33xx/board.o
130   CC      arch/arm/mach-omap2/am33xx/mux.o
131   CC      arch/arm/mach-omap2/am33xx/prcm-regs.o
132   CC      arch/arm/mach-omap2/am33xx/hw_data.o
133   CC      arch/arm/mach-omap2/am33xx/fdt.o
134   CC      arch/arm/mach-omap2/am33xx/clk_synthesizer.o
135   LD      arch/arm/mach-omap2/am33xx/built-in.o
136   CC      arch/arm/mach-omap2/reset.o
137   CC      arch/arm/mach-omap2/timer.o
138   CC      arch/arm/mach-omap2/utils.o
139   CC      arch/arm/mach-omap2/sysinfo-common.o
140   CC      arch/arm/mach-omap2/omap-cache.o
141   CC      arch/arm/mach-omap2/boot-common.o
142   AS      arch/arm/mach-omap2/lowlevel_init.o
143   CC      arch/arm/mach-omap2/mem-common.o
144   CC      arch/arm/mach-omap2/fdt-common.o
145   LD      arch/arm/mach-omap2/built-in.o
146   CC      board/ti/am335x/board.o
147   LD      board/ti/am335x/built-in.o
148   CC      board/ti/common/board_detect.o
149   LD      board/ti/common/built-in.o
150   CC      cmd/boot.o
151   CC      cmd/bootm.o
152   CC      cmd/help.o
153   CC      cmd/version.o
154   CC      cmd/blk_common.o
155   CC      cmd/source.o
156   CC      cmd/bdinfo.o
157   CC      cmd/bootefi.o
158   CC      cmd/bootz.o
159   CC      cmd/console.o
160   CC      cmd/echo.o
161   CC      cmd/eeprom.o
162   CC      cmd/elf.o
163   CC      cmd/exit.o
164   CC      cmd/ext4.o
165   CC      cmd/ext2.o
166   CC      cmd/fat.o
167   CC      cmd/fdt.o
168   CC      cmd/fs.o
169   CC      cmd/gpio.o
170   CC      cmd/i2c.o
171   CC      cmd/itest.o
172   CC      cmd/load.o
173   CC      cmd/mem.o
174   CC      cmd/mii.o
175   CC      cmd/mdio.o
176   CC      cmd/misc.o
177   CC      cmd/mmc.o
178   CC      cmd/net.o
179   CC      cmd/part.o
180   CC      cmd/pcmcia.o
181   CC      cmd/pxe.o
182   CC      cmd/sf.o
183   CC      cmd/spi.o
184   CC      cmd/time.o
185   CC      cmd/test.o
186   CC      cmd/usb.o
187   CC      cmd/disk.o
188   CC      cmd/fastboot.o
189   CC      cmd/ximg.o
190   CC      cmd/spl.o
191   CC      cmd/dfu.o
192   CC      cmd/gpt.o
193   CC      cmd/nvedit.o
194   LD      cmd/built-in.o
195   CC      common/init/board_init.o
196   LD      common/init/built-in.o
197   CC      common/main.o
198   CC      common/exports.o
199   CC      common/hash.o
200   CC      common/cli_hush.o
201   CC      common/autoboot.o
202   CC      common/board_f.o
203   CC      common/board_r.o
204   CC      common/board_info.o
205   CC      common/bootm.o
206   CC      common/bootm_os.o
207   CC      common/fdt_support.o
208   CC      common/miiphyutil.o
209   CC      common/usb.o
210   CC      common/usb_hub.o
211   CC      common/usb_storage.o
212   CC      common/splash.o
213   CC      common/menu.o
214   CC      common/update.o
215   CC      common/cli_readline.o
216   CC      common/cli_simple.o
217   CC      common/console.o
218   CC      common/dlmalloc.o
219   CC      common/malloc_simple.o
220   CC      common/image.o
221   CC      common/image-android.o
222   CC      common/image-fdt.o
223   CC      common/image-fit.o
224   CC      common/memsize.o
225   CC      common/stdio.o
226   CC      common/cli.o
227   CC      common/dfu.o
228   CC      common/command.o
229   CC      common/s_record.o
230   CC      common/xyzModem.o
231   LD      common/built-in.o
232   CC      disk/part.o
233   CC      disk/part_dos.o
234   CC      disk/part_iso.o
235   CC      disk/part_efi.o
236   LD      disk/built-in.o
237   LD      drivers/adc/built-in.o
238   LD      drivers/ata/built-in.o
239   CC      drivers/block/blk_legacy.o
240   LD      drivers/block/built-in.o
241   CC      drivers/bootcount/bootcount.o
242   CC      drivers/bootcount/bootcount_davinci.o
243   LD      drivers/bootcount/built-in.o
244   CC      drivers/core/device.o
245   CC      drivers/core/fdtaddr.o
246   CC      drivers/core/lists.o
247   CC      drivers/core/root.o
248   CC      drivers/core/uclass.o
249   CC      drivers/core/util.o
250   CC      drivers/core/device-remove.o
251   CC      drivers/core/dump.o
252   LD      drivers/core/built-in.o
253   CC      drivers/crypto/fsl/sec.o
254   LD      drivers/crypto/fsl/built-in.o
255   LD      drivers/crypto/rsa_mod_exp/built-in.o
256   LD      drivers/crypto/built-in.o
257   CC      drivers/dfu/dfu.o
258   CC      drivers/dfu/dfu_mmc.o
259   CC      drivers/dfu/dfu_ram.o
260   CC      drivers/dfu/dfu_tftp.o
261   LD      drivers/dfu/built-in.o
262   LD      drivers/firmware/built-in.o
263   CC      drivers/input/input.o
264   LD      drivers/input/built-in.o
265   LD      drivers/mailbox/built-in.o
266   LD      drivers/memory/built-in.o
267   LD      drivers/misc/built-in.o
268   CC      drivers/mmc/mmc.o
269   CC      drivers/mmc/mmc_legacy.o
270   CC      drivers/mmc/mmc_write.o
271   CC      drivers/mmc/omap_hsmmc.o
272   LD      drivers/mmc/built-in.o
273   LD      drivers/pcmcia/built-in.o
274   LD      drivers/phy/marvell/built-in.o
275   LD      drivers/pwm/built-in.o
276   LD      drivers/reset/built-in.o
277   CC      drivers/rtc/date.o
278   LD      drivers/rtc/built-in.o
279   LD      drivers/scsi/built-in.o
280   LD      drivers/soc/built-in.o
281   LD      drivers/sound/built-in.o
282   LD      drivers/spmi/built-in.o
283   LD      drivers/sysreset/built-in.o
284   LD      drivers/thermal/built-in.o
285   LD      drivers/tpm/built-in.o
286   LD      drivers/video/bridge/built-in.o
287   LD      drivers/video/sunxi/built-in.o
288   LD      drivers/video/built-in.o
289   CC      drivers/watchdog/omap_wdt.o
290   LD      drivers/watchdog/built-in.o
291   LD      drivers/built-in.o
292   LD      drivers/dma/built-in.o
293   CC      drivers/gpio/gpio-uclass.o
294   CC      drivers/gpio/omap_gpio.o
295   LD      drivers/gpio/built-in.o
296   CC      drivers/i2c/i2c_core.o
297   CC      drivers/i2c/omap24xx_i2c.o
298   LD      drivers/i2c/built-in.o
299   CC      drivers/mtd/mtdcore.o
300   CC      drivers/mtd/mtd_uboot.o
301   LD      drivers/mtd/built-in.o
302   LD      drivers/mtd/onenand/built-in.o
303   CC      drivers/mtd/spi/sf_probe.o
304   CC      drivers/mtd/spi/spi_flash.o
305   CC      drivers/mtd/spi/spi_flash_ids.o
306   CC      drivers/mtd/spi/sf.o
307   LD      drivers/mtd/spi/built-in.o
308   CC      drivers/net/cpsw.o
309   CC      drivers/net/cpsw-common.o
310   LD      drivers/net/built-in.o
311   CC      drivers/net/phy/phy.o
312   CC      drivers/net/phy/atheros.o
313   CC      drivers/net/phy/smsc.o
314   LD      drivers/net/phy/built-in.o
315   LD      drivers/pci/built-in.o
316   LD      drivers/power/built-in.o
317   LD      drivers/power/battery/built-in.o
318   LD      drivers/power/domain/built-in.o
319   LD      drivers/power/fuel_gauge/built-in.o
320   LD      drivers/power/mfd/built-in.o
321   CC      drivers/power/pmic/pmic_tps65217.o
322   CC      drivers/power/pmic/pmic_tps65910.o
323   LD      drivers/power/pmic/built-in.o
324   LD      drivers/power/regulator/built-in.o
325   CC      drivers/serial/serial-uclass.o
326   CC      drivers/serial/ns16550.o
327   LD      drivers/serial/built-in.o
328   CC      drivers/spi/spi.o
329   CC      drivers/spi/omap3_spi.o
330   LD      drivers/spi/built-in.o
331   LD      drivers/usb/common/built-in.o
332   LD      drivers/usb/dwc3/built-in.o
333   LD      drivers/usb/emul/built-in.o
334   LD      drivers/usb/eth/built-in.o
335   CC      drivers/usb/gadget/epautoconf.o
336   CC      drivers/usb/gadget/config.o
337   CC      drivers/usb/gadget/usbstring.o
338   CC      drivers/usb/gadget/g_dnl.o
339   CC      drivers/usb/gadget/f_dfu.o
340   CC      drivers/usb/gadget/f_fastboot.o
341   CC      drivers/usb/gadget/ether.o
342   CC      drivers/usb/gadget/rndis.o
343   LD      drivers/usb/gadget/built-in.o
344   LD      drivers/usb/gadget/udc/built-in.o
345   LD      drivers/usb/host/built-in.o
346   CC      drivers/usb/musb-new/musb_gadget.o
347   CC      drivers/usb/musb-new/musb_gadget_ep0.o
348   CC      drivers/usb/musb-new/musb_core.o
349   CC      drivers/usb/musb-new/musb_uboot.o
350   CC      drivers/usb/musb-new/musb_host.o
351   CC      drivers/usb/musb-new/musb_dsps.o
352   LD      drivers/usb/musb-new/built-in.o
353   LD      drivers/usb/musb/built-in.o
354   LD      drivers/usb/phy/built-in.o
355   LD      drivers/usb/ulpi/built-in.o
356   CC      env/common.o
357   CC      env/env.o
358   CC      env/attr.o
359   CC      env/callback.o
360   CC      env/flags.o
361   CC      env/mmc.o
362   LD      env/built-in.o
363   CC      fs/fs.o
364   CC      fs/ext4/ext4fs.o
365   CC      fs/ext4/ext4_common.o
366   CC      fs/ext4/dev.o
367   CC      fs/ext4/ext4_write.o
368   CC      fs/ext4/ext4_journal.o
369   CC      fs/ext4/crc16.o
370   LD      fs/ext4/built-in.o
371   CC      fs/fat/fat_write.o
372   LD      fs/fat/built-in.o
373   CC      fs/fs_internal.o
374   LD      fs/built-in.o
375   CC      lib/efi_loader/efi_image_loader.o
376   CC      lib/efi_loader/efi_boottime.o
377   CC      lib/efi_loader/efi_runtime.o
378   CC      lib/efi_loader/efi_console.o
379   CC      lib/efi_loader/efi_memory.o
380   CC      lib/efi_loader/efi_device_path_to_text.o
381   CC      lib/efi_loader/efi_device_path.o
382   CC      lib/efi_loader/efi_file.o
383   CC      lib/efi_loader/efi_variable.o
384   CC      lib/efi_loader/efi_bootmgr.o
385   CC      lib/efi_loader/efi_watchdog.o
386   CC      lib/efi_loader/efi_disk.o
387   CC      lib/efi_loader/efi_net.o
388   CC      lib/efi_loader/efi_smbios.o
389   LD      lib/efi_loader/built-in.o
390   CC      lib/efi_loader/helloworld.o
391   LD      lib/efi_loader/helloworld_efi.so
392   OBJCOPY lib/efi_loader/helloworld.efi
393 rm lib/efi_loader/helloworld_efi.so lib/efi_loader/helloworld.o
394   LD      lib/efi_selftest/built-in.o
395   CC      lib/libfdt/fdt.o
396   CC      lib/libfdt/fdt_wip.o
397   CC      lib/libfdt/fdt_strerror.o
398   CC      lib/libfdt/fdt_sw.o
399   CC      lib/libfdt/fdt_empty_tree.o
400   CC      lib/libfdt/fdt_addresses.o
401   CC      lib/libfdt/fdt_overlay.o
402   CC      lib/libfdt/fdt_ro.o
403   CC      lib/libfdt/fdt_rw.o
404   CC      lib/libfdt/fdt_region.o
405   LD      lib/libfdt/built-in.o
406   CC      lib/lzo/lzo1x_decompress.o
407   LD      lib/lzo/built-in.o
408   CC      lib/zlib/zlib.o
409   LD      lib/zlib/built-in.o
410   CC      lib/charset.o
411   CC      lib/crc7.o
412   CC      lib/crc8.o
413   CC      lib/crc16.o
414   CC      lib/fdtdec_common.o
415   CC      lib/smbios.o
416   CC      lib/initcall.o
417   CC      lib/lmb.o
418   CC      lib/ldiv.o
419   CC      lib/md5.o
420   CC      lib/net_utils.o
421   CC      lib/qsort.o
422   CC      lib/rc4.o
423   CC      lib/list_sort.o
424   CC      lib/sha1.o
425   CC      lib/sha256.o
426   CC      lib/gunzip.o
427   CC      lib/hashtable.o
428   CC      lib/errno.o
429   CC      lib/display_options.o
430   CC      lib/crc32.o
431   CC      lib/ctype.o
432   CC      lib/div64.o
433   CC      lib/hang.o
434   CC      lib/linux_compat.o
435   CC      lib/linux_string.o
436   CC      lib/membuff.o
437   CC      lib/slre.o
438   CC      lib/string.o
439   CC      lib/tables_csum.o
440   CC      lib/time.o
441   CC      lib/uuid.o
442   CC      lib/rand.o
443   CC      lib/vsprintf.o
444   CC      lib/panic.o
445   CC      lib/strto.o
446   CC      lib/strmhz.o
447   LD      lib/built-in.o
448   CC      net/checksum.o
449   CC      net/arp.o
450   CC      net/bootp.o
451   CC      net/eth_legacy.o
452   CC      net/eth_common.o
453   CC      net/net.o
454   CC      net/nfs.o
455   CC      net/ping.o
456   CC      net/tftp.o
457   LD      net/built-in.o
458   LD      test/built-in.o
459   CC      test/dm/cmd_dm.o
460   LD      test/dm/built-in.o
461   CC      examples/standalone/stubs.o
462   LD      examples/standalone/libstubs.o
463   CC      examples/standalone/hello_world.o
464   LD      examples/standalone/hello_world
465   OBJCOPY examples/standalone/hello_world.srec
466   OBJCOPY examples/standalone/hello_world.bin
467   LDS     u-boot.lds
468   LD      u-boot
469   OBJCOPY u-boot-nodtb.bin
470   COPY    u-boot.bin
471   MKIMAGE u-boot.img
472   OBJCOPY u-boot.srec
473   SYM     u-boot.sym
474 /bin/sh: 1: bc: not found
475   CC      spl/arch/arm/mach-omap2/am33xx/clock_am33xx.o
476   CC      spl/arch/arm/mach-omap2/am33xx/clock.o
477   CC      spl/arch/arm/mach-omap2/am33xx/sys_info.o
478   CC      spl/arch/arm/mach-omap2/am33xx/ddr.o
479   CC      spl/arch/arm/mach-omap2/am33xx/emif4.o
480   CC      spl/arch/arm/mach-omap2/am33xx/board.o
481   CC      spl/arch/arm/mach-omap2/am33xx/mux.o
482   CC      spl/arch/arm/mach-omap2/am33xx/prcm-regs.o
483   CC      spl/arch/arm/mach-omap2/am33xx/hw_data.o
484   CC      spl/arch/arm/mach-omap2/am33xx/fdt.o
485   CC      spl/arch/arm/mach-omap2/am33xx/clk_synthesizer.o
486   LD      spl/arch/arm/mach-omap2/am33xx/built-in.o
487   CC      spl/arch/arm/mach-omap2/reset.o
488   CC      spl/arch/arm/mach-omap2/timer.o
489   CC      spl/arch/arm/mach-omap2/utils.o
490   CC      spl/arch/arm/mach-omap2/sysinfo-common.o
491   CC      spl/arch/arm/mach-omap2/omap-cache.o
492   CC      spl/arch/arm/mach-omap2/boot-common.o
493   AS      spl/arch/arm/mach-omap2/lowlevel_init.o
494   CC      spl/arch/arm/mach-omap2/mem-common.o
495   CC      spl/arch/arm/mach-omap2/fdt-common.o
496   LD      spl/arch/arm/mach-omap2/built-in.o
497   CC      spl/arch/arm/cpu/armv7/cache_v7.o
498   AS      spl/arch/arm/cpu/armv7/cache_v7_asm.o
499   CC      spl/arch/arm/cpu/armv7/cpu.o
500   CC      spl/arch/arm/cpu/armv7/cp15.o
501   CC      spl/arch/arm/cpu/armv7/syslib.o
502   AS      spl/arch/arm/cpu/armv7/lowlevel_init.o
503   LD      spl/arch/arm/cpu/armv7/built-in.o
504   AS      spl/arch/arm/cpu/armv7/start.o
505   LD      spl/arch/arm/cpu/built-in.o
506   AS      spl/arch/arm/lib/vectors.o
507   AS      spl/arch/arm/lib/crt0.o
508   AS      spl/arch/arm/lib/setjmp.o
509   CC      spl/arch/arm/lib/spl.o
510   CC      spl/arch/arm/lib/zimage.o
511   CC      spl/arch/arm/lib/bootm-fdt.o
512   AS      spl/arch/arm/lib/memset.o
513   AS      spl/arch/arm/lib/memcpy.o
514   CC      spl/arch/arm/lib/sections.o
515   CC      spl/arch/arm/lib/stack.o
516   CC      spl/arch/arm/lib/interrupts.o
517   CC      spl/arch/arm/lib/reset.o
518   CC      spl/arch/arm/lib/cache.o
519   CC      spl/arch/arm/lib/cache-cp15.o
520   CC      spl/arch/arm/lib/psci-dt.o
521   LD      spl/arch/arm/lib/built-in.o
522   AS      spl/arch/arm/lib/ashldi3.o
523   AS      spl/arch/arm/lib/ashrdi3.o
524   CC      spl/arch/arm/lib/div0.o
525   AS      spl/arch/arm/lib/div64.o
526   AS      spl/arch/arm/lib/lib1funcs.o
527   AS      spl/arch/arm/lib/lshrdi3.o
528   AS      spl/arch/arm/lib/muldi3.o
529   AS      spl/arch/arm/lib/uldivmod.o
530   AR      spl/arch/arm/lib/lib.a
531   CC      spl/arch/arm/lib/eabi_compat.o
532   AS      spl/arch/arm/lib/crt0_arm_efi.o
533   CC      spl/arch/arm/lib/reloc_arm_efi.o
534   CC      spl/board/ti/am335x/mux.o
535   CC      spl/board/ti/am335x/board.o
536   LD      spl/board/ti/am335x/built-in.o
537   CC      spl/board/ti/common/board_detect.o
538   LD      spl/board/ti/common/built-in.o
539   CC      spl/common/spl/spl.o
540   CC      spl/common/spl/spl_ymodem.o
541   CC      spl/common/spl/spl_mmc.o
542   CC      spl/common/spl/spl_fat.o
543   CC      spl/common/spl/spl_ext.o
544   LD      spl/common/spl/built-in.o
545   CC      spl/common/init/board_init.o
546   LD      spl/common/init/built-in.o
547   CC      spl/common/xyzModem.o
548   CC      spl/common/fdt_support.o
549   CC      spl/common/console.o
550   CC      spl/common/dlmalloc.o
551   CC      spl/common/malloc_simple.o
552   CC      spl/common/image.o
553   CC      spl/common/image-android.o
554   CC      spl/common/image-fdt.o
555   CC      spl/common/memsize.o
556   CC      spl/common/stdio.o
557   CC      spl/common/cli.o
558   CC      spl/common/dfu.o
559   CC      spl/common/command.o
560   CC      spl/common/s_record.o
561   LD      spl/common/built-in.o
562   CC      spl/cmd/nvedit.o
563   LD      spl/cmd/built-in.o
564   CC      spl/env/common.o
565   CC      spl/env/env.o
566   CC      spl/env/attr.o
567   CC      spl/env/flags.o
568   CC      spl/env/callback.o
569   CC      spl/env/mmc.o
570   LD      spl/env/built-in.o
571   CC      spl/lib/sha1.o
572   CC      spl/lib/sha256.o
573   CC      spl/lib/libfdt/fdt.o
574   CC      spl/lib/libfdt/fdt_wip.o
575   CC      spl/lib/libfdt/fdt_strerror.o
576   CC      spl/lib/libfdt/fdt_sw.o
577   CC      spl/lib/libfdt/fdt_empty_tree.o
578   CC      spl/lib/libfdt/fdt_addresses.o
579   CC      spl/lib/libfdt/fdt_overlay.o
580   CC      spl/lib/libfdt/fdt_ro.o
581   CC      spl/lib/libfdt/fdt_rw.o
582   CC      spl/lib/libfdt/fdt_region.o
583   LD      spl/lib/libfdt/built-in.o
584   CC      spl/lib/crc16.o
585   CC      spl/lib/hashtable.o
586   CC      spl/lib/errno.o
587   CC      spl/lib/display_options.o
588   CC      spl/lib/crc32.o
589   CC      spl/lib/ctype.o
590   CC      spl/lib/div64.o
591   CC      spl/lib/hang.o
592   CC      spl/lib/linux_compat.o
593   CC      spl/lib/linux_string.o
594   CC      spl/lib/membuff.o
595   CC      spl/lib/slre.o
596   CC      spl/lib/string.o
597   CC      spl/lib/tables_csum.o
598   CC      spl/lib/time.o
599   CC      spl/lib/uuid.o
600   CC      spl/lib/rand.o
601   CC      spl/lib/tiny-printf.o
602   CC      spl/lib/panic.o
603   CC      spl/lib/strto.o
604   LD      spl/lib/built-in.o
605   CC      spl/disk/part.o
606   CC      spl/disk/part_dos.o
607   CC      spl/disk/part_iso.o
608   CC      spl/disk/part_efi.o
609   LD      spl/disk/built-in.o
610   CC      spl/drivers/block/blk_legacy.o
611   LD      spl/drivers/block/built-in.o
612   CC      spl/drivers/core/device.o
613   CC      spl/drivers/core/fdtaddr.o
614   CC      spl/drivers/core/lists.o
615   CC      spl/drivers/core/root.o
616   CC      spl/drivers/core/uclass.o
617   CC      spl/drivers/core/util.o
618   CC      spl/drivers/core/dump.o
619   LD      spl/drivers/core/built-in.o
620   CC      spl/drivers/gpio/gpio-uclass.o
621   CC      spl/drivers/gpio/omap_gpio.o
622   LD      spl/drivers/gpio/built-in.o
623   CC      spl/drivers/i2c/i2c_core.o
624   CC      spl/drivers/i2c/omap24xx_i2c.o
625   LD      spl/drivers/i2c/built-in.o
626   CC      spl/drivers/mmc/mmc.o
627   CC      spl/drivers/mmc/mmc_legacy.o
628   CC      spl/drivers/mmc/omap_hsmmc.o
629   LD      spl/drivers/mmc/built-in.o
630   LD      spl/drivers/power/built-in.o
631   CC      spl/drivers/power/pmic/pmic_tps65217.o
632   CC      spl/drivers/power/pmic/pmic_tps65910.o
633   LD      spl/drivers/power/pmic/built-in.o
634   LD      spl/drivers/power/regulator/built-in.o
635   CC      spl/drivers/serial/serial-uclass.o
636   CC      spl/drivers/serial/ns16550.o
637   LD      spl/drivers/serial/built-in.o
638   CC      spl/drivers/usb/musb-new/musb_gadget.o
639   CC      spl/drivers/usb/musb-new/musb_gadget_ep0.o
640   CC      spl/drivers/usb/musb-new/musb_core.o
641   CC      spl/drivers/usb/musb-new/musb_uboot.o
642   CC      spl/drivers/usb/musb-new/musb_host.o
643   CC      spl/drivers/usb/musb-new/musb_dsps.o
644   LD      spl/drivers/usb/musb-new/built-in.o
645   CC      spl/drivers/watchdog/omap_wdt.o
646   LD      spl/drivers/watchdog/built-in.o
647   LD      spl/drivers/built-in.o
648   LD      spl/dts/built-in.o
649   CC      spl/fs/ext4/ext4fs.o
650   CC      spl/fs/ext4/ext4_common.o
651   CC      spl/fs/ext4/dev.o
652   CC      spl/fs/ext4/ext4_write.o
653   CC      spl/fs/ext4/ext4_journal.o
654   CC      spl/fs/ext4/crc16.o
655   LD      spl/fs/ext4/built-in.o
656   CC      spl/fs/fat/fat_write.o
657   LD      spl/fs/fat/built-in.o
658   CC      spl/fs/fs_internal.o
659   LD      spl/fs/built-in.o
660   LDS     spl/u-boot-spl.lds
661   LD      spl/u-boot-spl
662   OBJCOPY spl/u-boot-spl-nodtb.bin
663   COPY    spl/u-boot-spl.bin
664   MKIMAGE MLO
665   MKIMAGE MLO.byteswap
666   CHK     include/config.h
667   CFG     u-boot.cfg
668   CFGCHK  u-boot.cfg

以spl/打头的都是spl相关的编译，在输出的后半部分，大家可以看到 475行和125 行编译的是同一个C文件，一个是u-boot，一个是u-boot-spl

SPL的其他配置项

由于现在u-boot同时使用Kconfig和老的头文件方式进行配置，下面很多配置在BBB中仍然是在头文件中配置的。

CONFIG_SPL_TEXT_BASE SPL的入口地址
CONFIG_SPL_LDSCRIPT SPL的链接脚本

除了板子特有的代码外，如果要使用u-boot通用的库或驱动，还需要定义 CONFIG_SPL_XXX_SUPPORT，当前支持如下配置

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_EXT_SUPPORT
CONFIG_SPL_LIBGENERIC_SUPPORT (lib/libgeneric.o)
CONFIG_SPL_POWER_SUPPORT (drivers/power/libpower.o)
CONFIG_SPL_NAND_SUPPORT (drivers/mtd/nand/libnand.o)
CONFIG_SPL_DRIVERS_MISC_SUPPORT (drivers/misc)
CONFIG_SPL_DMA_SUPPORT (drivers/dma/libdma.o)
CONFIG_SPL_POST_MEM_SUPPORT (post/drivers/memory.o)
CONFIG_SPL_NAND_LOAD (drivers/mtd/nand/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)

SPL 编译分析

u-boot在最近几年引入了Linux内核的Kconfig方式，主要介绍在源代码的 doc/README.kconfig 中，u-boot的目标是把所有配置都放到Kconfig中去，但是目前仍然有大量配置是写在头文件中的。在目前的情况下，C文件中可使用的配置文件是

include/generated/autoconf.h (generated by Kconfig for Normal)
include/configs/<target>.h (exists for all boards) 例如：bbb的target为 am335x_evm

Makefile中可使用的配置文件是

include/config/auto.conf (generated by Kconfig)
include/autoconf.mk (generated by the old config for Normal)
spl/include/autoconfig.mk (generated by the old config for SPL)
tpl/include/autoconfig.mk (generated by the old config for TPL) （注意：TPL跟SPL差不多，是第三级程序加载器，也是一个精简版的u-boot，很少用到，参见 doc/README.TPL）

板级相关的代码都在哪里

CONFIG_SYS_CPU="cpu" 将会编译 arch/<arch>/cpu/<cpu>
CONFIG_SYS_SOC="soc" 将会编译 arch/<arch>/cpu/<cpu>/<soc>
CONFIG_SYS_VENDOR="vendor" 将会编译 board/<vendor>/common/* 和 board/<vendor>/<board>/*
CONFIG_SYS_BOARD="board" 将会编译 board/<board>/* （或 board/<vendor>/<board>/* 如果 CONFIG_SYS_VENDOR 也定义了）
CONFIG_SYS_CONFIG_NAME="target" 将包含 include/configs/<target>.h
在 arch/<arch>/Kconfig 或 arch/<arch>/*/Kconfig 中应 source 板级相关的 Kconfig 文件来编译更多的文件

对于BBB开发板来说，可以查看 am335x_boneblack_defconfig：

<arch> = arm
<cpu> = armv7
<soc> = am33xx
<vendor> = ti
<board> = am335x
<target> = am335x_evm

对照上面，BBB板级相关的代码包括以下文件夹中

arch/arm/cpu/armv7
arch/arm/cpu/armv7/am33xx（无此文件夹）取而代之的是 arch/arm/mach-omap2/am33xx ，这是因为 arch/arm/mach-omap2/Kconfig 中 source 了 arch/arm/mach-omap2/am33xx/Kconfig 和 board/ti/am335x/Kconfig
board/ti/common
board/ti/am335x
配置的头文件为 includes/configs/am335x_evm.h

对照下上面实际编译输出，基本上差不多，除了多了一个 arch/arm/mach-omap2/* ，参见下文（machine-$(CONFIG_ARCH_OMAP2PLUS)）。

Makefile简述

u-boot根目录中的Makefile包含如下语句

ALL-$(CONFIG_SPL) += spl/u-boot-spl.bin

spl/u-boot-spl.bin: spl/u-boot-spl
	@:
spl/u-boot-spl: tools prepare 
		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb) 
		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_OF_EMBED)$(CONFIG_TPL_OF_PLATDATA),dts/dt.dtb)
	$(Q)$(MAKE) obj=spl -f $(srctree)/scripts/Makefile.spl all

而Makefile.spl首先包含了源代码树根目录的 config.mk，这个文件根据配置定义了ARCH、CPU、SOC、BOARD、VENDOR、CPUDIR、BOARDDIR等板级相关的变量，并包含了 ARCH、CPU、SOC、BOARD等板级相关的文件夹中的config.mk，以便添加或覆盖默认配置。配置确定后，Makefile.spl中定义了spl的各依赖，其中libs-y是spl需要编译的库（即子文件夹）

u-boot-spl-dirs    := $(patsubst %/,%,$(filter %/, $(libs-y)))

u-boot-spl-init := $(head-y)
# 注意：最后libs-y被转换为对应目录的 built-in.o，参见下文中对 Makefile.build 的分析
libs-y := $(patsubst %/, %/built-in.o, $(libs-y))
u-boot-spl-main := $(libs-y)

$(obj)/$(SPL_BIN): $(u-boot-spl-platdata) $(u-boot-spl-init) 
        $(u-boot-spl-main) $(obj)/u-boot-spl.lds FORCE
    $(call if_changed,u-boot-spl)

# May be overridden by arch/$(ARCH)/config.mk
# 这是 $(call if_changed,u-boot-spl) 实际执行的命令
quiet_cmd_u-boot-spl ?= LD      $@
      cmd_u-boot-spl ?= (cd $(obj) && $(LD) $(LDFLAGS) $(LDFLAGS_$(@F)) 
               $(patsubst $(obj)/%,%,$(u-boot-spl-init)) --start-group 
               $(patsubst $(obj)/%,%,$(u-boot-spl-main))  
               $(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) 
               --end-group 
               $(PLATFORM_LIBS) -Map $(SPL_BIN).map -o $(SPL_BIN))

其中 link script的查找方法是

ifeq ($(wildcard $(LDSCRIPT)),)
    LDSCRIPT := $(srctree)/board/$(BOARDDIR)/u-boot-spl.lds
endif
ifeq ($(wildcard $(LDSCRIPT)),)
    LDSCRIPT := $(srctree)/$(CPUDIR)/u-boot-spl.lds
endif
ifeq ($(wildcard $(LDSCRIPT)),)
    LDSCRIPT := $(srctree)/arch/$(ARCH)/cpu/u-boot-spl.lds
endif
ifeq ($(wildcard $(LDSCRIPT)),)
$(error could not find linker script)
endif

即，依次查找 BOARD、CPU、ARCH目录，第一个发现的u-boot-spl.lds文件。

大部分libs都是在Makefile.spl中直接定义的，如下

libs-y += $(if $(BOARDDIR),board/$(BOARDDIR)/)
libs-$(HAVE_VENDOR_COMMON_LIB) += board/$(VENDOR)/common/

libs-$(CONFIG_SPL_FRAMEWORK) += common/spl/
libs-y += common/init/

# Special handling for a few options which support SPL/TPL
ifeq ($(CONFIG_TPL_BUILD),y)
libs-$(CONFIG_TPL_LIBCOMMON_SUPPORT) += common/ cmd/ env/
libs-$(CONFIG_TPL_LIBGENERIC_SUPPORT) += lib/
else
libs-$(CONFIG_SPL_LIBCOMMON_SUPPORT) += common/ cmd/ env/
libs-$(CONFIG_SPL_LIBGENERIC_SUPPORT) += lib/
endif

libs-$(CONFIG_SPL_LIBDISK_SUPPORT) += disk/
libs-y += drivers/
libs-$(CONFIG_SPL_USB_GADGET_SUPPORT) += drivers/usb/dwc3/
libs-y += dts/
libs-y += fs/
libs-$(CONFIG_SPL_POST_MEM_SUPPORT) += post/drivers/
libs-$(CONFIG_SPL_NET_SUPPORT) += net/

但是有一部分是ARCH和CPU相关的是在 arch/arm/Makefile中定义的

# head-y 也是在这个文件中定义的
head-y := arch/arm/cpu/$(CPU)/start.o

# 注意：这就是为什么BBB会编译 mach-omap2 文件夹了
machine-$(CONFIG_ARCH_OMAP2PLUS)    += omap2

machdirs := $(patsubst %,arch/arm/mach-%/,$(machine-y))

libs-y += $(machdirs)

libs-y += arch/arm/cpu/$(CPU)/
libs-y += arch/arm/cpu/
libs-y += arch/arm/lib/

最后要说明下，这些lib-y的子文件夹是如何编译的

$(u-boot-spl-dirs): $(u-boot-spl-platdata)
    $(Q)$(MAKE) $(build)=$@

上面这句话非常简洁，但包含的内容却很多，注意是 $(build)=$@ 而不是 build=$@，build是kbuild.include中定义的一个变量，其值是

build := -f $(srctree)/scripts/Makefile.build obj

对u-boot-spl-dirs中的每一个子目录，展开了之后是

$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.build obj=目录

Makefile.build是Kbuild体系的核心文件，文件夹的编译都是通过这个文件来完成的，理解这个文件我认为需要知道几个要点：

这个文件包含了 Kbuild.include 和 Makefile.lib，这两个文件定义了很多实用函数、变量和通用的规则，一般位置找不到的东西都在这里
这个文件包含了 obj=目录 中的 Kbuild或Makefile文件，这样我们再写某个目录中的Kbuild或Makefile时就比较简单了，我们只需要定义
- obj-y：本目录中的源文件
- subdir-y：需要递归编译的子目录
- extra-y：额外要执行的编译目标
一个目录中的所有源文件及子目录的编译结果最终会LD成一个 built-in.o
subdir-y中的目录又会使用 Makefile.build 递归编译

# 至少有lib-y lib-m lib- 其中一个时才生成 lib.a
ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)
lib-target := $(obj)/lib.a
endif

# 至少有obj-y obj-m obj- subdir-m lib-y lib-m lib- 其中一个时才生成 built-in.o
ifneq ($(strip $(obj-y) $(obj-m) $(obj-) $(subdir-m) $(lib-target)),)
builtin-target := $(obj)/built-in.o
endif

modorder-target := $(obj)/modules.order

# We keep a list of all modules in $(MODVERDIR)

# KBUILD_BUILTIN 在顶层Makefile赋为了1，u-boot中不需要module，所以KBUILD_MODULES为0
__build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) 
	 $(if $(KBUILD_MODULES),$(obj-m) $(modorder-target)) 
	 $(subdir-ym) $(always)
	@:

以上这个部分我认为比较重要，未指定目标时，Makefile.build的默认目标是 __build，这个__build依赖

built-in.o
lib.a
extra-y
subdir-ym：这是子目录

SPL执行分析

（源代码根目录下 README 文件中有一节Board Initialisation Flow专门讲了启动流程）

SPL编译分析结束后，我们就知道了SPL由哪些源文件组成，分析起来要方便多了。首先当然是从head-y开始，即 arch/arm/cpu/armv7/start.S

save_boot_params：通过weak方式调用了 arch/arm/omach-omap2/lowlevel_init.S中的同名函数（存了r0到RAM）（暂时还没有搞清r0是啥）
关中断，进入SVC模式
cpu_init_cp15：Cache、MMU、TLB操作等（该清清，该禁禁，I-Cache一般是打开）
cpu_init_crit：调用 lowlevel_init，BBB板级未定义lowlevel_init，于是就调用了 arch/arm/cpu/armv7/lowlevel_init.S 中的 lowlevel_init函数，对SPL来说基本没干啥
_main：这个函数是重头戏，定义在 arch/arm/lib/crt0.S中，这个函数大概流程是这样的
- 初始化GD
- board_init_f
- 正常的u-boot此处会进行relocate，SPL不会，但是SPL可能会改变栈和GD的位置（参见 CONFIG_SPL_FRAMEWORK，common/spl/spl.c中的spl_relocate_stack_gd函数）
- 清零bss区
- board_init_r

GD（struct global_data）

为了简单起见，u-boot有一个全局的结构体数据，称为GD（指针为gd），很多数据都会丢到这个结构体里。

typedef struct global_data {
 //...
 struct arch_global_data arch;
 // ...
} gd_t;

这个结构体的定义在源代码目录的 include/asm-generic/global_data.h，这个文件应被arch的同名文件包含，以便可以定义不同的 struct arch_global_data 结构体，对于arm来说是 arch/arm/include/asm/global_data.h ，使用时采用 #include<asm/global_data.h>的方式就可以包含不同arch的同名文件了。

这么重要的结构体是在_main的一开始就被初始化为0了，代码在 common/init/board_init.c中的 board_init_f_alloc_reserve/board_init_f_init_reserve 两个函数里，通过在栈顶预留内存来达到给GD开辟空间的目的（因栈是向下增长的，所有不会和栈冲突）。

注：在arm架构中，gd的指针被赋给了r9寄存器，在需要使用gd地方，通过全局声明r9，使编译器不会使用r9寄存器作为临时变量，加快gd的访问速度。

https://gcc.gnu.org/onlinedocs/gcc/Global-Register-Variables.htm

在 arch/arm/config.mk 中

PLATFORM_RELFLAGS += -ffunction-sections -fdata-sections 
    -fno-common -ffixed-r9

注意： -ffixed-r9 就是上述链接中的 -ffixed-reg ，所有函数都不可以使用r9寄存器

board_init_f

GD初始化完成之后，就会调用 board_init_f，这个函数的目的是使board_init_r可以运行，主要是初始化DRAM和console串口，要注意下这个函数的运行环境：

GD指针可使用，因为GD已经初始化
栈是在SRAM中的
BSS还未清零，因为所有全局或静态变量都是不可以访问的，只能访问局部变量和GD

虽说这个函数以board打头，但是相似的板子可能会定义一个初始化的流程，在流程中大量使用虚函数和配置来抽象不同的板子。BBB的board_init_f是在 arch/arm/march-omap2/am33xx/board.c 中

#ifdef CONFIG_SPL_BUILD
void board_init_f(ulong dummy)
{
    hw_data_init();
    early_system_init();
    board_early_init_f();
    sdram_init();
    /* dram_init must store complete ramsize in gd->ram_size */
    gd->ram_size = get_ram_size(
            (void *)CONFIG_SYS_SDRAM_BASE,
            CONFIG_MAX_RAM_BANK_SIZE);
}
#endif

board_init_r

这个函数用来实现主要的代码逻辑，对于SPL来说，主要函数都是在 common/spl/spl.c 中实现了，具体大家直接看代码就是了

falcon

参见 doc/README.falcon，大概就是SPL=>u-boot=>Linux这个流程有点慢，falcon的功能就是不启动u-boot，直接启动Linux，相关的配置项是 CONFIG_SPL_OS_BOOT

这篇文章就写这么多了，把这些搞明白了，u-boot-spl就大体上理解了，更详细的还得大家自己回去读代码了，共勉！