u-boot 学习系列 1 - SPL
Contents
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/.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//cpu/
- CONFIG_SYS_SOC=“soc” 将会编译 arch//cpu//
- CONFIG_SYS_VENDOR=“vendor” 将会编译 board//common/* 和 board///*
- CONFIG_SYS_BOARD=“board” 将会编译 board//* （或 board///* 如果 CONFIG_SYS_VENDOR 也定义了）
- CONFIG_SYS_CONFIG_NAME=“target” 将包含 include/configs/.h
- 在 arch//Kconfig 或 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：
- = arm
- = armv7
- = am33xx
- = ti
- = am335x
- = 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！