我正在尝试按照 Chris Simmonds 的“掌握嵌入式 Linux 编程”中的说明为 Beagle Bone Black 构建 u-boot。我已经构建了交叉工具链，现在正在尝试使用该工具链构建 Das U-boot，但由于不支持 THUMB 指令（由编译器？由 BeagleBone？），构建失败。我正在使用 Denx 的 git 存储库 v2018.05 最新发布的版本。基于 Ubuntu 18.04 构建。
下面是控制台输出：
johann@mars:~/uboot-work/u-boot$ make CROSS_COMPILE=arm-cortex_a8-linux-gnueabihf- am335x_evm_defconfig
#
# configuration written to .config
#
johann@mars:~/uboot-work/u-boot$ make CROSS_COMPILE=arm-cortex_a8-linux-gnueabihf-
scripts/kconfig/conf --silentoldconfig Kconfig
CHK include/config.h
UPD include/config.h
CFG u-boot.cfg
GEN include/autoconf.mk
GEN include/autoconf.mk.dep
CFG spl/u-boot.cfg
GEN spl/include/autoconf.mk
CHK include/config/uboot.release
CHK include/generated/version_autogenerated.h
CHK include/generated/timestamp_autogenerated.h
UPD include/generated/timestamp_autogenerated.h
CC lib/asm-offsets.s
cc1: warning: target CPU does not support THUMB instructions
CHK include/generated/generic-asm-offsets.h
CC arch/arm/lib/asm-offsets.s
cc1: warning: target CPU does not support THUMB instructions
CHK include/generated/asm-offsets.h
HOSTCC scripts/dtc/dtc.o
HOSTCC scripts/dtc/flattree.o
HOSTCC scripts/dtc/fstree.o
HOSTCC scripts/dtc/data.o
HOSTCC scripts/dtc/livetree.o
HOSTCC scripts/dtc/treesource.o
HOSTCC scripts/dtc/srcpos.o
HOSTCC scripts/dtc/checks.o
HOSTCC scripts/dtc/util.o
SHIPPED scripts/dtc/dtc-lexer.lex.c
SHIPPED scripts/dtc/dtc-parser.tab.h
HOSTCC scripts/dtc/dtc-lexer.lex.o
SHIPPED scripts/dtc/dtc-parser.tab.c
HOSTCC scripts/dtc/dtc-parser.tab.o
HOSTLD scripts/dtc/dtc
HOSTCC tools/mkenvimage.o
HOSTCC tools/lib/crc32.o
HOSTLD tools/mkenvimage
HOSTCC tools/common/bootm.o
HOSTCC tools/lib/fdtdec.o
HOSTCC tools/fit_image.o
HOSTCC tools/image-host.o
HOSTCC tools/dumpimage.o
HOSTLD tools/dumpimage
HOSTCC tools/mkimage.o
HOSTLD tools/mkimage
CC arch/arm/cpu/armv7/cache_v7.o
cc1: warning: target CPU does not support THUMB instructions
{standard input}: Assembler messages:
{standard input}:42: Error: selected processor does not support `dsb sy' in ARM mode
make[1]: *** [arch/arm/cpu/armv7/cache_v7.o] Error 1
Makefile:1363: recipe for target 'arch/arm/cpu/armv7' failed
make: *** [arch/arm/cpu/armv7] Error 2
我也在看这本书，也遇到了同样的错误。我通过使用U-Boot的master分支解决了这个问题（日期：2019-03-19，操作系统：Ubuntu：18.04）。
但是，U-Boot的master分支没有am335x_boneblack_defconfig。我使用 am335x_evm_defconfig 代替。
尽管存在这些编译问题，这本书还是很棒的。
EDIT:
经过进一步调查，我发现根本原因是需要使用 ./ct-ng menuconfig 将 fpu 类型设置为“neon-vfpv3”。
正如书中指示将 float-abi 设置为 Hard 一样，fpu-type 也应该设置为 neon-vfpv3。否则，Linux 内核和 Barebox/U-boot 都不会被编译。
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