之前没有atf的时候，一般的bootflow是rom code ->uboot->kernel
有了atf后，其bootflow 改为rom code ->uboot->atf->kernel.加入没有secure os的话，这里的atf就仅仅之bl31.bin
在arm64的时候一般从rom code ->uboot的时候cpu处于el3,32bit模式，然后uboot中设置好warm reset的地址，这样在warm reset后就到atf中，将cpu且到64 bit后执行完bl31.bin 后，切到el1后返回到kernel中执行.
从bl31 中的bl31.ld.S 中SECTIONS定义可以知道，bl31的入口函数是在bl31_entrypoint
SECTIONS
{
. = BL31_BASE;
ASSERT(. == ALIGN(4096),
"BL31_BASE address is not aligned on a page boundary.")
#if SEPARATE_CODE_AND_RODATA
.text . : {
__TEXT_START__ = .;
*bl31_entrypoint.o(.text*)
*(.text*)
*(.vectors)
. = NEXT(4096);
__TEXT_END__ = .;
} >RAM
func bl31_entrypoint
#if !RESET_TO_BL31
mov x20, x0
mov x21, x1
el3_entrypoint_common \
_set_endian=0 \
_warm_boot_mailbox=0 \
_secondary_cold_boot=0 \
_init_memory=0 \
_init_c_runtime=1 \
_exception_vectors=runtime_exceptions
mov x0, x20
mov x1, x21
#else
el3_entrypoint_common \
_set_endian=1 \
_warm_boot_mailbox=!PROGRAMMABLE_RESET_ADDRESS \
_secondary_cold_boot=!COLD_BOOT_SINGLE_CPU \
_init_memory=1 \
_init_c_runtime=1 \
_exception_vectors=runtime_exceptions
mov x0, 0
mov x1, 0
#endif
//分别调用bl31_early_platform_setup 和 bl31_plat_arch_setup函数，完成后返回这里继续调用bl31_main
bl bl31_early_platform_setup
bl bl31_plat_arch_setup
bl bl31_main
adr x0, __DATA_START__
adr x1, __DATA_END__
sub x1, x1, x0
bl clean_dcache_range
adr x0, __BSS_START__
adr x1, __BSS_END__
sub x1, x1, x0
bl clean_dcache_range
//执行完bl31_main 后通过el3_exit 返回
b el3_exit
endfunc bl31_entrypoint
其中bl31_early_platform_setup和bl31_plat_arch_setup 都是对应的平台自己需要实现的函数。我们直接看公共的code
void bl31_main(void)
{
NOTICE("BL31: %s\n", version_string);
NOTICE("BL31: %s\n", build_message);
bl31_platform_setup();
bl31_lib_init();
INFO("BL31: Initializing runtime services\n");
runtime_svc_init();
//看起来bl32 是从这里跑起来的，执行完成后再回到这里继续执行bl31
if (bl32_init) {
INFO("BL31: Initializing BL32\n");
(*bl32_init)();
}
// 本例中这里就是准备kernel Image
bl31_prepare_next_image_entry();
console_flush();
//准备runtime service
bl31_plat_runtime_setup();
}