The usage of Low level debug

printhex8和printascii用法示例：

__error_p:
#ifdef CONFIG_DEBUG_LL
        adr r0, str_p1
        bl printascii
        mov r0, r9
        bl printhex8
        adr r0, str_p2
        bl printascii
        b __error
str_p1: .asciz "\nError: unrecognized/unsupported processor variant (0x"
str_p2: .asciz ").\n"
        .align
#endif
ENDPROC(__error_p)


CONFIG_DEBUG_LL的实现：
arch/arm/kernel/head.S

#ifdef CONFIG_DEBUG_LL
        ldr r7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflags
        /*
         * Map in IO space for serial debugging.
         * This allows debug messages to be output
         * via a serial console before paging_init.
         */
        ldr r3, [r8, #MACHINFO_PGOFFIO]
        add r0, r4, r3
        rsb r3, r3, #0x4000 @ PTRS_PER_PGD*sizeof(long)
        cmp r3, #0x0800 @ limit to 512MB
        movhi r3, #0x0800
        add r6, r0, r3
        ldr r3, [r8, #MACHINFO_PHYSIO]
        mov r3, r3, lsr #20 @ 1MB-aligned address
        mov r3, r3, lsl #20
        orr r3, r3, r7
1: str r3, [r0], #4
        add r3, r3, #1 << 20
        teq r0, r6
        bne 1b
#endif