[U-Boot] Query on CONFIG_SYS_THUMB_BUILD

[Victor Ascroft <victorascroft@gmail.com>]

Hello,

I am working with a Cortex A5 Freescale Vybrid Processor. Since a thumb build leads to a saving of almost 1 MB for my u-boot image and consequently to faster serial downloads I have been looking at this. Currently enabling this option leads to a hang. 

After some debugging I have narrowed the place of hang to "ldr pc, =board_init_r" in arch/arm/lib/crt0.S. My debugging procedure was to put a branch to a small function which just printed a small message with puts, just before the ldr instruction and then a printing a small message with puts just at the start of board_init_r in common/board_r.c . For a non thumb build, the two messages get printed and I can boot to the u-boot prompt. For a thumb build, only the first message before the ldr instruction gets printed. 

In crt0.S
bl debug_print
ldr pc, =board_init_r

In board_init_r
puts("In board_init_r\n"); // Right at start

void debug_print(void)
{
    // Defined in board file
    puts("Debug print\n");
}

My assembly knowledge is limited and after some consultation with a senior colleague, he told me things to check.

An object dump of the crt0.o shows a branch to an even address. For thumb, this is expected to be odd. To just try out, I did a change as below
ldr r3, =board_init_r
add r3, #1
bx r3

No change with this. My expectation was the compiler/linker/assembler would take care of the requirements, with the CONFIG_SYS_THUMB_BUILD. Frankly speaking I am not sure if this is the complete issue or only a part of it. I have seen patches with regards to OMAP send in by Aneesh V, which made changes of the form .type fn_name, %function to all the low level assembly functions, but, I couldn't dig up much more or variants thereof. Basically, from what I understand, this takes care of specifying .thumb_func for a thumb function or so to speak.

Any pointers?

Thanks & Regards,
Sanchayan Maity.