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.
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