[U-Boot] [PATCH] armv5te: make 'ret lr' produce iinterworking 'bx lr'

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Tom Rini <trini at konsulko.com> writes:

> On Mon, Feb 27, 2017 at 08:19:07PM +0100, Albert ARIBAUD wrote:
>> Current ARM assembler helper for the 'return to caller' pseudo-instruction
>> turns 'ret lr' into 'mov pc, lr' for ARMv5TE. This causes the core to remain
>> in its current ARM state even when the routine doing the 'ret' was called
>> from Thumb-1 state, triggering an undefined instruction exception.
>> 
>> This causes early run-time failures in all boards compiled using the Thumb-1
>> instruction set (for instance the Open-RD family).
>> 
>> ARMv5TE supports 'bx lr' which properly implements interworking and thus
>> correctly returns to Thumb-1 state from ARM state.
>> 
>> This change makes 'ret lr' turn into 'bx lr' for ARMv5TE.
>> 
>> Signed-off-by: Albert ARIBAUD <albert.u.boot at aribaud.net>
>> ---
>> Note: this patch supersedes patch "openrd: disable private arch memset,
>> memcpy and libgcc" dated Sun, 26 Feb 2017 16:29:32 +0100.
>> 
>>  arch/arm/include/asm/assembler.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
>> index ae1e42fc06..c56daf2a1f 100644
>> --- a/arch/arm/include/asm/assembler.h
>> +++ b/arch/arm/include/asm/assembler.h
>> @@ -59,7 +59,7 @@
>>  
>>  	.irp	c,,eq,ne,cs,cc,mi,pl,vs,vc,hi,ls,ge,lt,gt,le,hs,lo
>>  	.macro	ret\c, reg
>> -#if defined(__ARM_ARCH_5E__) || defined(__ARM_ARCH_5TE__)
>> +#if defined(__ARM_ARCH_5E__)
>>  	mov\c	pc, \reg
>>  #else
>>  	.ifeqs	"\reg", "lr"
>
> Emperical wins over theory, so I'll take this patch so we don't break
> things.  But looking at the kernel, the above is a test for
> __LINUX_ARM_ARCH__ < 6 instead.  But the kernel is hardly ever built and
> run in Thumb mode rather than ARM mode, so they wouldn't be tickling
> this particular issue.

The kernel needs Thumb-2, so the requirements are a bit different there.

> I guess my big question now is, when can / should we not just use 'bx
> lr' over 'mov pc, lr' ?

All cores with Thumb support the bx instruction.  The only time you want
to use "mov pc" is with non-Thumb v4 and v5 cores.

-- 
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