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

[Tom Rini]

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

-- 
Tom
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