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.b...@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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