On Thu, Mar 02, 2017 at 02:27:03PM +0000, Måns Rullgård wrote: > Tom Rini <tr...@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.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. > > The kernel needs Thumb-2, so the requirements are a bit different there.
Ah, right. > > 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. So, of ARM720T, ARM920T, ARM926EJS and ARM946ES, which fall into that category? Or is it harder to say than that? -- Tom
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