[PATCH] powerpc/32: Use r2 in wrtspr() instead of r0

[PATCH] powerpc/32: Use r2 in wrtspr() instead of r0

[Christophe Leroy]

wrtspr() is a function to write an arbitrary value in a special
register. It is used on 8xx to write to SPRN_NRI, SPRN_EID and
SPRN_EIE. Writing any value to one of those will play with MSR EE
and MSR RI regardless of that value.

r0 is used many places in the generated code and using r0 for
that creates an unnecessary dependency of this instruction with
preceding ones using r0 in a few places in vmlinux.

r2 is most likely the most stable register as it contains the
pointer to 'current'.

Using r2 instead of r0 avoids that unnecessary dependency.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/include/asm/reg.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
index e40a921d78f9..32a9020e93ab 100644
--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -1392,8 +1392,7 @@ static inline void mtmsr_isync(unsigned long val)
 				     : "r" ((unsigned long)(v)) \
 				     : "memory")
 #endif
-#define wrtspr(rn)	asm volatile("mtspr " __stringify(rn) ",0" : \
-				     : : "memory")
+#define wrtspr(rn)	asm volatile("mtspr " __stringify(rn) ",2" : : : "memory")
 
 static inline void wrtee(unsigned long val)
 {
-- 
2.25.0

Re: [PATCH] powerpc/32: Use r2 in wrtspr() instead of r0

[Michael Ellerman]

On Fri, 22 Jan 2021 07:15:03 +0000 (UTC), Christophe Leroy wrote:
> wrtspr() is a function to write an arbitrary value in a special
> register. It is used on 8xx to write to SPRN_NRI, SPRN_EID and
> SPRN_EIE. Writing any value to one of those will play with MSR EE
> and MSR RI regardless of that value.
> 
> r0 is used many places in the generated code and using r0 for
> that creates an unnecessary dependency of this instruction with
> preceding ones using r0 in a few places in vmlinux.
> 
> [...]

Applied to powerpc/next.

[1/1] powerpc/32: Use r2 in wrtspr() instead of r0
      https://git.kernel.org/powerpc/c/867e762480f4ad4106b16299a373fa23eccf5b4b

cheers