[PATCH] powerpc/4xx: Fix build errors from mfdcr()

lkp reported a build error in fsp2.o:

  CC      arch/powerpc/platforms/44x/fsp2.o
  {standard input}:577: Error: unsupported relocation against base

Which comes from:

  pr_err("GESR0: 0x%08x\n", mfdcr(base + PLB4OPB_GESR0));

Where our mfdcr() macro is stringifying "base + PLB4OPB_GESR0", and
passing that to the assembler, which obviously doesn't work.

The mfdcr() macro already checks that the argument is constant using
__builtin_constant_p(), and if not calls the out-of-line version of
mfdcr(). But in this case GCC is smart enough to notice that "base +
PLB4OPB_GESR0" will be constant, even though it's not something we can
immediately stringify into a register number.

Segher pointed out that passing the register number to the inline asm
as a constant would be better, and in fact it fixes the build error,
presumably because it gives GCC a chance to resolve the value.

While we're at it, change mtdcr() similarly.

Reported-by: kernel test robot <redacted>
Suggested-by: Segher Boessenkool <redacted>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/include/asm/dcr-native.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/include/asm/dcr-native.h b/arch/powerpc/include/asm/dcr-native.h
index 7141ccea8c94..a92059964579 100644
--- a/arch/powerpc/include/asm/dcr-native.h
+++ b/arch/powerpc/include/asm/dcr-native.h

@@ -53,8 +53,8 @@ static inline void mtdcrx(unsigned int reg, unsigned int val)
 #define mfdcr(rn)						\
 	({unsigned int rval;					\
 	if (__builtin_constant_p(rn) && rn < 1024)		\
-		asm volatile("mfdcr %0," __stringify(rn)	\
-		              : "=r" (rval));			\
+		asm volatile("mfdcr %0, %1" : "=r" (rval)	\
+			      : "n" (rn));			\
 	else if (likely(cpu_has_feature(CPU_FTR_INDEXED_DCR)))	\
 		rval = mfdcrx(rn);				\
 	else							\

@@ -64,8 +64,8 @@ static inline void mtdcrx(unsigned int reg, unsigned int val)
 #define mtdcr(rn, v)						\
 do {								\
 	if (__builtin_constant_p(rn) && rn < 1024)		\
-		asm volatile("mtdcr " __stringify(rn) ",%0"	\
-			      : : "r" (v)); 			\
+		asm volatile("mtdcr %0, %1"			\
+			      : : "n" (rn), "r" (v));		\
 	else if (likely(cpu_has_feature(CPU_FTR_INDEXED_DCR)))	\
 		mtdcrx(rn, v);					\
 	else							\

-- 
2.25.1

On Thu, Feb 18, 2021 at 11:30:58PM +1100, Michael Ellerman wrote:

lkp reported a build error in fsp2.o:

  CC      arch/powerpc/platforms/44x/fsp2.o
  {standard input}:577: Error: unsupported relocation against base

Which comes from:

  pr_err("GESR0: 0x%08x\n", mfdcr(base + PLB4OPB_GESR0));

Where our mfdcr() macro is stringifying "base + PLB4OPB_GESR0", and
passing that to the assembler, which obviously doesn't work.

The mfdcr() macro already checks that the argument is constant using
__builtin_constant_p(), and if not calls the out-of-line version of
mfdcr(). But in this case GCC is smart enough to notice that "base +
PLB4OPB_GESR0" will be constant, even though it's not something we can
immediately stringify into a register number.

Segher pointed out that passing the register number to the inline asm
as a constant would be better, and in fact it fixes the build error,
presumably because it gives GCC a chance to resolve the value.

While we're at it, change mtdcr() similarly.

Reported-by: kernel test robot <redacted>
Suggested-by: Segher Boessenkool <redacted>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Acked-by: Feng Tang <redacted>

Thanks!

On Thu, 18 Feb 2021 23:30:58 +1100, Michael Ellerman wrote:

lkp reported a build error in fsp2.o:

  CC      arch/powerpc/platforms/44x/fsp2.o
  {standard input}:577: Error: unsupported relocation against base

Which comes from:

[...]

Applied to powerpc/fixes.

[1/1] powerpc/4xx: Fix build errors from mfdcr()
      https://git.kernel.org/powerpc/c/eead089311f4d935ab5d1d8fbb0c42ad44699ada

cheers