[PATCH 3.16 126/148] perf regs: Make perf_reg_name() return "unknown" instead of NULL

[Ben Hutchings]

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

commit 5b596e0ff0e1852197d4c82d3314db5e43126bf7 upstream.

To avoid breaking the build on arches where this is not wired up, at
least all the other features should be made available and when using
this specific routine, the "unknown" should point the user/developer to
the need to wire this up on this particular hardware architecture.

Detected in a container mipsel debian cross build environment, where it
shows up as:

  In file included from /usr/mipsel-linux-gnu/include/stdio.h:867,
                   from /git/linux/tools/perf/lib/include/perf/cpumap.h:6,
                   from util/session.c:13:
  In function 'printf',
      inlined from 'regs_dump__printf' at util/session.c:1103:3,
      inlined from 'regs__printf' at util/session.c:1131:2:
  /usr/mipsel-linux-gnu/include/bits/stdio2.h:107:10: error: '%-5s' directive argument is null [-Werror=format-overflow=]
    107 |   return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

cross compiler details:

  mipsel-linux-gnu-gcc (Debian 9.2.1-8) 9.2.1 20190909

Also on mips64:

  In file included from /usr/mips64-linux-gnuabi64/include/stdio.h:867,
                   from /git/linux/tools/perf/lib/include/perf/cpumap.h:6,
                   from util/session.c:13:
  In function 'printf',
      inlined from 'regs_dump__printf' at util/session.c:1103:3,
      inlined from 'regs__printf' at util/session.c:1131:2,
      inlined from 'regs_user__printf' at util/session.c:1139:3,
      inlined from 'dump_sample' at util/session.c:1246:3,
      inlined from 'machines__deliver_event' at util/session.c:1421:3:
  /usr/mips64-linux-gnuabi64/include/bits/stdio2.h:107:10: error: '%-5s' directive argument is null [-Werror=format-overflow=]
    107 |   return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  In function 'printf',
      inlined from 'regs_dump__printf' at util/session.c:1103:3,
      inlined from 'regs__printf' at util/session.c:1131:2,
      inlined from 'regs_intr__printf' at util/session.c:1147:3,
      inlined from 'dump_sample' at util/session.c:1249:3,
      inlined from 'machines__deliver_event' at util/session.c:1421:3:
  /usr/mips64-linux-gnuabi64/include/bits/stdio2.h:107:10: error: '%-5s' directive argument is null [-Werror=format-overflow=]
    107 |   return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

cross compiler details:

  mips64-linux-gnuabi64-gcc (Debian 9.2.1-8) 9.2.1 20190909

Fixes: 2bcd355b71da ("perf tools: Add interface to arch registers sets")
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Link: https://lkml.kernel.org/n/tip-95wjyv4o65nuaeweq31t7l1s@xxxxxxxxxxxxxx
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 tools/perf/util/perf_regs.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/tools/perf/util/perf_regs.h
+++ b/tools/perf/util/perf_regs.h
@@ -16,7 +16,7 @@ int perf_reg_value(u64 *valp, struct reg
 
 static inline const char *perf_reg_name(int id __maybe_unused)
 {
-	return NULL;
+	return "unknown";
 }
 
 static inline int perf_reg_value(u64 *valp __maybe_unused,