libperf: Add debug output support

Add the perf_set_print() function to allow setting an output function
for warn/info/debug messages.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190721112506.12306-28-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/lib/include/perf/core.h

@@ -2,8 +2,21 @@
 #ifndef __LIBPERF_CORE_H
 #define __LIBPERF_CORE_H
 
+#include <stdarg.h>
+
 #ifndef LIBPERF_API
 #define LIBPERF_API __attribute__((visibility("default")))
 #endif
 
+enum libperf_print_level {
+	LIBPERF_WARN,
+	LIBPERF_INFO,
+	LIBPERF_DEBUG,
+};
+
+typedef int (*libperf_print_fn_t)(enum libperf_print_level level,
+				  const char *, va_list ap);
+
+LIBPERF_API void libperf_set_print(libperf_print_fn_t fn);
+
 #endif /* __LIBPERF_CORE_H */