[PATCH] perf tool: report user-friendly error from timechart

[Ramkumar Ramachandra <artagnon@gmail.com>]

Earlier, when 'timechart record' was invoked by a user without
permissions to debugfs:

$ perf timechart record
invalid or unsupported event: 'power:cpu_frequency'
Run 'perf list' for a list of valid events

 usage: perf record [<options>] [<command>]
    or: perf record [<options>] -- <command> [<options>]
[... detailed usage information ...]

This is highly user unfriendly, as the real problem is not reported to
the user correctly. Now, the user gets a much more friendly:

$ perf timechart record
Error:  No permissions to read $debugfs/tracing/events/power/cpu_frequency
Hint:   Change the permissions of debugfs: /sys/kernel/debug
        The directory will be present if your kernel was compiled with debugfs support.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 tools/perf/builtin-timechart.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/perf/builtin-timechart.c b/tools/perf/builtin-timechart.c
index c2e0231..ceaf9b8 100644
--- a/tools/perf/builtin-timechart.c
+++ b/tools/perf/builtin-timechart.c
@@ -1056,6 +1056,14 @@ static int __cmd_record(int argc, const char **argv)
 	}
 #endif
 
+	/* Perform a quick sanity check */
+	if (!is_valid_tracepoint("power:cpu_frequency")) {
+		fprintf(stderr, "Error:\tNo permissions to read $debugfs/tracing/events/power/cpu_frequency\n");
+		fprintf(stderr, "Hint:\tChange the permissions of debugfs: /sys/kernel/debug\n");
+		fprintf(stderr, "\tThe directory will be present if your kernel was compiled with debugfs support.\n");
+		return 1;
+	}
+
 	rec_argc = record_elems + argc - 1;
 	rec_argv = calloc(rec_argc + 1, sizeof(char *));
 
-- 
1.8.4.477.g5d89aa9