[PATCH] perf ftrace: Detect whether ftrace is enabled on system

[Changbin Du]

To make error messages more accurate, this change detects whether ftrace is
enabled on system by checking trace file "set_ftrace_pid".

Before:
~ # perf ftrace
failed to reset ftrace

After:
~ # perf ftrace
ftrace is not supported on this system

Signed-off-by: Changbin Du <changbin.du@xxxxxxxxxx>
---
 tools/perf/builtin-ftrace.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index eb30c8eca488..e1dc2768bc0f 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -1112,6 +1112,24 @@ static int parse_graph_tracer_opts(const struct option *opt,
 	return 0;
 }
 
+static bool is_ftrace_supported(void)
+{
+	char *file;
+	bool supported = false;
+
+	file = get_tracing_file("set_ftrace_pid");
+	if (!file) {
+		pr_debug("cannot get tracing file set_ftrace_pid\n");
+		return false;
+	}
+
+	if (!access(file, F_OK))
+		supported = true;
+
+	put_tracing_file(file);
+	return supported;
+}
+
 enum perf_ftrace_subcommand {
 	PERF_FTRACE_NONE,
 	PERF_FTRACE_TRACE,
@@ -1254,6 +1272,12 @@ int cmd_ftrace(int argc, const char **argv)
 		goto out_delete_filters;
 	}
 
+	if (!is_ftrace_supported()) {
+		pr_err("ftrace is not supported on this system\n");
+		ret = -ENOTSUP;
+		goto out_delete_filters;
+	}
+
 	ret = target__validate(&ftrace.target);
 	if (ret) {
 		char errbuf[512];
-- 
2.34.1