[PATCH v2] perf ftrace: Detect whether ftrace is enabled on system

[Changbin Du <changbin.du@huawei.com>]

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@huawei.com>

v2: rebase on perf-tools-next.
---
 tools/perf/builtin-ftrace.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index 88a87bf387d2..abcdc49b7a98 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -80,6 +80,24 @@ static bool check_ftrace_capable(void)
 	return false;
 }
 
+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;
+}
+
 static int __write_tracing_file(const char *name, const char *val, bool append)
 {
 	char *file;
@@ -1583,6 +1601,11 @@ int cmd_ftrace(int argc, const char **argv)
 	if (!check_ftrace_capable())
 		return -1;
 
+	if (!is_ftrace_supported()) {
+		pr_err("ftrace is not supported on this system\n");
+		return -ENOTSUP;
+	}
+
 	ret = perf_config(perf_ftrace_config, &ftrace);
 	if (ret < 0)
 		return -1;
-- 
2.34.1