[PATCH v3 12/13] perf tools: improve error handling of AMD Branch Sampling

[Stephane Eranian]

This patch improves the error message printed by perf when perf_event_open()
fails on AMD Zen3 when using the branch sampling feature. In the case of
EINVAL, there are two main reasons: frequency mode or period is smaller than
the depth of the branch sampling buffer (16). The patch checks the parameters
of the call and tries to print a relevant message to explain the error:

$ perf record -b -e cpu/branch-brs/ -c 10 ls Error: AMD Branch Sampling does
not support sampling period smaller than what is reported in
/sys/devices/cpu/caps/branches.

$ perf record -b -e cpu/branch-brs/ ls Error: AMD Branch Sampling does not
support frequency mode sampling, must pass a fixed sampling period via -c
option or cpu/branch-brs,period=xxxx/.

Improved by Kim Phillips <kim.phillips@xxxxxxx>
Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
---
 tools/perf/util/evsel.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 39e9063c0a80..af0bedbffff5 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2890,6 +2890,12 @@ static void detect_amd(void)
 	fclose(inf);
 }
 
+static bool is_amd_brs(struct evsel *evsel)
+{
+	return ((evsel->core.attr.config & 0xff) == 0xc4) &&
+	       (evsel->core.attr.sample_type & PERF_SAMPLE_BRANCH_STACK);
+}
+
 int evsel__open_strerror(struct evsel *evsel, struct target *target,
 			 int err, char *msg, size_t size)
 {
@@ -2992,6 +2998,14 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
 			return scnprintf(msg, size, "The 'aux_output' feature is not supported, update the kernel.");
 		if (is_amd && (evsel->core.attr.precise_ip || !strncmp(evsel->pmu_name, "ibs", 3)) && (evsel->core.attr.exclude_kernel))
 			return scnprintf(msg, size, "AMD IBS can't exclude kernel events.  Try running at a higher privilege level.");
+
+		if (is_amd && is_amd_brs(evsel)) {
+			if (evsel->core.attr.freq)
+				return scnprintf(msg, size, "AMD Branch Sampling does not support frequency mode sampling, must pass a fixed sampling period via -c option or cpu/branch-brs,period=xxxx/.");
+			/* another reason is that the period is too small */
+			return scnprintf(msg, size, "AMD Branch Sampling does not support sampling period smaller than what is reported in /sys/devices/cpu/caps/branches.");
+		}
+
 		break;
 	case ENODATA:
 		return scnprintf(msg, size, "Cannot collect data source with the load latency event alone. "
-- 
2.34.0.rc2.393.gf8c9666880-goog