[PATCH] cpufreq: cppc: discard out-of-range delivered_perf samples

[Breno Leitao]

cppc_cpufreq_get_rate() derives delivered_perf as:

    delivered_perf = reference_perf * delta_delivered / delta_reference

over a short udelay()-bounded window between two cppc_get_perf_ctrs()
calls. Per-read latency jitter on the underlying CPC register access
can skew the ratio, occasionally producing delivered_perf >
highest_perf. cppc_perf_to_khz() then linearly extrapolates above
(nominal_perf, nominal_freq), so the value reported via
/sys/.../cpufreq/cpuinfo_cur_freq exceeds cpuinfo_max_freq.

Observed on an arm64 host (governor=performance,
cpuinfo_max_freq=3339 MHz): 15 back-to-back reads returned values
between 2997 and 4230 MHz.

Treat an out-of-range sample as invalid and reuse the existing
out_invalid_counters fallback, which returns the platform's
desired_perf. This keeps cpuinfo_cur_freq within
[0, cpuinfo_max_freq] without reporting a value the hardware did
not deliver.

Signed-off-by: Breno Leitao <leitao at debian.org>
---
 drivers/cpufreq/cppc_cpufreq.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index 7e7f9dfb7a24c..dd92aa2bca464 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -814,6 +814,14 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
 	if (!delivered_perf)
 		goto out_invalid_counters;
 
+	/*
+	 * Sampling jitter on the CPC counter pair can produce
+	 * delivered_perf > highest_perf, which cppc_perf_to_khz() would
+	 * extrapolate to a frequency above cpuinfo_max_freq. Discard.
+	 */
+	if (delivered_perf > cpu_data->perf_caps.highest_perf)
+		goto out_invalid_counters;
+
 	return cppc_perf_to_khz(&cpu_data->perf_caps, delivered_perf);
 
 out_invalid_counters:

---
base-commit: 26fd6bff2c050196005312d1d306889220952a99
change-id: 20260501-cur_freq-fix-d569cf1d1052

Best regards,
--  
Breno Leitao <leitao at debian.org>