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From:   Yang Shi <yang@os.amperecomputing.com>
To:     rafael@kernel.org, viresh.kumar@linaro.org
Cc:     yang@os.amperecomputing.com, scott@os.amperecomputing.com,
        linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] cpufreq: CPPC: use 10ms delay instead of 2us to avoid high error
Date:   Tue, 28 Mar 2023 12:38:46 -0700
Message-Id: <20230328193846.8757-1-yang@os.amperecomputing.com>
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When testing CPPC cpufreq on our platform, we noticed the error may be quite
high and the high error may happen quite often.  For example, on a platform
with a maximum frequency of 2.8GHz when the CPUs were fully loaded (100% load),
we saw cpuinfo_cur_freq may show 4GHz, it means the error is > 40%.  And the
high error (> 1%) happened 256 times out of 2127 samples (sampled every 3
seconds) in an approximate 2hrs test.

We tried to enlarge the delay, and tested with 100us, 1ms and 10ms.  The
below is the results.

100us:
The highest error is 4GHz, 22 times out of 3623 samples

1ms:
The highest error is 3.3GHz, 3 times out of 2814 samples

10ms:
No high error anymore

Increase the measurement delay in cppc_cpufreq_get_rate to 10ms to avoid
high measurement errors.

Signed-off-by: Yang Shi <yang@os.amperecomputing.com>
---
 drivers/cpufreq/cppc_cpufreq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index 022e3555407c..c2bf65448d3d 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -851,7 +851,7 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
 	if (ret)
 		return ret;
 
-	udelay(2); /* 2usec delay between sampling */
+	mdelay(10); /* 10msec delay between sampling */
 
 	ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1);
 	if (ret)
-- 
2.39.2