linux-kernel - [PATCH] cpufreq: Honour transition_latency over transition_delay

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Message-Id: <20240227233452.405852-1-qyousef@layalina.io>
Date: Tue, 27 Feb 2024 23:34:52 +0000
From: Qais Yousef <qyousef@...alina.io>
To: "Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>
Cc: linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	"Pierre Gondois" <Pierre.Gondois@....com>,
	Qais Yousef <qyousef@...alina.io>,
	Pierre Gondois <pierre.gondois@....com>
Subject: [PATCH] cpufreq: Honour transition_latency over transition_delay_us

Some platforms like Arm's Juno can have a high transition latency that
can be larger than the 2ms cap introduced. If a driver report
a transition_latency that is higher than the cap, then use it as-is.

Update comment s/10/2/ to reflect the new cap of 2ms.

Reported-by: Pierre Gondois <pierre.gondois@....com>
Signed-off-by: Qais Yousef <qyousef@...alina.io>
---
 drivers/cpufreq/cpufreq.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 66cef33c4ec7..926a51cb7e52 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -576,8 +576,17 @@ unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy)
 
 	latency = policy->cpuinfo.transition_latency / NSEC_PER_USEC;
 	if (latency) {
+		unsigned int max_delay_us = 2 * MSEC_PER_SEC;
+
+		/*
+		 * If the platform already has high transition_latency, use it
+		 * as-is.
+		 */
+		if (latency > max_delay_us)
+			return latency;
+
 		/*
-		 * For platforms that can change the frequency very fast (< 10
+		 * For platforms that can change the frequency very fast (< 2
 		 * us), the above formula gives a decent transition delay. But
 		 * for platforms where transition_latency is in milliseconds, it
 		 * ends up giving unrealistic values.
@@ -586,7 +595,7 @@ unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy)
 		 * a reasonable amount of time after which we should reevaluate
 		 * the frequency.
 		 */
-		return min(latency * LATENCY_MULTIPLIER, (unsigned int)(2 * MSEC_PER_SEC));
+		return min(latency * LATENCY_MULTIPLIER, max_delay_us);
 	}
 
 	return LATENCY_MULTIPLIER;
-- 
2.34.1