linux-kernel - [PATCH 1/2] cpufreq: Add driver flag to avoid initial frequency verification

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Message-ID: <20250823001937.2765316-3-pmalani@google.com>
Date: Sat, 23 Aug 2025 00:17:48 +0000
From: Prashant Malani <pmalani@...gle.com>
To: open list <linux-kernel@...r.kernel.org>, 
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Viresh Kumar <viresh.kumar@...aro.org>
Cc: Beata Michalska <beata.michalska@....com>, Prashant Malani <pmalani@...gle.com>
Subject: [PATCH 1/2] cpufreq: Add driver flag to avoid initial frequency verification

Some cpufreq drivers have a get() function which can return an unreliable
frequency. This can cause issues when switching governors. For instance,
a CPU would be on performance governor and have it's frequency (and
policy->cur) set to max. When the governor is switched to userspace, the
policy->cur is re-used, but it is checked against the frequency returned
by the driver's get() function. If it's different, the frequency will
get set to the new (incorrect) value.

To avoid this, add a flag that avoids this verify step on governor start
if the cpufreq driver opts in to it.

Since there are no users of this flag, no functional changes are
introduced here.

Cc: Beata Michalska <beata.michalska@....com>
Signed-off-by: Prashant Malani <pmalani@...gle.com>
---
 drivers/cpufreq/cpufreq.c |  3 ++-
 include/linux/cpufreq.h   | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index b8937737d096..72e6552a40ea 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -2482,7 +2482,8 @@ int cpufreq_start_governor(struct cpufreq_policy *policy)
 
 	pr_debug("%s: for CPU %u\n", __func__, policy->cpu);
 
-	cpufreq_verify_current_freq(policy, false);
+	if (!(cpufreq_driver->flags & CPUFREQ_DONT_VERIFY_FREQ_ON_GOVERNOR_START))
+		cpufreq_verify_current_freq(policy, false);
 
 	if (policy->governor->start) {
 		ret = policy->governor->start(policy);
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 95f3807c8c55..1ebc12fcc905 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -474,6 +474,16 @@ struct cpufreq_driver {
  */
 #define CPUFREQ_NO_AUTO_DYNAMIC_SWITCHING	BIT(6)
 
+/*
+ * Set by drivers which want cpufreq core to avoid verifying that the current
+ * frequency of the policy matches the frequency returned by the driver's get()
+ * function. The get() function on certain drivers returns unreliable values,
+ * and this can result in the frequency (and consequently system performance)
+ * being reduced even though the governor didn't want the frequencies to be
+ * reduced.
+ */
+#define CPUFREQ_DONT_VERIFY_FREQ_ON_GOVERNOR_START	BIT(7)
+
 int cpufreq_register_driver(struct cpufreq_driver *driver_data);
 void cpufreq_unregister_driver(struct cpufreq_driver *driver_data);
 
-- 
2.51.0.rc2.233.g662b1ed5c5-goog