linux-kernel - [PATCH v1 2/4] cpufreq: CPPC: Avoid using CPUFREQ

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Message-ID: <22867781.EfDdHjke4D@rafael.j.wysocki>
Date: Thu, 25 Sep 2025 17:44:47 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Linux PM <linux-pm@...r.kernel.org>
Cc: Shawn Guo <shawnguo@...nel.org>, Qais Yousef <qyousef@...alina.io>,
 LKML <linux-kernel@...r.kernel.org>, Viresh Kumar <viresh.kumar@...aro.org>,
 Prashanth Prakash <pprakash@...eaurora.org>,
 Pierre Gondois <pierre.gondois@....com>,
 Mario Limonciello <mario.limonciello@....com>,
 Linux ACPI <linux-acpi@...r.kernel.org>
Subject:
 [PATCH v1 2/4] cpufreq: CPPC: Avoid using CPUFREQ_ETERNAL as transition delay

From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

If cppc_get_transition_latency() returns CPUFREQ_ETERNAL to indicate a
failure to retrieve the transition latency value from the platform
firmware, the CPPC cpufreq driver will use that value (converted to
microseconds) as the policy transition delay, but it is way too large
for any practical use.

Address this by making the driver use the cpufreq's default
transition latency value (in microseconds) as the transition delay
if CPUFREQ_ETERNAL is returned by cppc_get_transition_latency().

Fixes: d4f3388afd48 ("cpufreq / CPPC: Set platform specific transition_delay_us")
Cc: 5.19+ <stable@...r.kernel.org> # 5.19
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
 drivers/cpufreq/cppc_cpufreq.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -308,6 +308,16 @@ static int cppc_verify_policy(struct cpu
 	return 0;
 }

+static unsigned int get_transition_latency_from_cppc(unsigned int cpu)
+{
+	unsigned int transition_latency_ns = cppc_get_transition_latency(cpu);
+
+	if (transition_latency_ns == CPUFREQ_ETERNAL)
+		return CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS / NSEC_PER_USEC;
+
+	return transition_latency_ns / NSEC_PER_USEC;
+}
+
 /*
  * The PCC subspace describes the rate at which platform can accept commands
  * on the shared PCC channel (including READs which do not count towards freq
@@ -330,12 +340,12 @@ static unsigned int cppc_cpufreq_get_tra
 			return 10000;
 		}
 	}
-	return cppc_get_transition_latency(cpu) / NSEC_PER_USEC;
+	return get_transition_latency_from_cppc(cpu);
 }
 #else
 static unsigned int cppc_cpufreq_get_transition_delay_us(unsigned int cpu)
 {
-	return cppc_get_transition_latency(cpu) / NSEC_PER_USEC;
+	return get_transition_latency_from_cppc(cpu);
 }
 #endif