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Message-ID: <2264949.irdbgypaU6@rafael.j.wysocki>
Date: Fri, 26 Sep 2025 12:12:37 +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>,
Pierre Gondois <pierre.gondois@....com>,
Mario Limonciello <mario.limonciello@....com>,
Linux ACPI <linux-acpi@...r.kernel.org>, Jie Zhan <zhanjie9@...ilicon.com>,
rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
Alex Gaynor <alex.gaynor@...il.com>
Subject:
[PATCH v3 1/4] cpufreq: Make drivers using CPUFREQ_ETERNAL specify transition
latency
From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Commit a755d0e2d41b ("cpufreq: Honour transition_latency over
transition_delay_us") caused platforms where cpuinfo.transition_latency
is CPUFREQ_ETERNAL to get a very large transition latency whereas
previously it had been capped at 10 ms (and later at 2 ms).
This led to a user-observable regression between 6.6 and 6.12 as
described by Shawn:
"The dbs sampling_rate was 10000 us on 6.6 and suddently becomes
6442450 us (4294967295 / 1000 * 1.5) on 6.12 for these platforms
because the default transition delay was dropped [...].
It slows down dbs governor's reacting to CPU loading change
dramatically. Also, as transition_delay_us is used by schedutil
governor as rate_limit_us, it shows a negative impact on device
idle power consumption, because the device gets slightly less time
in the lowest OPP."
Evidently, the expectation of the drivers using CPUFREQ_ETERNAL as
cpuinfo.transition_latency was that it would be capped by the core,
but they may as well return a default transition latency value instead
of CPUFREQ_ETERNAL and the core need not do anything with it.
Accordingly, introduce CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS and make
all of the drivers in question use it instead of CPUFREQ_ETERNAL. Also
update the related Rust binding.
Fixes: a755d0e2d41b ("cpufreq: Honour transition_latency over transition_delay_us")
Closes: https://lore.kernel.org/linux-pm/20250922125929.453444-1-shawnguo2@yeah.net/
Reported-by: Shawn Guo <shawnguo@...nel.org>
Reviewed-by: Mario Limonciello (AMD) <superm1@...nel.org>
Reviewed-by: Jie Zhan <zhanjie9@...ilicon.com>
Cc: 6.6+ <stable@...r.kernel.org> # 6.6+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
v1 -> v3:
* Add updates of the Rust version of cpufreq-dt and Rust binding
* Update the changelog
* Add tags from Mario Limonciello and Jie Zhan
---
drivers/cpufreq/cpufreq-dt.c | 2 +-
drivers/cpufreq/imx6q-cpufreq.c | 2 +-
drivers/cpufreq/mediatek-cpufreq-hw.c | 2 +-
drivers/cpufreq/rcpufreq_dt.rs | 2 +-
drivers/cpufreq/scmi-cpufreq.c | 2 +-
drivers/cpufreq/scpi-cpufreq.c | 2 +-
drivers/cpufreq/spear-cpufreq.c | 2 +-
include/linux/cpufreq.h | 3 +++
rust/kernel/cpufreq.rs | 7 ++++---
9 files changed, 14 insertions(+), 10 deletions(-)
--- a/drivers/cpufreq/cpufreq-dt.c
+++ b/drivers/cpufreq/cpufreq-dt.c
@@ -104,7 +104,7 @@ static int cpufreq_init(struct cpufreq_p
transition_latency = dev_pm_opp_get_max_transition_latency(cpu_dev);
if (!transition_latency)
- transition_latency = CPUFREQ_ETERNAL;
+ transition_latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
cpumask_copy(policy->cpus, priv->cpus);
policy->driver_data = priv;
--- a/drivers/cpufreq/imx6q-cpufreq.c
+++ b/drivers/cpufreq/imx6q-cpufreq.c
@@ -442,7 +442,7 @@ soc_opp_out:
}
if (of_property_read_u32(np, "clock-latency", &transition_latency))
- transition_latency = CPUFREQ_ETERNAL;
+ transition_latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
/*
* Calculate the ramp time for max voltage change in the
--- a/drivers/cpufreq/mediatek-cpufreq-hw.c
+++ b/drivers/cpufreq/mediatek-cpufreq-hw.c
@@ -309,7 +309,7 @@ static int mtk_cpufreq_hw_cpu_init(struc
latency = readl_relaxed(data->reg_bases[REG_FREQ_LATENCY]) * 1000;
if (!latency)
- latency = CPUFREQ_ETERNAL;
+ latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
policy->cpuinfo.transition_latency = latency;
policy->fast_switch_possible = true;
--- a/drivers/cpufreq/rcpufreq_dt.rs
+++ b/drivers/cpufreq/rcpufreq_dt.rs
@@ -123,7 +123,7 @@ impl cpufreq::Driver for CPUFreqDTDriver
let mut transition_latency = opp_table.max_transition_latency_ns() as u32;
if transition_latency == 0 {
- transition_latency = cpufreq::ETERNAL_LATENCY_NS;
+ transition_latency = cpufreq::DEFAULT_TRANSITION_LATENCY_NS;
}
policy
--- a/drivers/cpufreq/scmi-cpufreq.c
+++ b/drivers/cpufreq/scmi-cpufreq.c
@@ -294,7 +294,7 @@ static int scmi_cpufreq_init(struct cpuf
latency = perf_ops->transition_latency_get(ph, domain);
if (!latency)
- latency = CPUFREQ_ETERNAL;
+ latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
policy->cpuinfo.transition_latency = latency;
--- a/drivers/cpufreq/scpi-cpufreq.c
+++ b/drivers/cpufreq/scpi-cpufreq.c
@@ -157,7 +157,7 @@ static int scpi_cpufreq_init(struct cpuf
latency = scpi_ops->get_transition_latency(cpu_dev);
if (!latency)
- latency = CPUFREQ_ETERNAL;
+ latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
policy->cpuinfo.transition_latency = latency;
--- a/drivers/cpufreq/spear-cpufreq.c
+++ b/drivers/cpufreq/spear-cpufreq.c
@@ -182,7 +182,7 @@ static int spear_cpufreq_probe(struct pl
if (of_property_read_u32(np, "clock-latency",
&spear_cpufreq.transition_latency))
- spear_cpufreq.transition_latency = CPUFREQ_ETERNAL;
+ spear_cpufreq.transition_latency = CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS;
cnt = of_property_count_u32_elems(np, "cpufreq_tbl");
if (cnt <= 0) {
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -32,6 +32,9 @@
*/
#define CPUFREQ_ETERNAL (-1)
+
+#define CPUFREQ_DEFAULT_TANSITION_LATENCY_NS NSEC_PER_MSEC
+
#define CPUFREQ_NAME_LEN 16
/* Print length for names. Extra 1 space for accommodating '\n' in prints */
#define CPUFREQ_NAME_PLEN (CPUFREQ_NAME_LEN + 1)
--- a/rust/kernel/cpufreq.rs
+++ b/rust/kernel/cpufreq.rs
@@ -39,7 +39,8 @@ use macros::vtable;
const CPUFREQ_NAME_LEN: usize = bindings::CPUFREQ_NAME_LEN as usize;
/// Default transition latency value in nanoseconds.
-pub const ETERNAL_LATENCY_NS: u32 = bindings::CPUFREQ_ETERNAL as u32;
+pub const DEFAULT_TRANSITION_LATENCY_NS: u32 =
+ bindings::CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS as u32;
/// CPU frequency driver flags.
pub mod flags {
@@ -400,13 +401,13 @@ impl TableBuilder {
/// The following example demonstrates how to create a CPU frequency table.
///
/// ```
-/// use kernel::cpufreq::{ETERNAL_LATENCY_NS, Policy};
+/// use kernel::cpufreq::{DEFAULT_TRANSITION_LATENCY_NS, Policy};
///
/// fn update_policy(policy: &mut Policy) {
/// policy
/// .set_dvfs_possible_from_any_cpu(true)
/// .set_fast_switch_possible(true)
-/// .set_transition_latency_ns(ETERNAL_LATENCY_NS);
+/// .set_transition_latency_ns(DEFAULT_TRANSITION_LATENCY_NS);
///
/// pr_info!("The policy details are: {:?}\n", (policy.cpu(), policy.cur()));
/// }
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