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cpufreq: Make drivers using CPUFREQ_ETERNAL specify transition latency

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Commit a755d0e2d4 ("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: a755d0e2d4 ("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@kernel.org>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Reviewed-by: Jie Zhan <zhanjie9@hisilicon.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: 6.6+ <stable@vger.kernel.org> # 6.6+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/2264949.irdbgypaU6@rafael.j.wysocki
[ rjw: Fix typo in new symbol name, drop redundant type cast from Rust binding ]
Tested-by: Shawn Guo <shawnguo@kernel.org> # with cpufreq-dt driver
Reviewed-by: Qais Yousef <qyousef@layalina.io>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Rafael J. Wysocki
2025-09-26 12:12:37 +02:00
parent d3f8f8d030
commit f97aef092e
9 changed files with 14 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

7
rust/kernel/cpufreq.rs
View File

@@ -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;
/// 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()));
/// }