Re: [PATCH] cpufreq: cap the default transition delay at 10 ms

Shawn Guo
2025-09-16T01:53:32Z

On Mon, Sep 15, 2025 at 03:18:44PM +0200, Rafael J. Wysocki wrote:
> The question is not about who's fault it is, but what's the best place
> to address this issue.
>
> I think that addressing it in cpufreq_policy_transition_delay_us() is
> a bit confusing because it is related to initialization and the new
> branch becomes pure overhead for the drivers that don't set
> cpuinfo.transition_latency to CPUFREQ_ETERNAL.
>
> However, addressing it at the initialization time would effectively
> mean that the core would do something like:
>
> if (policy->cpuinfo.transition_latency == CPUFREQ_ETERNAL)
> policy->cpuinfo.transition_latency =
> CPUFREQ_DEFAULT_TANSITION_LATENCY_NS;
>
> but then it would be kind of more straightforward to update everybody
> using CPUFREQ_ETERNAL to set cpuinfo.transition_latency to
> CPUFREQ_DEFAULT_TANSITION_LATENCY_NS directly (and then get rid of
> CPUFREQ_ETERNAL entirely).

So we fix the regression with an immediate change like below, and then
plan to remove CPUFREQ_ETERNAL entirely with another development series.
Do I get you right?

---8<---

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index d873ff9add49..e37722ce7aec 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -574,6 +574,10 @@ unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy)
if (policy->transition_delay_us)
return policy->transition_delay_us;

+ if (policy->cpuinfo.transition_latency == CPUFREQ_ETERNAL)
+ policy->cpuinfo.transition_latency =
+ CPUFREQ_DEFAULT_TANSITION_LATENCY_NS;
+
latency = policy->cpuinfo.transition_latency / NSEC_PER_USEC;
if (latency)
/*
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 7fe0981a7e46..7331bc06f161 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -36,6 +36,8 @@
/* Print length for names. Extra 1 space for accommodating '\n' in prints */
#define CPUFREQ_NAME_PLEN (CPUFREQ_NAME_LEN + 1)

+#define CPUFREQ_DEFAULT_TANSITION_LATENCY_NS NSEC_PER_MSEC
+
struct cpufreq_governor;

enum cpufreq_table_sorting {

--->8---

Shawn