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

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Message-ID: <aMfAQXE4sRjru9I_@dragon>
Date: Mon, 15 Sep 2025 15:29:05 +0800
From: Shawn Guo <shawnguo2@...h.net>
To: Qais Yousef <qyousef@...alina.io>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Shawn Guo <shawnguo@...nel.org>,
	stable@...r.kernel.org
Subject: Re: [PATCH] cpufreq: cap the default transition delay at 10 ms

On Sun, Sep 14, 2025 at 06:43:26PM +0100, Qais Yousef wrote:
> > > Why do you want to address the issue in the cpufreq core instead of
> > > doing that in the cpufreq-dt driver?
> > 
> > My intuition was to fix the regression at where the regression was
> > introduced by recovering the code behavior.
> 
> Isn't the right fix here is at the driver level still? We can only give drivers
> what they ask for. If they ask for something wrong and result in something
> wrong, it is still their fault, no?

I'm not sure.  The cpufreq-dt driver is following suggestion to use
CPUFREQ_ETERNAL, which has the implication that core will figure out
a reasonable default value for platforms where the latency is unknown.
And that was exactly the situation before the regression.  How does it
become the fault of cpufreq-dt driver?

> Alternatively maybe we can add special handling for CPUFREQ_ETERNAL value,
> though I'd suggest to return 1ms (similar to the case of value being 0). Maybe
> we can redefine CPUFREQ_ETERNAL to be 0, but not sure if this can have side
> effects.

Changing CPUFREQ_ETERNAL to 0 looks so risky to me.  What about adding
an explicit check for CPUFREQ_ETERNAL?

---8<---

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index fc7eace8b65b..053f3a0288bc 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -549,11 +549,15 @@ 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)
+               goto default_delay;
+
        latency = policy->cpuinfo.transition_latency / NSEC_PER_USEC;
        if (latency)
                /* Give a 50% breathing room between updates */
                return latency + (latency >> 1);
 
+default_delay:
        return USEC_PER_MSEC;
 }
 EXPORT_SYMBOL_GPL(cpufreq_policy_transition_delay_us);

--->8---

Shawn