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

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Message-ID: <CAJZ5v0i8L8w_ojua1ir3CGcwGSvE+3Jj0Sh5Cs1Yi8i4BX1Lbw@mail.gmail.com>
Date: Mon, 15 Sep 2025 15:18:44 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Shawn Guo <shawnguo2@...h.net>
Cc: Qais Yousef <qyousef@...alina.io>, "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 Mon, Sep 15, 2025 at 9:29 AM Shawn Guo <shawnguo2@...h.net> wrote:
>
> 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,

Fair enough.

Actually, there are a few other drivers that fall back to
CPUFREQ_ETERNAL if they cannot determine transition_latency.

> which has the implication that core will figure out a reasonable default value for
> platforms where the latency is unknown.

Is this expectation realistic, though?  I'm not sure.

The core can only use a hard-coded default fallback number, but would
that number be really suitable for all of the platforms in question?

> And that was exactly the situation before the regression.  How does it
> become the fault of cpufreq-dt driver?

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).

> > 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;

Can't USEC_PER_MSEC be just returned directly from here?

> +
>         latency = policy->cpuinfo.transition_latency / NSEC_PER_USEC;
>         if (latency)
>                 /* Give a 50% breathing room between updates */
>                 return latency + (latency >> 1);

Side note for self: The computation above can be done once at the
policy initialization time and transition_latency can be stored in us
(and only converted to ns when the corresponding sysfs attribute is
read).  It can be even set to USEC_PER_MSEC if zero.

> +default_delay:
>         return USEC_PER_MSEC;
>  }
>  EXPORT_SYMBOL_GPL(cpufreq_policy_transition_delay_us);
>
> --->8---