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

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Message-ID: <aMgEgvTyHEzaEJ1v@dragon>
Date: Mon, 15 Sep 2025 20:20:18 +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 Mon, Sep 15, 2025 at 11:02:07AM +0100, Qais Yousef wrote:
> On 09/15/25 15:29, Shawn Guo 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, 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?
> 
> Rafael and Viresh would know better, but amd-pstate chooses to fallback to
> specific values if cppc returned CPUFREQ_ETERNAL.
> 
> Have you tried to look why dev_pm_opp_get_max_transition_latency() returns
> 0 for your platform? I think this is the problem that was being masked before.

My platform doesn't scale voltage along with frequency, and the platform
DT doesn't specify 'clock-latency-ns' which is an optional property
after all.

Shawn