linux-kernel - Re: [PATCH] cpufreq, docs: (userspace governor) add that actual freq is >= scaling

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Message-ID: <CAJZ5v0gz3Y+RGqBf9E1hzq9rwfrryd98Xpk51DtLd-uck5y-rw@mail.gmail.com>
Date: Thu, 22 May 2025 11:47:04 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Russell Haley <yumpusamongus@...il.com>
Cc: Shashank Balaji <shashank.mahadasyam@...y.com>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Viresh Kumar <viresh.kumar@...aro.org>, Jonathan Corbet <corbet@....net>, linux-pm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Shinya Takumi <shinya.takumi@...y.com>
Subject: Re: [PATCH] cpufreq, docs: (userspace governor) add that actual freq
 is >= scaling_setspeed

On Thu, May 22, 2025 at 10:51 AM Russell Haley <yumpusamongus@...il.com> wrote:
>
>
> On 5/22/25 3:05 AM, Shashank Balaji wrote:
> > The userspace governor does not have the CPUFREQ_GOV_STRICT_TARGET flag, which
> > means the requested frequency may not strictly be followed. This is true in the
> > case of the intel_pstate driver with HWP enabled. When programming the
> > HWP_REQUEST MSR, the min_perf is set to `scaling_setspeed`, and the max_perf
> > is set to the policy's max. So, the hardware is free to increase the frequency
> > beyond the requested frequency.
> >
> > This behaviour can be slightly surprising, given the current wording "allows
> > userspace to set the CPU frequency". Hence, document this.
> >
>
> In my opinion, the documentation is correct, and it is the
> implementation in intel_pstate that is wrong. If the user wanted two
> separate knobs that control the minimum and maximum frequencies, they
> could leave intel_pstate in "active" mode and change scaling_min_freq
> and scaling_max_freq.
>
> If the user asks for the frequency to be set from userspace, the
> frequency had damn well better be set from userspace.

The userspace governor requests a frequency between policy->min and
policy->max on behalf of user space.  In intel_pstate this translates
to setting DESIRED_PERF to the requested value which is also the case
for the other governors.

There is no guarantee that the request will be granted by the
hardware, either way.