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

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Message-ID: <15871c67-0d18-430f-935e-261b2cda855b@gmail.com>
Date: Thu, 22 May 2025 03:50:55 -0500
From: Russell Haley <yumpusamongus@...il.com>
To: Shashank Balaji <shashank.mahadasyam@...y.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>, Jonathan Corbet <corbet@....net>
Cc: 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 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.

- Russell