linux-kernel - Re: [PATCH] cpufreq: amd_freq_sensitivity: Fix sensitivity clamping in amd_powersave_bias

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Message-Id: <3F396FA4-8DB6-451E-BF58-02646808FC3B@linux.dev>
Date: Fri, 19 Dec 2025 16:55:02 +0100
From: Thorsten Blum <thorsten.blum@...ux.dev>
To: David Laight <david.laight.linux@...il.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>,
 Thomas Renninger <trenn@...e.de>,
 Borislav Petkov <bp@...e.de>,
 Jacob Shin <jacob.shin@....com>,
 stable@...r.kernel.org,
 "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
 linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: amd_freq_sensitivity: Fix sensitivity clamping
 in amd_powersave_bias_target

On 2. Dec 2025, at 20:09, David Laight wrote:
> On Tue,  2 Dec 2025 13:44:28 +0100
> Thorsten Blum <thorsten.blum@...ux.dev> wrote:
> 
>> The local variable 'sensitivity' was never clamped to 0 or
>> POWERSAVE_BIAS_MAX because the return value of clamp() was not used. Fix
>> this by assigning the clamped value back to 'sensitivity'.
> 
> This actually makes no difference
> (assuming od_tuners->powersave_bias <= POWERSAVE_BIAS_MAX).
> The only use of 'sensitivity' is the test at the end of the diff.
> 
> So I think you could just delete the line.

The local variable 'sensitivity' is an 'int', while '->powersave_bias'
is an 'unsigned int'. If 'sensitivity' were ever negative, it would be
converted to an 'unsigned int', producing an incorrect result. That's
probably what the clamping was meant to prevent.

However, calculating 'sensitivity' can be simplified from:

	sensitivity = POWERSAVE_BIAS_MAX -
		(POWERSAVE_BIAS_MAX * (d_reference - d_actual) / d_reference);

to:

	sensitivity = POWERSAVE_BIAS_MAX * d_actual / d_reference;

which makes it clearer that, in practice, 'sensitivity' is never
negative. How about simplifying the formula as above, changing
'sensitivity' to 'unsigned int', and removing the clamping?

Thanks,
Thorsten