linux-kernel - Re: [PATCH v5 4/7] cpufreq: amd_pstate: fix wrong lowest perf fetch

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Date:   Thu, 01 Sep 2022 15:58:50 +0100
From:   Punit Agrawal <punit.agrawal@...edance.com>
To:     "Yuan, Perry" <Perry.Yuan@....com>
Cc:     Punit Agrawal <punit.agrawal@...edance.com>,
        "rafael.j.wysocki@...el.com" <rafael.j.wysocki@...el.com>,
        "Huang, Ray" <Ray.Huang@....com>,
        "viresh.kumar@...aro.org" <viresh.kumar@...aro.org>,
        "Sharma, Deepak" <Deepak.Sharma@....com>,
        "Limonciello, Mario" <Mario.Limonciello@....com>,
        "Fontenot, Nathan" <Nathan.Fontenot@....com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "Su, Jinzhou (Joe)" <Jinzhou.Su@....com>,
        "Huang, Shimmer" <Shimmer.Huang@....com>,
        "Du, Xiaojian" <Xiaojian.Du@....com>,
        "Meng, Li (Jassmine)" <Li.Meng@....com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/7] cpufreq: amd_pstate: fix wrong lowest perf fetch

"Yuan, Perry" <Perry.Yuan@....com> writes:

[...]

>> Perry Yuan <Perry.Yuan@....com> writes:
>> 
>> > Fix the wrong lowest perf value reading which is used for new des_perf
>> > calculation by governor requested, the incorrect min_perf will get
>> > incorrect des_perf to be set , that will cause the system frequency
>> > changing unexpectedly.
>> >
>> > Reviewed-by: Huang Rui <ray.huang@....com>
>> > Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
>> > Signed-off-by: Perry Yuan <Perry.Yuan@....com>
>> > Signed-off-by: Su Jinzhou <jinzhou.su@....com>
>> > ---
>> >  drivers/cpufreq/amd-pstate.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/cpufreq/amd-pstate.c
>> > b/drivers/cpufreq/amd-pstate.c index 5cdef6638681..183cdd4ba00e 100644
>> > --- a/drivers/cpufreq/amd-pstate.c
>> > +++ b/drivers/cpufreq/amd-pstate.c
>> > @@ -307,7 +307,7 @@ static int amd_pstate_target(struct cpufreq_policy
>> *policy,
>> >               return -ENODEV;
>> >
>> >       cap_perf = READ_ONCE(cpudata->highest_perf);
>> > -     min_perf = READ_ONCE(cpudata->lowest_nonlinear_perf);
>> > +     min_perf = READ_ONCE(cpudata->lowest_perf);
>> >       max_perf = cap_perf;
>> >
>> >       freqs.old = policy->cur;
>> 
>> This looks to be a pretty big change (lowest nonlinear vs lowest). Does the patch
>> need to be backported to older kernels?
>
> The patch fixes the min perf initial value, the correct min perf is lowest_perf which is captured through MSR_AMD_CPPC_CAP1 register or the cppc_get_perf_caps().
> Yes, the patch will need to be backported to other kernel branch as issue fix.

Great, thanks for confirming!

[...]