RE: [PATCH v5 4/7] cpufreq: amd_pstate: fix wrong lowest perf fetch

Yuan, Perry
2022-08-31T08:53:44Z

[AMD Official Use Only - General]

Hi Agrawal,

> -----Original Message-----
> From: Punit Agrawal <punit.agrawal@bytedance.com>
> Sent: Monday, August 15, 2022 11:05 PM
> To: Yuan, Perry <Perry.Yuan@amd.com>
> Cc: rafael.j.wysocki@intel.com; Huang, Ray <Ray.Huang@amd.com>;
> viresh.kumar@linaro.org; Sharma, Deepak <Deepak.Sharma@amd.com>;
> Limonciello, Mario <Mario.Limonciello@amd.com>; Fontenot, Nathan
> <Nathan.Fontenot@amd.com>; Deucher, Alexander
> <Alexander.Deucher@amd.com>; Su, Jinzhou (Joe) <Jinzhou.Su@amd.com>;
> Huang, Shimmer <Shimmer.Huang@amd.com>; Du, Xiaojian
> <Xiaojian.Du@amd.com>; Meng, Li (Jassmine) <Li.Meng@amd.com>; linux-
> pm@vger.kernel.org; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH v5 4/7] cpufreq: amd_pstate: fix wrong lowest perf fetch
>
> [CAUTION: External Email]
>
> Hi Perry,
>
> Perry Yuan <Perry.Yuan@amd.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@amd.com>
> > Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> > Signed-off-by: Perry Yuan <Perry.Yuan@amd.com>
> > Signed-off-by: Su Jinzhou <jinzhou.su@amd.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.
Sorry to response late.

Perry.