Subject: Re: [PATCH] cpufreq: cppc: discard out-of-range delivered_perf samples

From: Breno Leitao <[email-masked]>
Date: Fri, 8 May 2026 01:17:57 -0700

Hello Jie,

On Fri, May 08, 2026 at 11:23:18AM +0800, Jie Zhan wrote:
> On 5/2/2026 12:32 AM, Breno Leitao wrote:
> > cppc_cpufreq_get_rate() derives delivered_perf as:
> > 
> >     delivered_perf = reference_perf * delta_delivered / delta_reference
> > 
> > over a short udelay()-bounded window between two cppc_get_perf_ctrs()
> > calls. Per-read latency jitter on the underlying CPC register access
> > can skew the ratio, occasionally producing delivered_perf >
> > highest_perf. cppc_perf_to_khz() then linearly extrapolates above
> > (nominal_perf, nominal_freq), so the value reported via
> > /sys/.../cpufreq/cpuinfo_cur_freq exceeds cpuinfo_max_freq.
> > 
> > Observed on an arm64 host (governor=performance,
> > cpuinfo_max_freq=3339 MHz): 15 back-to-back reads returned values
> > between 2997 and 4230 MHz.
> Hi Breno,
> 
> Frequency sampling discrepancy through 'cpuinfo_cur_freq' in the
> cppc_cpufreq driver has been an issue discussed for a very long time.

Thanks for the heads-up.

> If the CPPC feedback counters are FFH on your platform, can you have a look
> at [1] and see if that helps?

Have you forgot to get the [1] link here?

Thanks for looking at this,
--breno