linux-kernel - Re: [PATCH] arm64: Provide an AMU-based version of arch_freq_get_on

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Message-ID: <ZICNkXfBQUiT/BvK@e120325.cambridge.arm.com>
Date:   Wed, 7 Jun 2023 15:00:49 +0100
From:   Beata Michalska <beata.michalska@....com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        mark.rutland@....com, will@...nel.org, rafael@...nel.org,
        viresh.kumar@...aro.org, ionela.voinescu@....com,
        sumitg@...dia.com, yang@...amperecomputing.com
Subject: Re: [PATCH] arm64: Provide an AMU-based version of
 arch_freq_get_on_cpu

On Wed, Jun 07, 2023 at 10:58:56AM +0100, Sudeep Holla wrote:
> On Tue, Jun 06, 2023 at 04:57:54PM +0100, Beata Michalska wrote:
> > With the Frequency Invariance Engine (FIE) being already wired up with
> > sched tick and making use of relevant (core counter and constant
> > counter) AMU counters, getting the current frequency for a given CPU
> > on supported platforms, can be achieved by utilizing the frequency scale
> > factor which reflects an average CPU frequency for the last tick period
> > length.
> > 
> > With that at hand, arch_freq_get_on_cpu dedicated implementation
> > gets enrolled into cpuinfo_cur_freq policy sysfs attribute handler,
> > which is expected to represent the current frequency of a given CPU,
> > as obtained by the hardware. This is exactly the type of feedback that
> > cycle counters provide.
> > 
> > In order to avoid calling arch_freq_get_on_cpu from the scaling_cur_freq
> > attribute handler for platforms that do provide cpuinfo_cur_freq, and
> > yet keeping things intact for those platform that do not, its use gets
> > conditioned on the presence of cpufreq_driver (*get) callback (which also
> > seems to be the case for creating cpuinfo_cur_freq attribute).
> >
> 
> LGTM,
> 
> Reviewed-by: Sudeep Holla <sudeep.holla@....com>
> 
Thanks for the review.
> However I fail to understand if both the changes are dependent ?
> Can this be split into 2 patches ? I fail to see the dependency, what
> am I missing ? Even if there is some dependency to get arch value
> (arch_freq_get_on_cpu() from show_cpuinfo_cur_freq()), you can push
> that change first followed by the arm64 change as 2 different change.
> 
I guess I could split the patch into two parts:
1. adding implementation for the arch_freq_get_on_cpu
2. wiring it up with the cpufreq relevant attrib handlers

or the other way round (if that's what you have in mind).

Will wait a bit for any further comments before pushing new v.

---
BR
B.

> -- 
> Regards,
> Sudeep