Message-ID: <Zjnm7LzrYLCbz-XX@arm.com>
Date: Tue, 7 May 2024 10:31:40 +0200
From: Beata Michalska <beata.michalska@....com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Vanshidhar Konda <vanshikonda@...amperecomputing.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	ionela.voinescu@....com, sudeep.holla@....com, will@...nel.org,
	catalin.marinas@....com, vincent.guittot@...aro.org,
	sumitg@...dia.com, yang@...amperecomputing.com,
	lihuisong@...wei.com
Subject: Re: [PATCH v4 4/4] cpufreq: Use arch specific feedback for
 cpuinfo_cur_freq

On Mon, Apr 29, 2024 at 02:55:15PM +0530, Viresh Kumar wrote:
> On 26-04-24, 12:45, Beata Michalska wrote:
> > It seems that we might need to revisit the discussion we've had around
> > scaling_cur_freq and cpuinfo_cur_freq and the use of arch_freq_get_on_cpu.
> > As Vanshi has raised, having both utilizing arch specific feedback for
> > getting current frequency is bit problematic and might be confusing at best.
> > As arch_freq_get_on_cpu is already used by show_scaling_cur_freq there are not
> > many options we are left with, if we want to kee all archs aligned:
> > we can either try to rework show_scaling_cur_freq and it's use of
> > arch_freq_get_on_cpu, and move it to cpuinfo_cur_freq, which would align with
> > relevant docs, though that will not work for x86, or we keep it only there and
> > skip updating cpuinfo_cur_freq, going against the guidelines. Other options,
> > purely theoretical, would involve making arch_freq_get_on_cpu aware of type of
> > the info requested (hw vs sw) or adding yet another arch-specific implementation,
> > and those are not really appealing alternatives to say at least.
> > What's your opinion on this one ?
> 
> Hi Beata / Vanshidhar,
> 
> Lets forget for once what X86 and ARM may have done and think about it
> once again. I also had a chat with Vincent today about this.
> 
> The documentation says it clearly, cpuinfo_cur_freq is the one
> received from hardware and scaling_cur_freq is the one requested from
> software.
> 
> Now, I know that X86 has made both of them quite similar and I
> suggested to make them all aligned (and never received a reply on my
> previous message).
> 
> There are few reasons why it may be worth keeping the definition (and
> behavior) of the sysfs files as is, at least for ARM:
> - First is that the documentation says so.
> - There is no point providing the same information via both the
>   interfaces, there are two interfaces here for a reason.
> - There maybe tools around which depend on the documented behavior.
> - From userspace, currently there is only one way to know the exact
>   frequency that the cpufreq governors have requested from a platform,
>   i.e. the value from scaling_cur_freq. If we make it similar to
>   cpuinfo_cur_freq, then userspace will never know about the requested
>   frequency and the eventual one and if they are same or different.
> 
> Lets keep the behavior as is and update only cpuinfo_cur_freq with
> arch_freq_get_on_cpu().
> 
> Makes sense ?
>
First of all - apologies for late reply.

It all makes sense, though to clarify things up, for my own benefit, and to
avoid any potential confusion ....

Adding arch_freq_get_on_cpu to cpuinfo_cur_freq does seem to be the right
approach - no argue on this one. Doing that though means we need a way to
skip calling arch_freq_get_on_cpu() from show_scaling_cur_freq(), to avoid
having both providing the same information when that should not be the case.
In the initial approach [1], that was handled by checking whether the cpufreq
driver supports 'get' callback (and thus cpuinfo_cur_freq). In case it didn't,
things remained unchanged for scaling_cur_freq. That does not seem to be a viable
option though, as there are at least few drivers, that will support both:
cpuinfo_cur_freq alongside scaling_cur_freq (+ APERF/MPERF) and for those,
we would hit the initial problem of both relying on arch_freq_get_on_cpu.
So I guess we need another way of avoiding calling arch_freq_get_on_cpu
for show_scaling_cur_freq (and most probably avoid calling that one for
cpuinfo_cur_freq). Quick idea on how to not bring arch specificity into
cpufreq generic code would be to introduce a new flag for cpufreq drivers though
that seems a bit stretched. Will ponder a bit about that but in the meantime
suggestions are more than welcomed.

Aside: I will most probably send the changes separately from this series to not
mix things any further.

---
[1] https://lore.kernel.org/all/20230606155754.245998-1-beata.michalska@arm.com/
---
BR
Beata


> -- 
> viresh

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