linux-kernel - Re: Re: [PATCH v4 4/4] cpufreq: Use arch specific feedback for cpuinfo_cur

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Date: Wed, 1 May 2024 07:46:26 -0700
From: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
To: Viresh Kumar <viresh.kumar@...aro.org>, 
	Beata Michalska <beata.michalska@....com>
Cc: 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: 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 ?

I had the same concerns. It probably makes sense explicity note this in
the next version of the patch series; in the future readers may be
confused by x86 and ARM behave differntly on scaling_cur_freq.

Thanks,
Vanshi

>
>-- 
>viresh