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

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Message-ID: <20240429092515.2ehk4ifcul6mbaxh@vireshk-i7>
Date: Mon, 29 Apr 2024 14:55:15 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Beata Michalska <beata.michalska@....com>
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 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 ?

-- 
viresh