linux-kernel - Re: [PATCH 4/8] cpufreq,vexpress-spc: fix Frequency Invariance (FI) for bL switching

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Date:   Thu, 2 Jul 2020 12:41:52 +0100
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     rjw@...ysocki.net, catalin.marinas@....com, sudeep.holla@....com,
        will@...nel.org, linux@...linux.org.uk, valentin.schneider@....com,
        mingo@...hat.com, peterz@...radead.org, dietmar.eggemann@....com,
        linux-pm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Liviu Dudau <liviu.dudau@....com>
Subject: Re: [PATCH 4/8] cpufreq,vexpress-spc: fix Frequency Invariance (FI)
 for bL switching

Hi,

On Thursday 02 Jul 2020 at 08:35:51 (+0530), Viresh Kumar wrote:
> On 01-07-20, 15:07, Ionela Voinescu wrote:
> > On Wednesday 01 Jul 2020 at 16:16:19 (+0530), Viresh Kumar wrote:
> > > Is there anyone who cares for this driver and EAS ? I will just skip doing the
> > > FIE thing here and mark it skipped.
> > 
> > That is a good question. The vexpress driver is still used for TC2, but
> > I don't know of any users of this bL switcher functionality that's part
> > of the driver. I think there were a few people wondering recently if
> > it's still used [1].
> 
> Even if it is used by some, there is no need, I believe, to enable
> freq-invariance for it, which wasn't enabled until now.
>

It was enabled until now, but it was partially broken. If you look over
the driver you'll see arch_set_freq_scale() being called for both
is_bL_switching_enabled() and for when it's not [1].

For !is_bL_switching_enabled() this is fine. But for
is_bL_switching_enabled(), it is broken as described in 4/8.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/cpufreq/vexpress-spc-cpufreq.c?h=v5.8-rc3#n203

> And considering that we are going to enable the flag only for the
> interested parties now, as from the discussion on 1/8, this shouldn't
> be required.
> 

If we just don't want frequency invariance for
is_bL_switching_enabled(), I can just guard the setting of the flag
suggested by Rafael at 1/8 by !CONFIG_BL_SWITCHER.

I'll proceed to do that and remove the fix at 4/8.

Many thanks!
Ionela.

> -- 
> viresh