linux-kernel - Re: [Eas-dev] [PATCH V3 2/3] cpufreq: schedutil: Process remote callback for shared policies

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Date:   Thu, 20 Jul 2017 14:22:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Saravana Kannan <skannan@...eaurora.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...hat.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, smuckle.linux@...il.com,
        eas-dev@...ts.linaro.org
Subject: Re: [Eas-dev] [PATCH V3 2/3] cpufreq: schedutil: Process remote
 callback for shared policies

On Thu, Jul 13, 2017 at 07:02:37PM -0700, Saravana Kannan wrote:
> In all Qualcomm chipsets (well, at least the ones that have been used in
> Android devices so far), we can switch the frequency of any CPU from any
> other CPU. If we can do that even without fast switching, why wouldn't any
> theoretical fast switching be incapable of supporting this? Is this a
> limitation specific to x86 that we are assuming all architectures and
> platforms are going to have?

So the typical implementation of fast switching we're thinking of is the
CPU writing the DVFS request into a machine register. Now machine
registers are typically per logical CPU.

What style of fast switching were you thinking of?