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

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Message-ID: <fec9b074-49d2-18ea-a259-4b1b372deae7@arm.com>
Date:   Thu, 20 Jul 2017 16:11:38 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Saravana Kannan <skannan@...eaurora.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        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 20/07/17 13:22, Peter Zijlstra wrote:
> 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.
> 

But, if ARM decides to architect and move to it to a system/machine
register, we will end up with the same limitation :( IMO.

For now with SCMI kind of interface, there's no such limitation as
yoalready mentioned in the follow up email.
-- 
Regards,
Sudeep