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

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  Re: [Eas-dev] [PATCH V3 2/3] cpufreq: schedutil: Process remote  callback for shared policies Peter Zijlstra <peterz@infradead.org> - 2017-07-20 14:30 +0200
    Re: [Eas-dev] [PATCH V3 2/3] cpufreq: schedutil: Process remote  callback for shared policies Sudeep Holla <sudeep.holla@arm.com> - 2017-07-20 17:20 +0200
    Re: [Eas-dev] [PATCH V3 2/3] cpufreq: schedutil: Process remote callback  for shared policies Saravana Kannan <skannan@codeaurora.org> - 2017-07-26 23:00 +0200

#1692884 — 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?

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#1693024

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

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#1697555 — Re: [Eas-dev] [PATCH V3 2/3] cpufreq: schedutil: Process remote callback for shared policies

On 07/20/2017 05:22 AM, Peter Zijlstra wrote:
> 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.

Writing to a memory addressable register. AFAIK, ARM has no support for 
a machine register for DVFS request. So, even if any ARM licensee wants 
to add one, it won't be possible.

Also, even if we have an ARM CPU with a machine register, rejecting a 
valid frequency switch just because it happened to come on a different 
CPU seem silly (you can have a huge performance hit due to that). A much 
better solution is to just make an IPI to the right CPU and execute the 
machine register write on the right CPU.

-Saravana

-- 
Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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