linux-kernel - Re: [PATCH V2] cpufreq: Make sure CPU is running on a freq from freq-table

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Date:	Mon, 25 Nov 2013 09:43:59 -0800
From:	Dirk Brandewie <dirk.brandewie@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Patch Tracking <patches@...aro.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nishanth Menon <nm@...com>, Carlos Hernandez <ceh@...com>
Subject: Re: [PATCH V2] cpufreq: Make sure CPU is running on a freq from freq-table

On 11/25/2013 09:01 AM, Viresh Kumar wrote:
> On 25 November 2013 22:08, Dirk Brandewie <dirk.brandewie@...il.com> wrote:
>> IMHO this issue should be fixed in the scaling driver for the platform.
>>
>> The scaling driver sets policy->cur and fills in the frequency table and has
>
> Not anymore, policy->cur is set in the core for most of the drivers now.
> Drivers just provide ->get() callbacks.
>
>> the ability to adjust the frequency of the CPU.
>
> I believe this kind of decisions should stay with the core, drivers should
> just provide the backend instead of intelligence..
>


This is a platform specific bug fix AFAICT and belongs in a platform
specific piece of code


>> Letting this leak up into the core
>> is wrong IMHO and just widens the window where the CPU will be running at
>> the wrong frequency set by the bootloader.
>
> It doesn't stay there for long enough.. we get to this point in
> cpufreq core just
> after calling ->init(), and if the CPU is working without issues until
> now, it will
> stay stable for few more milliseconds.
>

And this is where the scaling driver should detect and fix the issue in ->init()
on platforms we know about today, What happens if platforms in the future are
more sensitive to the issue?

>> Shouldn't there be a check to see if the problem exists at all?  Otherwise
>> the core is setting a policy for ALL platform even those where the issue
>> does
>> not exist.
>
> That is taken care of by __cpufreq_driver_target(). It checks if we are
> already running at requested frequency or not (after getting the next
> higher frequency)... If current freq is present in table,
> cpufreq_frequency_table_target() will return current frequency only for
> policy->cur -1. And so we will not end up configuring hardware
> unnecessarily.
>

The core should not be working around bootloader bugs IMHO.  Silently
fixing it is evil IMHO at a minimum the core should complain LOUDLY
about this happening otherwise the bootloaders will have no incentive to
get their act together.

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