linux-kernel - Re: [PATCH 1/2] cpufreq: Return error if ->get() failed in cpufreq_update

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Message-ID: <530C2FD7.4000105@linux.vnet.ibm.com>
Date:	Tue, 25 Feb 2014 11:23:27 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.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>,
	Pierre Ossman <pierre-list@...man.eu>
Subject: Re: [PATCH 1/2] cpufreq: Return error if ->get() failed in cpufreq_update_policy()

On 02/25/2014 10:11 AM, Viresh Kumar wrote:
> On 18 February 2014 07:49, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> On 18 February 2014 03:30, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>>> On Monday, February 17, 2014 02:25:34 PM Srivatsa S. Bhat wrote:
>>>> Why go to no_policy when we can actually set things right?
>>>>
>>>> Anyway, I am not arguing against this strongly. I just wanted to share my
>>>> thoughts, since this is the approach that made more sense to me.
>>>
>>> And I agree with that.  In particular, since we're going to set the new
>>> policy *anyway* at this point, we can adjust the current frequency just fine
>>> in the process, can't we?
>>
>> Though I still feel that it wouldn't be the right thing to do as get()
>> just can't
>> return zero. Actually I was planning to place a WARN() over its return value
>> of zero.

A WARN() would definitely be good.

>>
>> Anyway, as two of the three are in favor of this we can get that in.. But how
>> would that work?
>>
>> - What frequency should we call cpufreq_driver_target ?
>> - Remember that it wouldn't do anything if policy->cur is same as new freq.
>> - Also remember that drivers need special attention if new freq is > old
>> freq or vice versa. As they will change voltage before or after change here.
>> And because we actually don't know what frequency we are at currently, how
>> will we decide that?
> 

Hmm, that's a good point. However, lets first think about the simple scenario
that the driver _is_ able to detect the current frequency from the hardware
(a non-zero, sane value) say X KHz, and that frequency is different from what
the cpufreq subsystem thinks it is (Y KHz).

In the current code, when we observe this, we send out a notification and try
to adjust to X KHz. Instead, what I'm suggesting is to invoke the driver to
set the frequency to Y KHz, since that's what the cpufreq subsystems wants the
frequency to be at.

As for the case where the driver reports the frequency to be 0 KHz, we can
print a WARN() and try to force set the frequency to Y KHz. But if that turns
out to be too tricky to handle, we can just put a WARN() and error out, as you
proposed earlier.
 
Regards,
Srivatsa S. Bhat

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