linux-kernel - Re: [RFC v2 01/11] OPP: Don't overwrite rounded clk rate

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Message-ID: <a97f991d-539a-ce22-834b-ce19944e4f57@codeaurora.org>
Date:   Mon, 17 Jun 2019 09:55:00 +0530
From:   Rajendra Nayak <rnayak@...eaurora.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     swboyd@...omium.org, vincent.guittot@...aro.org,
        mturquette@...libre.com, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-serial@...r.kernel.org, linux-spi@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-scsi@...r.kernel.org,
        ulf.hansson@...aro.org, dianders@...omium.org, rafael@...nel.org
Subject: Re: [RFC v2 01/11] OPP: Don't overwrite rounded clk rate



On 6/17/2019 9:47 AM, Viresh Kumar wrote:
> On 17-06-19, 09:37, Rajendra Nayak wrote:
>>
>>
>> On 6/17/2019 9:20 AM, Viresh Kumar wrote:
>>> On 14-06-19, 10:57, Viresh Kumar wrote:
>>>> Hmm, so this patch won't break anything and I am inclined to apply it again :)
>>>>
>>>> Does anyone see any other issues with it, which I might be missing ?
>>>
>>> I have updated the commit log a bit more to clarify on things, please let me
>>> know if it looks okay.
>>>
>>>       opp: Don't overwrite rounded clk rate
>>>       The OPP table normally contains 'fmax' values corresponding to the
>>>       voltage or performance levels of each OPP, but we don't necessarily want
>>>       all the devices to run at fmax all the time. Running at fmax makes sense
>>>       for devices like CPU/GPU, which have a finite amount of work to do and
>>>       since a specific amount of energy is consumed at an OPP, its better to
>>>       run at the highest possible frequency for that voltage value.
>>>       On the other hand, we have IO devices which need to run at specific
>>>       frequencies only for their proper functioning, instead of maximum
>>>       possible frequency.
>>>       The OPP core currently roundup to the next possible OPP for a frequency
>>>       and select the fmax value. To support the IO devices by the OPP core,
>>>       lets do the roundup to fetch the voltage or performance state values,
>>>       but not use the OPP frequency value. Rather use the value returned by
>>>       clk_round_rate().
>>>       The current user, cpufreq, of dev_pm_opp_set_rate() already does the
>>>       rounding to the next OPP before calling this routine and it won't
>>>       have any side affects because of this change.
>>
>> Looks good to me. Should this also be documented someplace that dev_pm_opp_set_rate()
>> would not be able to distinguish between its users trying to scale CPU/GPU's vs IO
>> devices, so its the callers responsibility to round it accordingly before calling the
>> API?
> 
> diff --git a/drivers/opp/core.c b/drivers/opp/core.c
> index 0fbc77f05048..bae94bfa1e96 100644
> --- a/drivers/opp/core.c
> +++ b/drivers/opp/core.c
> @@ -751,8 +751,11 @@ static int _set_required_opps(struct device *dev,
>    * @dev:        device for which we do this operation
>    * @target_freq: frequency to achieve
>    *
> - * This configures the power-supplies and clock source to the levels specified
> - * by the OPP corresponding to the target_freq.
> + * This configures the power-supplies to the levels specified by the OPP
> + * corresponding to the target_freq, and programs the clock to a value <=
> + * target_freq, as rounded by clk_round_rate(). Device wanting to run at fmax
> + * provided by the opp, should have already rounded to the target OPP's
> + * frequency.
>    */

Perfect, thanks.

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