linux-kernel - Re: [PATCH v8 00/18] Tegra124 CL-DVFS / DFLL clocksource + cpufreq

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Message-ID: <552D8246.8040207@kapsi.fi>
Date:	Wed, 15 Apr 2015 00:10:30 +0300
From:	Mikko Perttunen <mikko.perttunen@...si.fi>
To:	Michael Turquette <mturquette@...aro.org>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>
CC:	Thierry Reding <thierry.reding@...il.com>, swarren@...dotorg.org,
	gnurou@...il.com, pdeschrijver@...dia.com, rjw@...ysocki.net,
	viresh.kumar@...aro.org, pwalmsley@...dia.com, vinceh@...dia.com,
	pgaikwad@...dia.com, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, tuomas.tynkkynen@....fi
Subject: Re: [PATCH v8 00/18] Tegra124 CL-DVFS / DFLL clocksource + cpufreq

On 04/15/2015 12:06 AM, Michael Turquette wrote:
> Quoting Mikko Perttunen (2015-04-14 12:40:36)
>> On 04/14/2015 08:21 PM, Boris Brezillon wrote:
>>> Hi Mikko,
>>>
>>> On Tue, 14 Apr 2015 14:25:59 +0300
>>> Mikko Perttunen <mikko.perttunen@...si.fi> wrote:
>>>
>>>> On 04/11/2015 12:11 AM, Michael Turquette wrote:
>>>>> Quoting Thierry Reding (2015-03-11 03:07:43)
>>>>>> Hi Mike,
>>>>>>
>>>>>> Have you had a chance to look at these changes to the Tegra clock
>>>>>> driver? If you're fine with it, I'd like to take these patches through
>>>>>> the Tegra tree because the rest of the series depends on them. I can
>>>>>> provide a stable branch in case we need to base other Tegra clock
>>>>>> changes on top of this.
>>>>>
>>>>> Hi Thierry,
>>>>>
>>>>> Clock patches (and corresponding DT binding descriptions and changes to
>>>>> DTS) look fine to me. Please add:
>>>>>
>>>>> Acked-by: Michael Turquette <mturquette@...aro.org>
>>>>>
>>>>> I did have a question about the beahvior of clk_put in one of Mikko's
>>>>> patches but it should not gate this series. I'm just trying to find out
>>>>> if we have a bug in the framework or if the Tegra driver is a special
>>>>> case.
>>>>>
>>>>> Also I do not think a stable branch is necessary.
>>>>>
>>>>> Regards,
>>>>> Mike
>>>>>
>>>>
>>>> Looks like in the meantime, this has been partially broken by
>>>> 03bc10ab5b0f "clk: check ->determine/round_rate() return value in
>>>> clk_calc_new_rates". The highest rates supported by the DFLL clock have
>>>> 1 in the MSB, so those cannot be entered after the aforementioned patch,
>>>> as the return value of round_rate is interpreted as an error. Avenues
>>>> that I can see: 1) revert the above patch 2) restrict the cpu clock rate
>>>> to those with 0 in the MSB 3) move to 64-bit clock rates.
>>>
>>> How about changing ->determine_rate() and ->round_rate() prototypes so
>>> that they always return 0 or an error code and passing the adjusted_rate
>>> as an argument ?
>>>
>>> Something like that:
>>>
>>>        int (*round_rate)(struct clk_hw *hw, unsigned long *rate,
>>>                          unsigned long *parent_rate);
>>>        int (*determine_rate)(struct clk_hw *hw, unsigned long *rate,
>>>                              unsigned long min_rate,
>>>                              unsigned long max_rate,
>>>                              unsigned long *best_parent_rate,
>>>                              struct clk_hw **best_parent_hw);
>>>
>>> I know this implies a lot of changes (in all clock drivers and in the
>>> core infrastructure), but I really think we should not mix error codes
>>> and clock frequencies (even if we decide to move to a 64 bits rate
>>> approach).
>>
>> This sounds like a good idea, too.
>
> I've had this idea as well, which is to never return rates but only
> error codes, and rates are passed by reference like in your example
> above. Clearly the *best_parent_rate stuff already functions this way.
> Would be cool to use a programming language that supported complex
> return types ;-)

Algebraic data type pipe dreams.. :)

>
>>
>>>
>>> Anyway, IMHO the only alternative to this solution is solution #3,
>>> because #1 implies re-introducing another bug where
>>> ->round_rate()/->determine_rate() are silently ignored, and #2 implies
>>> lying about your DFLL capabilities.
>>>
>>
>> Yeah, #1 and #2 weren't really meant as realistic options to end up with :)
>
> Yes, seems that we're heading towards #3. In the mean time option #1.5
> (the one where we change the round_rate/determine_rate semantics) is
> probably a good idea and can resolve this issue in the shorter term
> compared to signed 64-bit rates (and will be necessary anyhow if we use
> unsigned 64-bit rates).
>
> I'll add this to the high priority todo list since the Tegra EMC stuff
> won't go for 4.1 but will very likely go for 4.2.

Wonderful :)

Thanks,
Mikko

>
> Regards,
> Mike
>
>>
>>> Mike, what's your opinion ?
>>>
>>> Best Regards,
>>>
>>> Boris
>>>
>>
>> Cheers,
>> Mikko.
>>

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