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

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Date:	Tue, 14 Apr 2015 22:40:36 +0300
From:	Mikko Perttunen <mikko.perttunen@...si.fi>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
CC:	Michael Turquette <mturquette@...aro.org>,
	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 04/14/2015 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.

>
> 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 :)

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

Cheers,
Mikko.

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