[PATCH/RFC 3/4] of/clk: Register clocks suitable for Runtime PM with the PM core

[Ulf Hansson]

On 2 May 2014 16:58, Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> Hi Ulf, Tomasz,
>
> On Fri, May 2, 2014 at 10:13 AM, Ulf Hansson <ulf.hansson at linaro.org> wrote:
>>>>> +static int of_clk_register(struct device *dev, struct clk *clk)
>>>>> +{
>>>>> +       int error;
>>>>> +
>>>>> +       if (!dev->pm_domain) {
>>>>> +               error = pm_clk_create(dev);
>>>>> +               if (error)
>>>>> +                       return error;
>>>>> +
>>>>> +               dev->pm_domain = &of_clk_pm_domain;
>>>>
>>>>
>>>> I am concerned about how this will work in conjunction with the
>>>> generic power domain.
>>>>
>>>> A device can't reside in more than one pm_domain; thus I think it
>>>> would be better to always use the generic power domain and not have a
>>>> specific one for clocks. Typically the genpd should invoke
>>>> pm_clk_resume|suspend from it's runtime PM callbacks.
>>>
>>> I'm not sure about this. A typical use case would be to gate clocks ASAP and
>>> then wait until device is idle long enough to consider turning off the power
>>> domain worthwhile. Also sometimes we may want to gate the clocks, but
>>> prevent power domain from being powered off to retain hardware state (e.g.
>>> because there is no way to read it and restore later).
>>
>> So, in principle you prefer to have driver's handle clock gating to
>> save power from their runtime PM callbacks, instead of from the power
>> domain, right? Just to clarify, that's my view as well.
>
> If there's both a gate clock and a power domain, and the driver's Runtime PM
> callbacks handle clock gating, who's handling the power domain?

This is my view, not sure everybody agrees :-)

1. If you have a hardware power domain you need to implement a
pm_domain (preferably use the generic power domain).

2. If you don't have a hardware power domain, but still cares about
having a centralized solution for dev_pm_qos - you may use the generic
power domain, since it supports this.

3. If none of the above, you don't need a pm_domain at all.

Kind regards
Ulf Hansson

>
> Gr{oetje,eeting}s,
>
>                         Geert (still trying to fit all pieces of the
> puzzle together)
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds