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

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Date:	Tue, 6 May 2014 09:58:13 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Tomasz Figa <tomasz.figa@...il.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Magnus Damm <magnus.damm@...il.com>,
	Simon Horman <horms@...ge.net.au>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Felipe Balbi <balbi@...com>,
	Mike Turquette <mturquette@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-omap <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH/RFC 3/4] of/clk: Register clocks suitable for Runtime PM
 with the PM core

On 2 May 2014 16:58, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> Hi Ulf, Tomasz,
>
> On Fri, May 2, 2014 at 10:13 AM, Ulf Hansson <ulf.hansson@...aro.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@...ux-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
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