linux-kernel - Re: [PATCH v10 1/5] PM / Runtime: Allow accessing irq_safe if no PM

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Date:	Mon, 10 Nov 2014 15:11:45 +0100
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Russell King <linux@....linux.org.uk>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dmaengine@...r.kernel.org, Lars-Peter Clausen <lars@...afoo.de>,
	Michal Simek <michal.simek@...inx.com>,
	Kevin Hilman <khilman@...nel.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v10 1/5] PM / Runtime: Allow accessing irq_safe if no PM_RUNTIME

On 7 November 2014 15:50, Alan Stern <stern@...land.harvard.edu> wrote:
> On Fri, 7 Nov 2014, Krzysztof Kozlowski wrote:
>
>> > Well, that is a good reason to introduce a wrapper around power.irq_safe in my
>> > view.
>> >
>> > And define the wrapper so that it always returns false for CONFIG_PM_RUNTIME
>> > unset.
>> >
>> > This way not only you wouldn't need to move the flag from under the #ifdef,
>> > but also you would make the compiler skip the relevant pieces of code
>> > entiretly for CONFIG_PM_RUNTIME unset.
>>
>> Few days ago I would be happy with your opinion :), but know I think
>> this is better solution than wrapper. Consider case:
>> 1. PM_RUNTIME unset.
>> 2. System suspends.
>> 3. The pl330 in its suspend callback calls force_runtime_suspend which
>> leads us to amba/bus.
>> 4. The amba/bus.c in runtime suspend checks for irq_safe (it is FALSE),
>> so it disables and unprepares the clock.
>> 5. The pl330 in probe requested irq_safe so it assumes amba/bus will
>> only disable the clock. So the pl330 unprepares the clock. Again.
>
> To me, this sounds like a good reason to avoid using
> force_runtime_suspend().  In fact, it sounds like a good reason to
> avoid relying on the runtime PM mechanism to handle non-runtime-PM
> things (like a system suspend callback).  If CONFIG_PM_RUNTIME isn't
> enabled then the runtime PM stack simply should not be used.

There are an important advantage of using the pm_runtime_force_suspend() here.

For the driver to handle clock gating at system PM suspend, it first
needs to bring the device into full power, through
pm_runtime_get_sync(). Otherwise it's not safe to gate the clock,
since it may already be gated.

Kind regards
Uffe
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