linux-kernel - Re: [PATCH v7 1/5] PM / Runtime: Add getter for quering the IRQ safe option

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Date:	Thu, 25 Sep 2014 09:57:52 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Pavel Machek <pavel@....cz>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dmaengine@...r.kernel.org, Grant Likely <grant.likely@...aro.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Michal Simek <michal.simek@...inx.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v7 1/5] PM / Runtime: Add getter for quering the IRQ safe option

On 24 September 2014 21:52, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Wed, Sep 24, 2014 at 03:47:17PM -0400, Alan Stern wrote:
>> On Wed, 24 Sep 2014, Pavel Machek wrote:
>>
>> > On Wed 2014-09-24 15:50:08, Krzysztof Kozlowski wrote:
>> > > Add a simple getter pm_runtime_is_irq_safe() for quering whether runtime
>> > > PM IRQ safe was set or not.
>> > >
>> > > Various bus drivers implementing runtime PM may use choose to suspend
>> > > differently based on IRQ safeness status of child driver (e.g. do not
>> > > unprepare the clock if IRQ safe is not set).
>> > >
>> > > Signed-off-by: Krzysztof Kozlowski <k.kozlowski@...sung.com>
>> > > Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>
>> >
>> > Are you sure this is good interface?
>> >
>> > "Tell me if another function works this or that way".
>> >
>> > That's certainly not traditional interface, and it seems dangerous to
>> > me. Callbacks now have different semantic requirements based on value
>> > of some flag...
>> >
>> > Would it be possible to have two sets of callbacks, one irq safe and
>> > one not?
>>
>> Or maybe add a flag to the bus-specific device structures, indicating
>> specifically whether or not the clock should be unprepared during a
>> runtime suspend.  Then individual drivers could set this flag or not,
>> independent of the irq-safe setting.
>
> What you're proposing is _less_ safe, because with your proposal, you
> now have the possibility that drivers will tell runtime PM that it has
> IRQ safe callbacks, but the bus code tries to prepare/unprepare the
> clock, which causes a might-sleep-if warning.
>
> This is fragile.

I agree.

I would also like to feed another argument into this discussion on why
I like the new API. Currently the generic power domain accesses
"dev->power.irq_safe" directly.

Kind regards
Uffe
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