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

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Message-ID: <20140924195222.GB5182@n2100.arm.linux.org.uk>
Date:	Wed, 24 Sep 2014 20:52:22 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	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, dmaengine@...r.kernel.org,
	Ulf Hansson <ulf.hansson@...aro.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-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 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.

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