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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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@samsung.com>
> > > Reviewed-by: Ulf Hansson <ulf.hansson@linaro.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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