Re: subtle pm_runtime_put_sync race and sdio functions

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On Wed, 26 Jan 2011, Kevin Hilman wrote:

> >> Consider this instead:  Since A is required to be functional before B
> >> can be used, A must be registered before B and hence B gets suspended
> >> before A.  Therefore during the prepare phase we can runtime-resume A
> >> and leave it powered up; when B needs to suspend, it won't matter that
> >> the runtime-PM calls are ineffective.
> >
> > We don't really need to do that, because the runtime resume _is_ functional
> > during system suspend.

Not asynchronous runtime resume, because the workqueue is frozen.  But 
that's not the issue here.

> >  The only thing missing is a ->suspend() callback for A
> > (and a corresponding ->resume() callback to make sure A will be available to
> > B during system resume).
> >
> 
> OK, I'm finally back to debugging this problem and looking for a final
> solution.
> 
> I agree that what is needed is ->suspend() and ->resume() callbacks for
> A, but the question remains how to implement them.
> 
> In my case, A doesn't need runtime callbacks, but *does* require that
> the subsystem callbacks are called because the subsystem actually does
> all the real PM work.  On OMAP, the device PM code (clock mgmt, device
> low-power states, etc.) is common for all on-chip devices, so is handled
> in common code at the subsystem level (in this case, platform_bus.)
> 
> Therefore, what is ideally needed is the ability for A's suspend to
> simply call pm_runtime_suspend() so the subsystem can do the work.
> However, since runtime transitions are locked out by this time, that
> doesn't work.  IOW, what is needed is a way for a system suspend to say
> "please finish the runtime suspend that was already requested."

Calling the runtime_suspend method directly is the way to do it.

> What I've done to work around this in driver A is to manually check
> pm_runtime_suspended() and directly call the subsystem's runtime
> suspend/resume (patch below[1].  NOTE, I've used the _noirq methods to
> ensure device A is available when device B needs it.)

Hmm.  The pm_runtime_suspended() check may not be needed (if A were
suspended already then B would have encountered problems).  But
including it doesn't hurt.

Using the _noirq method isn't a good idea, unless you know for certain 
that the runtime_suspend handler doesn't need to sleep.  Using the 
normal suspend method should work okay, because B always has to suspend 
before A.

> While this works, I'm not crazy about it since it requires the driver
> know about the subsystem (in this case the bus) where the real PM work
> is done.  IMO, it would be much more intuitive (and readable) if the
> driver's suspend hooks could simply trigger a runtime suspend (either a
> new one, or one already requested.)

This isn't clear to me.  Isn't the driver registered on the bus in 
question?  Can't the driver therefore call the bus's runtime_suspend 
routine directly, instead of dereferencing the bus->pm->runtime_suspend 
pointer?

Alan Stern

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