Re: [linux-pm] Issue: Runtime API usage in wake-up device irq_handler during wakeup from system-wide-suspend.

[Tero Kristo <t-kristo@ti.com>]

Hi,

On Sat, 2011-08-27 at 21:42 +0200, Alan Stern wrote:
> On Sat, 27 Aug 2011, Santosh wrote:
> 
> > On Saturday 27 August 2011 07:31 PM, Alan Stern wrote:
> > > On Sat, 27 Aug 2011, Santosh wrote:
> > >
> > >> I might be wrong here, but after discussion with Govindraj on this
> > >> issue, it seems there is a flaw in the way OMAP chain handler
> > >> handling the child interrupts.
> > >>
> > >> On OMAP, we have special interrupt wakeup source at PRCM level and
> > >> many devices can trigger wakeup from low power via this common
> > >> interrupt source. The common interrupt source upon wakeup from low
> > >> power state, decodes the source of interrupt and based on that
> > >> source, calls the respective device ISR directly.
> > >>
> > >> The issue I see here is, the ISR on _a_ device (UART in this case)
> > >> is happening even before UART resume and DPM resume has been completed.
> > >> If this is the case, then it is surely asking for trouble. Because not
> > >> just clocks, but even driver state machine is already in suspend state
> > >> when the ISR is called.
> > >
> > > If the driver state machine is in the suspend state when the ISR is
> > > called, then the ISR should realize it is handling a wakeup event
> > > instead of a normal I/O event.  All it needs to do is turn off the
> > > interrupt source; the event itself will be taken care of during the
> > > device's resume callback.
> > >
> > Good point but the ISR is called as a function call and not real
> > hardware event so no need to turn-off the source in the child
> > ISR. Parent ISR will clear the source anyways.
> > 
> > But the intention here is to record the event for the child.
> 
> In that case the ISR only has to record the event.
> 
> > I mean for UART wakeup, the received character should be
> > extracted. If not done, UART will loose that character because
> > the event is lost. So not sure how the event will be taken
> > care during resume callback. Could you elaborate bit more on
> > last comment please?
> 
> The resume callback routine must check to see if an event was recorded.
> If one was, the routine must see whether a character was received while 
> the system was asleep and extract the character from the UART.  (It 
> probably won't hurt to do this even if an event wasn't recorded.)
> 
> Alan Stern
> 

After thinking about this problem and looking at possible ways to fix
it, I am planning to change the PRCM chain handler to be a driver, which
gets suspended along with the rest of the system. This means the PRCM
interrupt would get disabled also during this time, and it would be
enabled in the driver->complete() call, which should happen after rest
of the drivers have been able to enable their PM runtime in the
driver->resume() call chain. Do you see any problems with this approach?
The only issue I am seeing myself is if some driver decides to do
resume() in the complete() pm-op and potentially screwing the ordering
here...

-Tero



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