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

[Kevin Hilman <khilman@ti.com>]

Tero Kristo <t-kristo@ti.com> writes:

> 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?

How will the system wakeup from retention or off-mode if the PRCM IRQ is
disabled?

Besides that, I don't like this approach because it leaves a rather long
window during which no PRCM-triggered wakeup events can happen.  This is
not good since we also want potential wakeup events that happen *during*
system suspend to be able to cancel the suspend.

> 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...

Personally, I think Alan's approach is the only scalable approach.  This
has to be handled by the drivers.

If the driver's ISR does a pm_runtime_get_sync() and it fails (in this
case, with -EACCESS since runtime PM is disabled), the driver should
handle that handle as Alan described above.

Kevin