Re: [PATCH/RFC] rtc: rtc-twl: Fixed nested IRQ handling in resume from suspend

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Thomas,

On Saturday 13 September 2014 21:12:16 Thomas Gleixner wrote:
> On Sat, 13 Sep 2014, Laurent Pinchart wrote:
> > The TWL RTC interrupt is a double-nested threaded interrupt, handled
> > through the TWL SIH (Secondary Interrupt Handler) and PIH (Primary
> > Interrupt Handler).
> > 
> > When the system is woken up from suspend by a TWL RTC alarm interrupt,
> > the TWL PIH and SIH are enabled first (due to the normal IRQ enabling
> > sequence for the PIH and to the IRQF_EARLY_RESUME flag for the SIH)
> > before the TWL RTC interrupt gets enabled. This results on the interrupt
> > being processed by the TWL primary interrupt handler, forwarded to the
> > nested SIH, and then marked as pending for the RTC by handle_nested_irq
> > called from the SIH.
> > 
> > The RTC interrupt then eventually gets reenabled the kernel, which will
> > try to call its top half interrupt handler. As the interrupt is a nested
> > threaded IRQ, its primary handler has been set to the
> > irq_nested_primary_handler function, which is never supposed to be
> > called and generates a WARN_ON, without waking the IRQ thread up.
> > 
> > Fix this by setting the IRQF_EARLY_RESUME for the TWL RTC interrupt to
> > ensure it gets enabled before the parent handlers try to process it.
> > 
> > This is likely a bit of a hack, I have a feeling that a more generic
> > solution that would fix the problem for all nested threaded IRQs enabled
> > as a wake up source by enable_irq_wake would be better.
> 
> Indeed. It's a hack. This is not the first abuse of IRQF_EARLY_RESUME
> which is used to "fix" ordering issues with nested thread handlers.
> 
> I haven't come around yet to analyze the issue and come up with a proper
> core side mechanism to handle that case. I put it on the "look at it while
> trapped in a tin can" list.

Should this patch be applied in the meantime, or do you think you will be 
trapped in a tin can in the not too distant future ?

-- 
Regards,

Laurent Pinchart