From mboxrd@z Thu Jan  1 00:00:00 1970
From: tglx@linutronix.de (Thomas Gleixner)
Date: Fri, 18 Feb 2011 12:42:37 +0100 (CET)
Subject: [PATCH] ARM: gic: use handle_fasteoi_irq for SPIs
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To: linux-arm-kernel@lists.infradead.org
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On Fri, 18 Feb 2011, Will Deacon wrote:
> > >> Right, so to get back to the original discussion about how to handle
> > >> chained handlers if the high-level flow type of the IRQ chip is altered
> > >> it seems that there are two options:
> > >>
> > >> 1.) Update all of the chained handlers to use the new flow-control
> > >> 2.) Retain backwards compatibility if a chained handler decides to
> > >>     use the old method of flow control (specifically, leave an ack
> > >>     implementation in the GIC code after moving to fasteoi).
> > >>
> > >> Obviously, I'd rather go with option (2) and let platforms migrate
> > >> over time if they choose to. Now, given that the ack function is really
> > >> not something you want to do in a virtualised environment (because it
> > >> pokes the distributor), is it worth sticking a
> > >> WARN_ON_ONCE(cpu_has_virtualisation()); in the ack code?
> > >
> > > #2 is less painful and just works. The fasteoi stuff does not use ack
> > > IIRC so it wont hurt.
> > 
> > On an MSM we use handle_percpu_irq for PPIs, if we have ack and eoi we
> > will end up EOI ing the interrupt twice so #2 wont work. Also all the
> > cascaded handlers would have assumed that the ack function masks the
> > interrupt, it is best we fix all of them to use eoi at the end (just
> > like handle_fasteoi_irq). Please tell me how you guys locate such
> > cascaded handlers?
> 
> I don't think the cascaded handlers would have assumed that because ack
> just sends EOI - it doesn't do any masking. We do have a problem with
> the percpu_irq flow though (the GIC reference manual says that EOIing a
> non-active interrupt is UNPREDICTABLE).
> 
> Another easy hack is to set IRQ_PER_CPU in the irq_desc->status for PPI
> interrupts and then check this in the ack routine. It's pretty ugly, but
> it doesn't affect the common case and it at least postpones the platform
> changes.

Conditionals in irq_chip callbacks are almost always a sign of
doom. Don't do that.

How many chained handlers need to be fixed, when the whole gic stuff
switches to eoi ?

Thanks,

	tglx