Re: [PATCH v4 3/5] irqchip: Add DT binding doc for the virtual irq demuxer chip

[Boris Brezillon]

On Wed, 11 Feb 2015 15:55:47 +0100
"Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx> wrote:

> On Wednesday, February 11, 2015 01:24:37 PM Boris Brezillon wrote:
> > Hi Mark,
> > 
> > On Wed, 11 Feb 2015 11:11:06 +0000
> > Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > 
> > > On Wed, Feb 11, 2015 at 08:53:39AM +0000, Boris Brezillon wrote:
> > > > Hi Mark,
> > > > 
> > > > On Tue, 10 Feb 2015 20:48:36 +0000
> > > > Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > > 
> > > > > On Tue, Feb 10, 2015 at 03:52:01PM +0000, Boris Brezillon wrote:
> > > > > > Hi Mark,
> > > > > > 
> > > > > > On Tue, 10 Feb 2015 15:36:28 +0000
> > > > > > Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > > > > 
> > > > > > > Hi Boris,
> > > > > > > 
> > > > > > > On Thu, Jan 29, 2015 at 10:33:38AM +0000, Boris Brezillon wrote:
> > > > > > > > Add documentation for the virtual irq demuxer.
> > > > > > > > 
> > > > > > > > Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>
> > > > > > > > Acked-by: Nicolas Ferre <nicolas.ferre@xxxxxxxxx>
> > > > > > > > ---
> > > > > > > >  .../bindings/interrupt-controller/dumb-demux.txt   | 41 ++++++++++++++++++++++
> > > > > > > >  1 file changed, 41 insertions(+)
> > > > > > > >  create mode 100644 Documentation/devicetree/bindings/interrupt-controller/dumb-demux.txt
> > > > > > > > 
> > > > > > > > diff --git a/Documentation/devicetree/bindings/interrupt-controller/dumb-demux.txt b/Documentation/devicetree/bindings/interrupt-controller/dumb-demux.txt
> > > > > > > > new file mode 100644
> > > > > > > > index 0000000..b9a7830
> > > > > > > > --- /dev/null
> > > > > > > > +++ b/Documentation/devicetree/bindings/interrupt-controller/dumb-demux.txt
> > > > > > > > @@ -0,0 +1,41 @@
> > > > > > > > +* Virtual Interrupt Demultiplexer
> > > > > > > > +
> > > > > > > > +This virtual demultiplexer simply forward all incoming interrupts to its
> > > > > > > > +enabled/unmasked children.
> > > > > > > > +It is only intended to be used by hardware that do not provide a proper way
> > > > > > > > +to demultiplex a source interrupt, and thus have to wake all their children
> > > > > > > > +up so that they can possibly handle the interrupt (if needed).
> > > > > > > > +This can be seen as an alternative to shared interrupts when at least one
> > > > > > > > +of the interrupt children is a timer (and require the irq to stay enabled
> > > > > > > > +on suspend) while others are not. This will prevent calling irq handlers of
> > > > > > > > +non timer devices while they are suspended.
> > > > > > > 
> > > > > > > This sounds like a DT-workaround for a Linux implementation problem, and
> > > > > > > I don't think this the right way to solve your problem.
> > > > > > 
> > > > > > I understand your concern, but why are you answering while I asked for
> > > > > > DT maintainers reviews for several days (if not several weeks).
> > > > > > 
> > > > > > > 
> > > > > > > Why does this have to be in DT at all? Why can we not fix the core to
> > > > > > > handle these details?
> > > > > > 
> > > > > > We already discussed that with Rob and Thomas, and hiding such a
> > > > > > demuxer chip is not an easy task.
> > > > > > I'm open to any suggestion to do that, though I'd like you (I mean DT
> > > > > > guys) to provide a working implementation (or at least a viable concept)
> > > > > > that would silently demultiplex an irq.
> > > > > > 
> > > > > > > 
> > > > > > > I am very much not keen on this binding.
> > > > > > 
> > > > > > Yes, but do you have anything else to propose.
> > > > > > We're experiencing this warning for 2 releases now, and this is time to
> > > > > > find a solution (even if it's not a perfect one).
> > > > > 
> > > > > Thoughts on the patch below?
> > > > 
> > > > That's pretty much what I proposed in my first attempt to solve this
> > > > problem [1] (except for a few things commented below).
> > > > Anyway, Thomas suggested to go for the "dumb/virt irq demultiplexer"
> > > > approach instead.
> > > 
> > > There is one fundamental difference in that this patch does not require
> > > drivers to be updated (the new flag is only used internally). Which
> > > avoids having to patch every single driver that could possibly be used
> > > in combination with one wanting interrupts during suspend.
> > 
> > Actually, that was one of the requirements expressed by Thomas (Thomas,
> > correct me if I'm wrong).
> > The point was to force shared irq users to explicitly specify that they
> > are mixing !IRQF_NO_SUSPEND and IRQF_NO_SUSPEND because they have no
> > other choice.
> > 
> > With your patch, there's no way to inform users that they are
> > erroneously setting the IRQF_NO_SUSPEND flag on one of their shared
> > interrupt.
> > 
> > > 
> > > Any used which did not explicitly request with IRQF_NO_SUSPEND will not
> > > receive interrupts during suspend.
> > > 
> > > [...]
> > > 
> > > > > +static irqreturn_t __handle_irq_event_percpu(unsigned int irq, struct irqaction *action)
> > > > > +{
> > > > > +	/*
> > > > > +	 * During suspend we must not call potentially unsafe irq handlers.
> > > > > +	 * See suspend_suspendable_actions.
> > > > > +	 */
> > > > > +	if (unlikely(action->flags & IRQF_NO_ACTION))
> > > > > +		return IRQ_NONE;
> > > > 
> > > > Thomas was trying to avoid any new conditional code in the interrupt
> > > > handling path, that's why I added a suspended_action list in my
> > > > proposal.
> > > > Even if your 'unlikely' statement make things better I'm pretty sure it
> > > > adds some latency.
> > > 
> > > I can see that we don't want to add more code here to keep things
> > > clean/pure, but I find it hard to believe that a single bit test and
> > > branch (for data that should be hot in the cache) are going to add
> > > measurable latency to a path that does pointer chasing to get to the
> > > irqaction in the first place. I could be wrong though, and I'm happy to
> > > benchmark.
> > 
> > Again, I don't have enough experience to say this is (or isn't)
> > impacting irq handling latency, I'm just reporting what Thomas told me.
> > 
> > > 
> > > It would be possible to go for your list shuffling approach here while
> > > still keeping the flag internal and all the logic hidden away in
> > > kernel/irq/pm.c. I wasn't sure how actions could be manipulated during
> > > suspend, which made me wary of moving them to a separate list.
> > 
> > Moving them to a temporary list on suspend and restoring them on
> > resume should not be a problem.
> > The only drawback I see is that actions might be reordered after the
> > first resume (anyway, relying on shared irq action order is dangerous
> > IMHO).
> 
> We considered doing that too and saw some drawbacks (in addition to the
> reordering of actions you've mentioned).  It added just too much complexity
> to the IRQ suspend-resume code.
> 
> I, personally, would be fine with adding an IRQ flag to silence the
> warning mentioned by Alexandre.  Something like IRQD_TIMER_SHARED that would
> be set automatically if someone requested IRQF_TIMER | IRQF_SHARED.

Yep, but that won't prevent irq handler from being called (even when
they are suspended), and IIRC, that was one of Thomas' concerns.
This shouldn't be a problem for the at91 platform though (actually, this
is the current behavior).


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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