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

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Wed, 11 Feb 2015 16:17:20 +0100
"Rafael J. Wysocki" <rjw@rjwysocki.net> wrote:

> On Wednesday, February 11, 2015 02:43:45 PM Mark Rutland wrote:
> > [...]
> > 
> > > > > > > +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.
> > > 
> > > Thoughts?
> > 
> > Even if the timer driver does that, we still require the other handlers
> > sharing the line to do the right thing across suspend, no? So either
> > their actions need to be masked at suspend time, or the handlers need to
> > detect when they're called during suspend and return early.
> 
> Well, the issue at hand is about things that share an IRQ with a timer AFAICS.
> 
> That is odd enough already and I'd say everyone in that situation needs to
> be prepared to take the pain (including having to check if the device is not
> suspended in their interrupt handlers).
> 
> And quite frankly they need to do that already, because we've never suspended
> timer IRQs.
> 
> > So for the flag at request time approach to work, all the drivers using
> > the interrupt would have to flag they're safe in that context.
> 
> Something like IRQF_"I can share the line with a timer" I guess?  That wouldn't
> hurt and can be checked at request time even.
> 
> > I'm not averse to having that (only a few drivers shuold be affected and
> > we can sanity check them now).
> 
> Right.

Okay, if everyone agrees on this solution, then I'm fine with that too
(even if I'm a bit disappointed to have spent so much time on this
problem to eventually end-up with a simple IRQF_SHARED_TIMER flag :-().


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