Message-ID: <2875371.t6c6mhUP9i@vostro.rjw.lan>
Date:	Wed, 11 Feb 2015 16:39:48 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/5] irqchip: Add DT binding doc for the virtual irq demuxer chip

On Wednesday, February 11, 2015 04:03:17 PM Boris Brezillon wrote:
> On Wed, 11 Feb 2015 16:17:20 +0100
> "Rafael J. Wysocki" <rjw@...ysocki.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 :-().

IRQD_SHARED_TIMER (that needs to be an IRQ flag, not an irqaction one).

And spending time on things that never go in happens a lot in the core
development.  I've sent several versions of the wakeup interrupts handling
rework and then Thomas wrote the final one himself. :-)

The goal is to find a way that will address everyone's needs and that's
not always starightforward.

Rafael

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