Re: [RFC PATCH v1] irqchip: add support for SMP irq router

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 6 Jul 2016, Marc Zyngier wrote:
> On 05/07/16 20:24, Thomas Gleixner wrote:
> > On Tue, 5 Jul 2016, Marc Zyngier wrote:
> >> Hardcoded? No way. You simply implement a route allocator in your
> >> driver, assigning them as needed. And yes, if you have more than 24
> >> interrupts, they get muxed.
> > 
> > There is one caveat though. Under some circumstances (think RT) you want to
> > configure which interrupts get muxed and which not. We really should have that
> > option, but yes for anything which has less than 24 autorouting is the way to
> > go.
> 
> Good point. I can see two possibilities for that:
> 
> - either we describe this DT with some form of hint, indicating what are
> the inputs that can be muxed to a single output. Easy, but the DT guys
> are going to throw rocks at me for being Linux-specific.

That's not necessarily Linux specific. The problem arises with any other OS as
well.

> - or we have a way to express QoS in the irq subsystem, and a driver can
> request an interrupt with a "make it fast" flag. Of course, everybody
> and his dog are going to ask for it, and we're back to square one.

That and the driver does not know about the particular application
scenario/system configuration.
 
> Do we have a way to detect which interrupt is more likely to be
> sensitive to muxing? My hunch is that if it is requested with
> IRQF_SHARED, then it is effectively muxable. Thoughts?

That's too late. request_irq happens _after_ the interrupt is set up and the
routing established.

Thanks,

	tglx