linux-kernel - Re: [RFC PATCH v1 2/9] genirq: Support shared per_cpu

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Date:   Tue, 26 Sep 2017 14:00:34 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paul Burton <paul.burton@...tec.com>
cc:     Ralf Baechle <ralf@...ux-mips.org>, dianders@...omium.org,
        James Hogan <james.hogan@...tec.com>,
        Brian Norris <briannorris@...omium.org>,
        Jason Cooper <jason@...edaemon.net>, jeffy.chen@...k-chips.com,
        Marc Zyngier <marc.zyngier@....com>,
        linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org,
        tfiga@...omium.org
Subject: Re: [RFC PATCH v1 2/9] genirq: Support shared per_cpu_devid
 interrupts

On Mon, 25 Sep 2017, Thomas Gleixner wrote:

> On Thu, 7 Sep 2017, Paul Burton wrote:
> > Up until now per_cpu_devid interrupts have not supported sharing. On
> > MIPS we have some percpu interrupts which are shared in many systems -
> > a single CPU interrupt line may be used to indicate a timer interrupt,
> > performance counter interrupt or fast debug channel interrupt. We have
> > up until now supported this with a series of hacks, wherein drivers call
> > each other's interrupt handlers & our MIPS GIC irqchip driver includes a
> > hack which configures the interrupt(s) for all CPUs. In order to allow
> > this mess to be cleaned up, this patch introduces support for shared
> > per_cpu_devid interrupts.
> > 
> > The major portion of this is supporting per_cpu_devid interrupts in
> > __handle_irq_event_percpu() and then making use of this, via
> > handle_irq_event_percpu(), from handler_percpu_devif_irq() to invoke the
> > handler for all actions associated with the shared interrupt. This does
> > have a few side effects worth noting:
> > 
> >  - per_cpu_devid interrupts will now add to the entropy pool via
> >    add_interrupt_randomness(), where they previously did not.
> > 
> >  - per_cpu_devid interrupts will record timings when IRQS_TIMINGS is
> >    set, via record_irq_time(), where they previously did not.
> > 
> >  - per_cpu_devid interrupts will handle an IRQ_WAKE_THREAD return from
> >    their handlers to wake a thread, where they previously did not.
> 
> That's broken because it lacks the magic synchronization which is described
> in the comment in __irq_wake_thread().

Aside of that to make that work at all would require per cpu threads and
not a single systemwide thread.

Thanks,

	tglx