[RFC PATCH] arm: imx: Workaround i.MX6 PMU interrupts muxed to one SPI

[tglx@linutronix.de (Thomas Gleixner)]

On Fri, 21 Nov 2014, Daniel Thompson wrote:
> On 20/11/14 23:30, Thomas Gleixner wrote:
> > On Thu, 20 Nov 2014, Daniel Thompson wrote:
> >> +/*
> >> + * The PMU IRQ lines of all cores are muxed onto a single interrupt.
> >> + * Rotate the interrupt around the cores if the current CPU cannot
> >> + * figure out why the interrupt has been triggered.
> >> + */
> >> +static irqreturn_t imx6q_pmu_handler(int irq, void *dev, irq_handler_t handler)
> >> +{
> >> +	irqreturn_t ret = handler(irq, dev);
> >> +	int next;
> >> +
> >> +	if (ret == IRQ_NONE && num_online_cpus() > 1) {
> > 
> > What guarantees that ret == IRQ_HANDLED is a sign for 'this is only
> > for this particular core' interrupt ?
> 
> It isn't guaranteed. We rely on re-entering the interrupt handler if
> more than one PMU is raising the interrupt simultaneously.
> 
> > 
> >> +		next = cpumask_next(smp_processor_id(), cpu_online_mask);
> >> +		if (next > nr_cpu_ids)
> >> +			next = cpumask_next(-1, cpu_online_mask);
> >> +		irq_set_affinity(irq, cpumask_of(next));
> >> +	}
> > 
> > Aside of the fact, that the hardware designers who came up with such a
> > brainfart should be put on sane drugs, this is just silly.
> > 
> > Rotating that thing around will introduce arbitrary latencies and
> > dependencies on other interrupts to be handled.
> 
> To be honest I viewed the only real merits of the rotation workaround to
> be that it is simple and minimally invasive. I am in total agreement
> that there are profiling use cases that it will handle badly (although
> there are a useful set which this workaround is sufficient to support).
> 
> > So if there is really no way to figure out which of the cores is the
> > actual target of the PMU interrupt
> 
> PMU is only accessible via the bus if you are a external debugger
> (signals external to the cluster control the register windowing). From
> the kernel we have to use the co-processor interface and can only see
> our own PMU.
> 
> > then you should simply broadcast
> > that interrupt to a designated per cpu vector async from the one which
> > handles it in the first place and be done with it. That's the only
> > sane option you have.
> 
> As it happens I was planning to do some work on rebroadcasting next
> anyway regardless of this discussion because I can't call
> irq_set_affinity() from a FIQ handler...
> 
> Options I considered to rebroadcast are either direct use of an (new and
> ARM specific?) IPI or use of smp_call_function() from a tasklet. I was

> inclined to rule out the tasklet because it has the potential for far
> greater timing jitter than rotating the affinity (doesn't it?).

You cannot schedule a tasklet from FIQ. The only options you have are:

    - Async IPI (direct call into the architecture code). I have no
      idea whether thats possible on ARm

    - irq_work

Thanks,

	tglx