[PATCH 2/2] arm-cci: ensure perf synchronisation

[Punit Agrawal]

Mark Rutland <mark.rutland at arm.com> writes:

> On Mon, Jul 04, 2016 at 11:22:05AM +0100, Will Deacon wrote:
>> On Tue, Jun 28, 2016 at 06:50:18PM +0100, Mark Rutland wrote:
>> > Currently the IRQ core is permitted to make the CCI PMU IRQ handler
>> > threaded, and will allow userspace to change the CPU affinity of the
>> > interrupt behind our back. Both of these could violate our
>> > synchronisation requirements with the core perf code, which relies upon
>> > strict CPU affinity and disabling of interrupts to guarantee mutual
>> > exclusion in some cases.
>> 
>> Minor nit, but I think $subject is particularly unhelpful for these two
>> patches. How about "arm-ccX: fix PMU interrupt flags"?
>
> Sure, I'll move over to that wording.
>
>> > @@ -881,7 +881,8 @@ static int pmu_request_irq(struct cci_pmu *cci_pmu, irq_handler_t handler)
>> >  	 * This should allow handling of non-unique interrupt for the counters.
>> >  	 */
>> >  	for (i = 0; i < cci_pmu->nr_irqs; i++) {
>> > -		int err = request_irq(cci_pmu->irqs[i], handler, IRQF_SHARED,
>> 
>> Why is this shared and who is the line shared with? We should check that
>> we don't have contradictory IRQ flags in the other irq request path(s).
>
> Hmm... I thought that was so the driver could request the same IRQ
> multiple times in the case of muxing, but I see we've always had the
> is_duplicate_irq logic.
>
> The IRQF_SHARED flags has also been there since day one, so I'm not sure
> if that's needed for some platform or whether that was added out of
> habit.
>
> Punit, do you recall if/why IRQF_SHARED was used?

AFAIR, there was no platform that needed the flag. The IRQF_SHARED flag
was included to allow sharing in the eventuality that such a platform
did show up.

I worked with the assumption that adding the flag was part of good
practice for a driver that didn't need exclusive interrupt.

Anyways, I haven't come across any platform that's needed that
functionality yet. I think it can be safely dropped.

Thanks,
Punit

>
> I'll take a look at dts and see if I can get rid of it.
>
> Thanks,
> Mark.
>
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