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

Mark Rutland [off-list ref] writes:

On Mon, Jul 04, 2016 at 11:22:05AM +0100, Will Deacon wrote:

quoted

On Tue, Jun 28, 2016 at 06:50:18PM +0100, Mark Rutland wrote:

quoted

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.

quoted

quoted

@@ -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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