linux-kernel - Re: [PATCH] driver/perf: Add PMU driver for the ARM DMC-620 memory controller.

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Message-ID: <3efa118a-5c85-6af9-e676-44087f1d398e@arm.com>
Date:   Wed, 1 Apr 2020 12:57:03 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Tuan Phan <tuanphan@...amperecomputing.com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Tuan Phan <tuanphan@...eremail.onmicrosoft.com>
Subject: Re: [PATCH] driver/perf: Add PMU driver for the ARM DMC-620 memory
 controller.

On 2020-04-01 12:27 pm, Mark Rutland wrote:
> On Wed, Apr 01, 2020 at 12:12:23PM +0100, Robin Murphy wrote:
>> On 2020-04-01 11:27 am, Will Deacon wrote:
>>> On Wed, Apr 01, 2020 at 10:52:26AM +0100, Mark Rutland wrote:
>>>> On Tue, Mar 31, 2020 at 03:14:59PM -0700, Tuan Phan wrote:
>>>>> I looked at the SMMUv3 PMU driver and it also uses IRQF_SHARED. SMMUv3
>>>>> PMU and DMC620 PMU are very much similar in which counters can be
>>>>> accessed by any cores using memory map. Any special reasons
>>>>> IRQF_SHARED works with SMMUv3 PMU driver?
>>>>
>>>> No; I believe that is a bug in the SMMUv3 PMU driver. If the IRQ were
>>>> shared, and another driver that held the IRQ changed the affinity,
>>>> things would go very wrong.
>>>
>>> I *think* the idea is that the SMMUv3 PMU driver manages multiple PMCG
>>> devices, which may all share an irq line, rather than the irq line being
>>> shared by some other driver that might change the affinity. So I suspect
>>> dropping IRQF_SHARED will break things.
>>
>> Each PMCG is conceptually a distinct PMU with its own interrupt - for
>> instance, MMU-600 has one PMCG for its TCU and one for each TBU, each with a
>> distinct interrupt output signal. Of course, integrators can and will mash
>> them all together into a single SPI (particularly since they're all part of
>> "the SMMU"), but that boils down to the same case as here.
>>
>> This is going to continue to happen, so we could really do with figuring out
>> a way to let MMIO system PMU drivers properly cope with shared interrupts in
>> general :/
> 
> It does seem so, but I think we can only reasonably do that where it's
> only being shared across instances of the same driver, rather than when
> the IRQ is muxed across completely independent drivers. I'd like to
> avoid that latter case if we can.
> 
> The driver would have to handle migration on a cross-instance basis.
> e.g. all the contexts need to be migrated before the IRQ is, to avoid a
> screaming IRQ on the target CPU, or the IRQ handler on the target racing
> with migration from the source.
> 
> Is there a neat way to do that in a driver without using IRQF_SHARED, so
> that we don't end up accidentally sharing with other drivers? We can
> probably librify the code to handle this under drivers/pmu/.

I can envision a fairly straightforward approach of flipping things 
upside-down such that we register a hotplug instance for the IRQ rather 
than the PMU, then handle the association of PMUs to IRQs internally to 
the driver. I believe I need to support this case in my CMN PMU driver 
too, so I'll prototype something there and see how it looks.

Robin.