Re: [PATCH v2] spi: bcm2835: Enable shared interrupt support

[Florian Fainelli <f.fainelli@gmail.com>]

On 6/15/2020 12:09 PM, Robin Murphy wrote:
> On 2020-06-08 12:41, Lukas Wunner wrote:
>> On Mon, Jun 08, 2020 at 12:11:11PM +0100, Robin Murphy wrote:
>>> And all in code that has at least one obvious inefficiency left on
>>> the table either way.
>>
>> Care to submit a patch to overcome that inefficiency?
> 
> I'll have a quick go, but without any way to measure performance impact
> (or even test for correctness) I don't fancy going too deep based purely
> on disassembly and ARM11 cycle timings.
> 
>>> This thread truly epitomises Knuth's "premature optimisation"
>>> quote... ;)
>>
>> The thread came about because it can be determined at compile time
>> whether the interrupt is going to be shared:
> 
> ...which is exactly my point - "because it can be" is anything but proof
> that avoiding a trivial check makes enough measurable difference to
> justify putting in the effort to do so.
> 
>> On the BCM2835 (Raspberry Pi 1), CONFIG_ARCH_MULTI_V6 is set and this
>> SoC doesn't have multiple bcm2835-spi instances, so no shared interrupt.
>>
>> The question is how to discern BCM2836/BCM2837 (Raspberry Pi 2/3), which
>> do not have multiple instances, and BCM2711 (Raspberry Pi 4) which does.
> 
> Hmm, how much relative importance does that have? On a 700MHz ARM11 it's
> obviously desirable to spend as little time in the IRQ handler as
> possible in order to have time left to do anything else, but on the
> other SoCs even if the IRQ remains permanently asserted it can still
> only consume 25% of the available CPU capacity, at which point the
> impact of 2-3 cycles either way at 1GHz+ seems pretty much immeasurable.
> 
>> The Raspberry Pi Foundation compiles BCM2711 kernels with
>> CONFIG_ARM_LPAE=y,
>> but Florian considered that kludgy as a discriminator and opted for
>> runtime-detection via the compatible string instead.  If you've got
>> a better idea please come forward.
>>
>> Is "optimize shared IRQ support away if IS_ENABLED(CONFIG_ARCH_MULTI_V6),
>> else leave it in" the best we can do?
> 
> In all honesty I'm starting to think it seriously might be :)

Or how about this: we slightly re-structure the interrupt handler such
that all possible interrupt conditions are explicitly handled and
terminate with a return IRQ_HANDLED, and those which are not, including
in the case of a "spurious" (because the interrupt was triggered for
another SPI controller instance), then we finish the function with
IRQ_NONE. This would not impact the performance for the BCM2835/36/37
which would still have a single controller/single interrupt line to handle.
-- 
Florian