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SubjectRe: [PATCH v2] spi: bcm2835: Enable shared interrupt support
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On 2020-06-05 14:46, Robin Murphy wrote:
> On 2020-06-05 14:20, Mark Brown wrote:
>> On Fri, Jun 05, 2020 at 12:34:36PM +0100, Robin Murphy wrote:
>>> On 2020-06-04 22:28, Florian Fainelli wrote:
>>
>>>> For the BCM2835 case which is deemed performance critical, we would
>>>> like
>>>> to continue using an interrupt handler which does not have the extra
>>>> comparison on BCM2835_SPI_CS_INTR.
>>
>>> FWIW, if I'm reading the patch correctly, then with sensible codegen
>>> that
>>> "overhead" should amount to a bit test on a live register plus a
>>> not-taken
>>> conditional branch - according to the 1176 TRM that should add up to a
>>> whopping 2 cycles. If that's really significant then I'd have to wonder
>>> whether you want to be at the mercy of the whole generic IRQ stack at
>>> all,
>>> and should perhaps consider using FIQ instead.
>>
>> Yes, and indeed the compiler does seem to manage that.  It *is* non-zero
>> overhead though.
>
> True, but so's the existing level of pointer-chasing indirection that
> with some straightforward refactoring could be taken right out of the
> critical path and confined to just the conditional complete() call.
> That's the kind of thing leaving me unconvinced that this is code where
> every single cycle counts ;)

Ha, and in fact having checked a build out of curiosity, this patch
as-is actually stands to make things considerably worse. At least with
GCC 8.3 and bcm2835_defconfig, bcm2835_spi_interrupt_common() doesn't
get inlined, which means bcm2835_spi_interrupt() pushes/pops a stack
frame and makes an out-of-line call to bcm2835_spi_interrupt_common(),
resulting in massively *more* work than the extra two instructions of
simply inlining the test.

So yes, the overhead of inlining the test vs. the alternative is indeed
non-zero. It's just also negative :D

Robin.

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