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

[Robin Murphy]

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.