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.
