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

[Mark Brown]

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.

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