On Mon, Jun 08, 2020 at 12:11:11PM +0100, Robin Murphy wrote: > Again, 2 cycles. The overhead of a static key alone is at least 50% of that. > And that's not even considering whether the change in code layout caused by > doubling up the IRQ handler might affect I-cache or branch predictor > behaviour, where a single miss stands to more than wipe out any perceived > saving. And all in code that has at least one obvious inefficiency left on > the table either way. > This thread truly epitomises Knuth's "premature optimisation" quote... ;) In fairness the main reason this driver is so heavily tuned already (and has lead to some really nice improvements in the core) is that there are a number of users hitting 100% CPU utilization driving SPI devices on some of the older RPi hardware, IIRC around IIO type applications mostly. I do tend to agree that this particular optimization is a bit marginal but there has been a lot of effort put into this.
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