On Fri, Dec 09, 2016 at 07:23:17PM +0100, Mason wrote: > On 09/12/2016 18:56, Vinod Koul wrote: > > Right, but in that case the fallback would be PIO mode, and if that is > > not availble (IIRC some f your devices don't) then reject the usage with > > EAGAIN. > Maybe I'm missing something, but I don't see how that would help. > Take the NAND Flash controller driver, for instance. PIO is not > an option, because the ECC engine is tied to DMA. > And failing with -EAGAIN doesn't help the busy looping situation. > The caller should be put on some kind of queue to wait for a > "channel ready" event. Even without the tie into ECC being an issue it seems like falling back to PIO could produce poor results at the system level - it's likely that the PIO would be very expensive and take longer than waiting would've done, increasing the burden on the CPU and slowing things down overall for userspace.
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