[ Dropping Mans to preserve his peace-of-mind ] On 09/12/2016 18:56, Vinod Koul wrote: > On Fri, Dec 09, 2016 at 06:34:15PM +0100, Mason wrote: >> On 09/12/2016 18:17, Vinod Koul wrote: >> >>> On Fri, Dec 09, 2016 at 11:25:57AM +0100, Sebastian Frias wrote: >>>> >>>> What concrete solution do you propose? >>> >>> I have already proposed two solutions. >>> >>> A) Request a channel only when you need it. Obviously we can't do virtual >>> channels with this (though we should still use virt-channels framework). >>> The sbox setup and teardown can be done as part of channel request and >>> freeup. PL08x already does this. >>> >>> Downside is that we can only have as many consumers at a time as channels. >>> >>> I have not heard any technical reason for not doing this apart from drivers >>> grab the channel at probe, which is incorrect and needs to be fixed >>> irrespective of the problem at hand. >>> >>> This is my preferred option. >> >> There is one important drawback with this solution. If a driver calls >> dma_request_chan() when no channels are currently available, it will >> get -EBUSY. If there were a flag in dma_request_chan to be put to >> sleep (with timeout) until a channel is available, then it would >> work. But busy waiting in the client driver is a waste of power. > > 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. Regards. -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
