On Wed, Jan 27, 2021 at 12:29:08AM +0100, Ricardo Ribalda wrote: > - Is there any platform where dma_alloc_noncontiguos can fail? > This is, !ops->alloc_noncontiguous and !dev->coherent_dma_mask > If yes then we need to add a function to let the driver know in > advance that it has to use the coherent allocator (usb_alloc_coherent > for uvc) dev->coherent_dma_mask is set by the driver. So the only reason why dma_alloc_noncontiguos will fail is because is because it can't allocate any memory. > - In dma_alloc_noncontiguos, on the dma_alloc_pages fallback. If we > have a device where the dma happens in only one direction, could not > get more performance with DMA_FROM/TO_DEVICE instead of > DMA_BIDIRECTIONAL ? Yes, we could probably do that. > > > Then I have tried to use the API, and I have encountered a problem: on > uvcvideo the device passed to the memory allocator is different for > DMA_PAGES and NON_CONTIGUOUS: > https://github.com/ribalda/linux/blob/042cd497739f71c8d4a83a67ee970369e2baca4a/drivers/media/usb/uvc/uvc_video.c#L1236 > > I need to dig a bit tomorrow to figure out why this is, I have > hardware to test both paths, so it should not be too difficult. I always found the USB dma alloc API a little weird, but we might have to follow the scheme of the usb coherent wrappers there.
