Hi Christoph On Wed, Jan 27, 2021 at 4:56 PM . Christoph Hellwig <hch@xxxxxx> wrote: > > 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. I have used the current API here: https://git.kernel.org/pub/scm/linux/kernel/git/ribalda/linux.git/log/?h=uvc-noncontiguous And I think the result is very clean. Great work! I have tested it in X86 and in arm64, with similar performance as the previous patchset. Maybe you want to cherry pick that commit into your series I can also send the patch to the list for review if you prefer so. At least in 5.11 rc5 I the same dmadev worked in arm64 and x86. Best regards! -- Ricardo Ribalda
