On Fri, Aug 2, 2019 at 9:13 PM Shik Chen <shik@xxxxxxxxxxxx> wrote: > > Similar to the commit 1161db6776bd ("media: usb: pwc: Don't use coherent > DMA buffers for ISO transfer") [1] for the pwc driver. Use streaming DMA > APIs to transfer buffers and sync them explicitly, because accessing > buffers allocated by usb_alloc_coherent() is slow on platforms without > hardware coherent DMA. > > Tested on x86-64 (Intel Celeron 4305U) and armv7l (Rockchip RK3288) with > Logitech Brio 4K camera at 1920x1080 using the WebRTC sample site [3]. > > | | URB (us) | Decode (Gbps) | CPU (%) | > |------------------|------------|---------------|---------| > | x86-64 coherent | 53 +- 20 | 50.6 | 0.24 | > | x86-64 streaming | 55 +- 19 | 50.1 | 0.25 | > | armv7l coherent | 342 +- 379 | 1.8 | 2.16 | > | armv7l streaming | 99 +- 98 | 11.0 | 0.36 | > > The performance characteristics are improved a lot on armv7l, and > remained (almost) unchanged on x86-64. The code used for measurement can > be found at [2]. > > [1] https://git.kernel.org/torvalds/c/1161db6776bd > [2] https://crrev.com/c/1729133 > [3] https://webrtc.github.io/samples/src/content/getusermedia/resolution/ > > Signed-off-by: Shik Chen <shik@xxxxxxxxxxxx> > --- > The allocated buffer could be as large as 768K when streaming 4K video. > Ideally we should use some generic helper to allocate non-coherent > memory in a more efficient way, such as https://lwn.net/Articles/774429/ > ("make the non-consistent DMA allocator more userful"). > > But I cannot find any existing helper so a simple kzalloc() is used in > this patch. The logic to figure out the DMA addressable GFP flags is > similar to __dma_direct_alloc_pages() without the optimistic retries: > https://elixir.bootlin.com/linux/v5.2.5/source/kernel/dma/direct.c#L96 > > drivers/media/usb/uvc/uvc_video.c | 65 +++++++++++++++++++++---------- > 1 file changed, 45 insertions(+), 20 deletions(-) > > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c > index 8fa77a81dd7f2c..962c35478896c4 100644 > --- a/drivers/media/usb/uvc/uvc_video.c > +++ b/drivers/media/usb/uvc/uvc_video.c > @@ -1539,6 +1539,8 @@ static void uvc_video_complete(struct urb *urb) > * Process the URB headers, and optionally queue expensive memcpy tasks > * to be deferred to a work queue. > */ > + dma_sync_single_for_cpu(&urb->dev->dev, urb->transfer_dma, > + urb->transfer_buffer_length, DMA_FROM_DEVICE); > stream->decode(uvc_urb, buf, buf_meta); > > /* If no async work is needed, resubmit the URB immediately. */ > @@ -1565,18 +1567,51 @@ static void uvc_free_urb_buffers(struct uvc_streaming *stream) > if (!uvc_urb->buffer) > continue; > > -#ifndef CONFIG_DMA_NONCOHERENT > - usb_free_coherent(stream->dev->udev, stream->urb_size, > - uvc_urb->buffer, uvc_urb->dma); > -#else > + dma_unmap_single(&stream->dev->udev->dev, uvc_urb->dma, > + stream->urb_size, DMA_FROM_DEVICE); > kfree(uvc_urb->buffer); > -#endif > - uvc_urb->buffer = NULL; > } > > stream->urb_size = 0; > } > > +static gfp_t uvc_alloc_gfp_flags(struct device *dev) > +{ > + u64 mask = dma_get_mask(dev); > + > + if (dev->bus_dma_mask) > + mask &= dev->bus_dma_mask; > + > + if (mask < DMA_BIT_MASK(32) && IS_ENABLED(CONFIG_ZONE_DMA)) > + return GFP_DMA; > + > + if (mask < DMA_BIT_MASK(64)) { > + if (IS_ENABLED(CONFIG_ZONE_DMA32)) > + return GFP_DMA32; We're hitting issues with this on 64-bit ARM platform, where ZONE_DMA32 is enabled (default), the kmalloc allocation with GFP_DMA32 fails. There is no slab cache for GFP_DMA32, so those calls are expected to fail, AFAICT there are no other (working) kmalloc(..., .. | GFP_DMA32) users in the kernel, so I don't think we want to add a cache. If this helps, some discussion here why the cache wasn't added: https://lore.kernel.org/patchwork/patch/1009563/#1198622 This function looks out of place in a high-level driver, but from your comment above (below ---), I guess we may have to live with that until the kernel provides a better API? For the time being, looking at how __dma_direct_alloc_pages works, could you use alloc_pages_node (or dma_alloc_contiguous?) instead of kmalloc, that would let you use GFP_DMA32 as needed? > + if (IS_ENABLED(CONFIG_ZONE_DMA)) > + return GFP_DMA; > + } > + > + return 0; > +} > + > +static char *uvc_alloc_urb_buffer(struct device *dev, size_t size, > + gfp_t gfp_flags, dma_addr_t *dma_handle) > +{ > + void *buffer = kzalloc(size, gfp_flags | uvc_alloc_gfp_flags(dev)); > + > + if (!buffer) > + return NULL; > + > + *dma_handle = dma_map_single(dev, buffer, size, DMA_FROM_DEVICE); > + if (dma_mapping_error(dev, *dma_handle)) { > + kfree(buffer); > + return NULL; > + } > + > + return buffer; > +} > + > /* > * Allocate transfer buffers. This function can be called with buffers > * already allocated when resuming from suspend, in which case it will > @@ -1607,18 +1642,14 @@ static int uvc_alloc_urb_buffers(struct uvc_streaming *stream, > > /* Retry allocations until one succeed. */ > for (; npackets > 1; npackets /= 2) { > + stream->urb_size = psize * npackets; > + > for (i = 0; i < UVC_URBS; ++i) { > struct uvc_urb *uvc_urb = &stream->uvc_urb[i]; > > - stream->urb_size = psize * npackets; > -#ifndef CONFIG_DMA_NONCOHERENT > - uvc_urb->buffer = usb_alloc_coherent( > - stream->dev->udev, stream->urb_size, > + uvc_urb->buffer = uvc_alloc_urb_buffer( > + &stream->dev->udev->dev, stream->urb_size, > gfp_flags | __GFP_NOWARN, &uvc_urb->dma); > -#else > - uvc_urb->buffer = > - kmalloc(stream->urb_size, gfp_flags | __GFP_NOWARN); > -#endif > if (!uvc_urb->buffer) { > uvc_free_urb_buffers(stream); > break; > @@ -1728,12 +1759,8 @@ static int uvc_init_video_isoc(struct uvc_streaming *stream, > urb->context = uvc_urb; > urb->pipe = usb_rcvisocpipe(stream->dev->udev, > ep->desc.bEndpointAddress); > -#ifndef CONFIG_DMA_NONCOHERENT > urb->transfer_flags = URB_ISO_ASAP | URB_NO_TRANSFER_DMA_MAP; > urb->transfer_dma = uvc_urb->dma; > -#else > - urb->transfer_flags = URB_ISO_ASAP; > -#endif > urb->interval = ep->desc.bInterval; > urb->transfer_buffer = uvc_urb->buffer; > urb->complete = uvc_video_complete; > @@ -1793,10 +1820,8 @@ static int uvc_init_video_bulk(struct uvc_streaming *stream, > > usb_fill_bulk_urb(urb, stream->dev->udev, pipe, uvc_urb->buffer, > size, uvc_video_complete, uvc_urb); > -#ifndef CONFIG_DMA_NONCOHERENT > urb->transfer_flags = URB_NO_TRANSFER_DMA_MAP; > urb->transfer_dma = uvc_urb->dma; > -#endif > > uvc_urb->urb = urb; > } > -- > 2.22.0.770.g0f2c4a37fd-goog >