On 2020-11-24 15:38, Ricardo Ribalda wrote:
On architectures where the is no coherent caching such as ARM use the
dma_alloc_noncontiguos API and handle manually the cache flushing using
dma_sync_single().

With this patch on the affected architectures we can measure up to 20x
performance improvement in uvc_video_copy_data_work().

Signed-off-by: Ricardo Ribalda <riba...@chromium.org>
---
  drivers/media/usb/uvc/uvc_video.c | 74 ++++++++++++++++++++++++++-----
  drivers/media/usb/uvc/uvcvideo.h  |  1 +
  2 files changed, 63 insertions(+), 12 deletions(-)

diff --git a/drivers/media/usb/uvc/uvc_video.c 
b/drivers/media/usb/uvc/uvc_video.c
index a6a441d92b94..9e90b261428a 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -1490,6 +1490,11 @@ static void uvc_video_encode_bulk(struct uvc_urb 
*uvc_urb,
        urb->transfer_buffer_length = stream->urb_size - len;
  }
+static inline struct device *stream_to_dmadev(struct uvc_streaming *stream)
+{
+       return stream->dev->udev->bus->controller->parent;
+}
+
  static void uvc_video_complete(struct urb *urb)
  {
        struct uvc_urb *uvc_urb = urb->context;
@@ -1539,6 +1544,11 @@ 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.
         */
+       if (uvc_urb->pages)
+               dma_sync_single_for_cpu(stream_to_dmadev(stream),
+                                       urb->transfer_dma,
+                                       urb->transfer_buffer_length,
+                                       DMA_FROM_DEVICE);

This doesn't work. Even in iommu-dma, the streaming API still expects to work on physically-contiguous memory that could have been passed to dma_map_single() in the first place. As-is, this will invalidate transfer_buffer_length bytes from the start of the *first* physical page, and thus destroy random other data if lines from subsequent unrelated pages are dirty in caches.

The only feasible way to do a DMA sync on disjoint pages in a single call is with a scatterlist.

Robin.

        stream->decode(uvc_urb, buf, buf_meta);
/* If no async work is needed, resubmit the URB immediately. */
@@ -1566,8 +1576,15 @@ static void uvc_free_urb_buffers(struct uvc_streaming 
*stream)
                        continue;
#ifndef CONFIG_DMA_NONCOHERENT
-               usb_free_coherent(stream->dev->udev, stream->urb_size,
-                                 uvc_urb->buffer, uvc_urb->dma);
+               if (uvc_urb->pages) {
+                       vunmap(uvc_urb->buffer);
+                       dma_free_noncontiguous(stream_to_dmadev(stream),
+                                              stream->urb_size,
+                                              uvc_urb->pages, uvc_urb->dma);
+               } else {
+                       usb_free_coherent(stream->dev->udev, stream->urb_size,
+                                         uvc_urb->buffer, uvc_urb->dma);
+               }
  #else
                kfree(uvc_urb->buffer);
  #endif
@@ -1577,6 +1594,47 @@ static void uvc_free_urb_buffers(struct uvc_streaming 
*stream)
        stream->urb_size = 0;
  }
+#ifndef CONFIG_DMA_NONCOHERENT
+static bool uvc_alloc_urb_buffer(struct uvc_streaming *stream,
+                                struct uvc_urb *uvc_urb, gfp_t gfp_flags)
+{
+       struct device *dma_dev = dma_dev = stream_to_dmadev(stream);
+
+       if (!dma_can_alloc_noncontiguous(dma_dev)) {
+               uvc_urb->buffer = usb_alloc_coherent(stream->dev->udev,
+                                                    stream->urb_size,
+                                                    gfp_flags | __GFP_NOWARN,
+                                                    &uvc_urb->dma);
+               return uvc_urb->buffer != NULL;
+       }
+
+       uvc_urb->pages = dma_alloc_noncontiguous(dma_dev, stream->urb_size,
+                                                &uvc_urb->dma,
+                                                gfp_flags | __GFP_NOWARN, 0);
+       if (!uvc_urb->pages)
+               return false;
+
+       uvc_urb->buffer = vmap(uvc_urb->pages,
+                              PAGE_ALIGN(stream->urb_size) >> PAGE_SHIFT,
+                              VM_DMA_COHERENT, PAGE_KERNEL);
+       if (!uvc_urb->buffer) {
+               dma_free_noncontiguous(dma_dev, stream->urb_size,
+                                      uvc_urb->pages, uvc_urb->dma);
+               return false;
+       }
+
+       return true;
+}
+#else
+static bool uvc_alloc_urb_buffer(struct uvc_streaming *stream,
+                                struct uvc_urb *uvc_urb, gfp_t gfp_flags)
+{
+       uvc_urb->buffer = kmalloc(stream->urb_size, gfp_flags | __GFP_NOWARN);
+
+       return uvc_urb->buffer != NULL;
+}
+#endif
+
  /*
   * Allocate transfer buffers. This function can be called with buffers
   * already allocated when resuming from suspend, in which case it will
@@ -1607,19 +1665,11 @@ 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,
-                               gfp_flags | __GFP_NOWARN, &uvc_urb->dma);
-#else
-                       uvc_urb->buffer =
-                           kmalloc(stream->urb_size, gfp_flags | __GFP_NOWARN);
-#endif
-                       if (!uvc_urb->buffer) {
+                       if (!uvc_alloc_urb_buffer(stream, uvc_urb, gfp_flags)) {
                                uvc_free_urb_buffers(stream);
                                break;
                        }
diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
index a3dfacf069c4..3e3ef1f1daa5 100644
--- a/drivers/media/usb/uvc/uvcvideo.h
+++ b/drivers/media/usb/uvc/uvcvideo.h
@@ -532,6 +532,7 @@ struct uvc_urb {
char *buffer;
        dma_addr_t dma;
+       struct page **pages;
unsigned int async_operations;
        struct uvc_copy_op copy_operations[UVC_MAX_PACKETS];

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