Re: [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer

[Matwey V. Kornilov]

ÐÑ, 21 ÐÐÐ. 2018 Ð. Ð 20:06, Matwey V. Kornilov <matwey@xxxxxxxxxx>:
>
> DMA cocherency slows the transfer down on systems without hardware
> coherent DMA.
> Instead we use noncocherent DMA memory and explicit sync at data receive
> handler.
>
> Based on previous commit the following performance benchmarks have been
> carried out. Average memcpy() data transfer rate (rate) and handler
> completion time (time) have been measured when running video stream at
> 640x480 resolution at 10fps.
>
> x86_64 based system (Intel Core i5-3470). This platform has hardware
> coherent DMA support and proposed change doesn't make big difference here.
>
>  * kmalloc:            rate = (2.0 +- 0.4) GBps
>                        time = (5.0 +- 3.0) usec
>  * usb_alloc_coherent: rate = (3.4 +- 1.2) GBps
>                        time = (3.5 +- 3.0) usec
>
> We see that the measurements agree within error ranges in this case.
> So theoretically predicted performance downgrade cannot be reliably
> measured here.
>
> armv7l based system (TI AM335x BeagleBone Black @ 300MHz). This platform
> has no hardware coherent DMA support. DMA coherence is implemented via
> disabled page caching that slows down memcpy() due to memory controller
> behaviour.
>
>  * kmalloc:            rate =  (114 +- 5) MBps
>                        time =   (84 +- 4) usec
>  * usb_alloc_coherent: rate = (28.1 +- 0.1) MBps
>                        time =  (341 +- 2) usec
>
> Note, that quantative difference leads (this commit leads to 4 times
> acceleration) to qualitative behavior change in this case. As it was
> stated before, the video stream cannot be successfully received at AM335x
> platforms with MUSB based USB host controller due to performance issues
> [1].
>
> [1] https://www.spinics.net/lists/linux-usb/msg165735.html
>
> Signed-off-by: Matwey V. Kornilov <matwey@xxxxxxxxxx>

Ping

> ---
>  drivers/media/usb/pwc/pwc-if.c | 57 ++++++++++++++++++++++++++++++++----------
>  1 file changed, 44 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/media/usb/pwc/pwc-if.c b/drivers/media/usb/pwc/pwc-if.c
> index 72d2897a4b9f..1360722ab423 100644
> --- a/drivers/media/usb/pwc/pwc-if.c
> +++ b/drivers/media/usb/pwc/pwc-if.c
> @@ -159,6 +159,32 @@ static const struct video_device pwc_template = {
>  /***************************************************************************/
>  /* Private functions */
>
> +static void *pwc_alloc_urb_buffer(struct device *dev,
> +                                 size_t size, dma_addr_t *dma_handle)
> +{
> +       void *buffer = kmalloc(size, GFP_KERNEL);
> +
> +       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;
> +}
> +
> +static void pwc_free_urb_buffer(struct device *dev,
> +                               size_t size,
> +                               void *buffer,
> +                               dma_addr_t dma_handle)
> +{
> +       dma_unmap_single(dev, dma_handle, size, DMA_FROM_DEVICE);
> +       kfree(buffer);
> +}
> +
>  static struct pwc_frame_buf *pwc_get_next_fill_buf(struct pwc_device *pdev)
>  {
>         unsigned long flags = 0;
> @@ -306,6 +332,11 @@ static void pwc_isoc_handler(struct urb *urb)
>         /* Reset ISOC error counter. We did get here, after all. */
>         pdev->visoc_errors = 0;
>
> +       dma_sync_single_for_cpu(&urb->dev->dev,
> +                               urb->transfer_dma,
> +                               urb->transfer_buffer_length,
> +                               DMA_FROM_DEVICE);
> +
>         /* vsync: 0 = don't copy data
>                   1 = sync-hunt
>                   2 = synched
> @@ -428,16 +459,15 @@ static int pwc_isoc_init(struct pwc_device *pdev)
>                 urb->dev = udev;
>                 urb->pipe = usb_rcvisocpipe(udev, pdev->vendpoint);
>                 urb->transfer_flags = URB_ISO_ASAP | URB_NO_TRANSFER_DMA_MAP;
> -               urb->transfer_buffer = usb_alloc_coherent(udev,
> -                                                         ISO_BUFFER_SIZE,
> -                                                         GFP_KERNEL,
> -                                                         &urb->transfer_dma);
> +               urb->transfer_buffer_length = ISO_BUFFER_SIZE;
> +               urb->transfer_buffer = pwc_alloc_urb_buffer(&udev->dev,
> +                                                           urb->transfer_buffer_length,
> +                                                           &urb->transfer_dma);
>                 if (urb->transfer_buffer == NULL) {
>                         PWC_ERROR("Failed to allocate urb buffer %d\n", i);
>                         pwc_isoc_cleanup(pdev);
>                         return -ENOMEM;
>                 }
> -               urb->transfer_buffer_length = ISO_BUFFER_SIZE;
>                 urb->complete = pwc_isoc_handler;
>                 urb->context = pdev;
>                 urb->start_frame = 0;
> @@ -488,15 +518,16 @@ static void pwc_iso_free(struct pwc_device *pdev)
>
>         /* Freeing ISOC buffers one by one */
>         for (i = 0; i < MAX_ISO_BUFS; i++) {
> -               if (pdev->urbs[i]) {
> +               struct urb *urb = pdev->urbs[i];
> +
> +               if (urb) {
>                         PWC_DEBUG_MEMORY("Freeing URB\n");
> -                       if (pdev->urbs[i]->transfer_buffer) {
> -                               usb_free_coherent(pdev->udev,
> -                                       pdev->urbs[i]->transfer_buffer_length,
> -                                       pdev->urbs[i]->transfer_buffer,
> -                                       pdev->urbs[i]->transfer_dma);
> -                       }
> -                       usb_free_urb(pdev->urbs[i]);
> +                       if (urb->transfer_buffer)
> +                               pwc_free_urb_buffer(&urb->dev->dev,
> +                                                   urb->transfer_buffer_length,
> +                                                   urb->transfer_buffer,
> +                                                   urb->transfer_dma);
> +                       usb_free_urb(urb);
>                         pdev->urbs[i] = NULL;
>                 }
>         }
> --
> 2.16.4
>


--
With best regards,
Matwey V. Kornilov