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SubjectRe: [PATCH v3 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer
Date
Hi Matwey,

Thank you for the patch.

On Thursday, 9 August 2018 12:33:07 EEST Matwey V. Kornilov wrote:
> 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@sai.msu.ru>
> ---
> drivers/media/usb/pwc/pwc-if.c | 47 ++++++++++++++++++++++++++++-----------
> 1 file changed, 35 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/media/usb/pwc/pwc-if.c b/drivers/media/usb/pwc/pwc-if.c
> index 72d2897a4b9f..17f2015a75bb 100644
> --- a/drivers/media/usb/pwc/pwc-if.c
> +++ b/drivers/media/usb/pwc/pwc-if.c
> @@ -159,6 +159,31 @@ 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) {

There are several violations of the Linux coding style here, could you please
run your patch through checkpatch.pl to fix them ?

> + void* buffer = kmalloc(size, GFP_KERNEL);
> +
> + if (buffer == NULL) {
> + goto fail_kmalloc;
> + }

No need for a goto, you can just return NULL here.

> + *dma_handle = dma_map_single(dev, buffer, size, DMA_FROM_DEVICE);
> + if (dma_mapping_error(dev, *dma_handle)) {
> + goto fail_dma_map_single;

And you can inline the error handling code here as it's used from a single
location.

> + }
> +
> + return buffer;
> +
> +fail_dma_map_single:
> + kfree(buffer);
> +fail_kmalloc:
> + return NULL;
> +}
> +
> +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 +331,8 @@ 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 +455,13 @@ 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 +512,14 @@ 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);
> + if (urb->transfer_buffer) {
> + pwc_free_urb_buffer(&urb->dev->dev, urb->transfer_buffer_length,
> urb->transfer_buffer, urb->transfer_dma); }
> - usb_free_urb(pdev->urbs[i]);
> + usb_free_urb(urb);
> pdev->urbs[i] = NULL;
> }
> }

--
Regards,

Laurent Pinchart



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