Re: [PATCH v3 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer

[Laurent Pinchart]

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@xxxxxxxxxx>
> ---
>  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