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

[Laurent Pinchart]

Hi Matwey,

(CC'ing Christoph)

Thank you for the patch, and my apologies for the late answer. For the record, 
I don't think this should have been blocked by lack of time on my side: even 
if I'm one of the core developers in the subsystem, and possibly have the most 
experience with USB webcams recently due to uvcvideo, patches shouldn't be 
blocked indefinitely by the absence of reply from anyone, me included.

(On a side note, thanks to Ezequiel for pointing my attention to this patch in 
our face to face meeting during ELCE, side channels are very useful in this 
kind of situation, including IRC.)

On Tuesday, 21 August 2018 20:06:29 EET 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 | 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;

As discussed before, we're clearly missing a proper non-coherent memory 
allocation API. As much as I would like to see a volunteer for this, I don't 
think it's a reason to block the performance improvement we get from this 
patch.

This being said, I'm a bit concerned about the allocation of 16kB blocks from 
kmalloc(), and believe that the priority of the non-coherent memory allocation 
API implementation should be increased. Christoph, you mentioned in a recent 
discussion on this topic that you are working on removing the existing non-
coherent DMA allocation API, what is your opinion on how we should gllobally 
solve the problem that this patch addresses ?

> +}
> +
> +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);
> +

As explained before as well, I think we need dma_sync_single_for_device() 
calls, and I know they would degrade performances until we fix the problem on 
the DMA mapping API side. This is not a reason to block the patch either. I 
would appreciate, however, if a comment could be added to the place where 
dma_sync_single_for_device() should be called, to explain the problem.

Is there anyone who would like to volunteer to drive the effort on the DMA 
mapping API side ? I'm unfortunately too short on time to address this myself 
:-(

>  	/* 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;
>  		}
>  	}

The rest of the patch looks fine, so

Reviewed-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>

(if possible with a comment about dma_sync_single_for_device() added)

-- 
Regards,

Laurent Pinchart