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

[Ezequiel Garcia <ezequiel@collabora.com>]

+ Laurent

On Sun, 2018-06-17 at 17:36 +0300, Matwey V. Kornilov wrote:
> DMA cocherency slows the transfer down on systems without hardware
> coherent DMA.
> 
> 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 = (4.4 +- 1.0) GBps
>                        time = (2.4 +- 1.2) usec
>  * usb_alloc_coherent: rate = (4.1 +- 0.9) GBps
>                        time = (2.5 +- 1.0) usec
> 
> We see that the measurements agree well within error ranges in this case.
> So no performance downgrade is introduced.
> 
> armv7l based system (TI AM335x BeagleBone Black). 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 =  (190 +-  30) MBps
>                        time =   (50 +-  10) usec
>  * usb_alloc_coherent: rate =   (33 +-   4) MBps
>                        time = (3000 +- 400) usec
> 
> Note, that quantative difference leads (this commit leads to 5 times
> acceleration) to qualitative behavior change in this case. As it was
> stated before, the video stream can not 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
> 

This is quite interesting! I have receive similar complaints
from users wanting to use stk1160 on BBB and Raspberrys,
without much luck on either, due to insufficient isoc bandwidth.

I'm guessing other ARM platforms could be suffering
from the same issue.

Note that stk1160 and uvcvideo drivers use kmalloc on platforms
where DMA_NONCOHERENT is defined, but this is not the case
on ARM platforms.

So, what is the benefit of using consistent 
for these URBs, as opposed to streaming?

If the choice is simply platform dependent,
can't we somehow detect which mapping should
be prefered?

> Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
> ---
>  drivers/media/usb/pwc/pwc-if.c | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/media/usb/pwc/pwc-if.c b/drivers/media/usb/pwc/pwc-if.c
> index 5775d1f60668..6a3cd9680a7f 100644
> --- a/drivers/media/usb/pwc/pwc-if.c
> +++ b/drivers/media/usb/pwc/pwc-if.c
> @@ -427,11 +427,8 @@ static int pwc_isoc_init(struct pwc_device *pdev)
>  		urb->interval = 1; // devik
>  		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_flags = URB_ISO_ASAP;
> +		urb->transfer_buffer = kmalloc(ISO_BUFFER_SIZE, GFP_KERNEL);
>  		if (urb->transfer_buffer == NULL) {
>  			PWC_ERROR("Failed to allocate urb buffer %d\n", i);
>  			pwc_isoc_cleanup(pdev);
> @@ -491,10 +488,7 @@ static void pwc_iso_free(struct pwc_device *pdev)
>  		if (pdev->urbs[i]) {
>  			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);
> +				kfree(pdev->urbs[i]->transfer_buffer);
>  			}
>  			usb_free_urb(pdev->urbs[i]);
>  			pdev->urbs[i] = NULL;