2018-06-22 15:04 GMT+03:00 Matwey V. Kornilov <matwey@xxxxxxxxxx>: > DMA cocherency slows the transfer down on systems without hardware > coherent DMA. > Ping. > 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 > > Signed-off-by: Matwey V. Kornilov <matwey@xxxxxxxxxx> > --- > 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 72d2897a4b9f..339a285600d1 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; > -- > 2.16.4 > -- With best regards, Matwey V. Kornilov. Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia 119234, Moscow, Universitetsky pr-k 13, +7 (495) 9392382
