[PATCH v2 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer

["Matwey V. Kornilov" <matwey@sai.msu.ru>]

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

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 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