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

[Ezequiel Garcia]

On Tue, 2018-09-11 at 21:58 +0300, Matwey V. Kornilov wrote:
> ÐÑ, 28 ÐÐÐ. 2018 Ð. Ð 10:17, Matwey V. Kornilov <matwey.kornilov@xxxxxxxxx>:
> > 
> > ÐÑ, 21 ÐÐÐ. 2018 Ð. Ð 20:06, Matwey V. Kornilov <matwey@xxxxxxxxxx>:
> > > 
> > > 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>
> > 
> > Ping
> > 

We are already using slab buffers in em28xx, so I think this change
makes sense, until we have proper a non-coherent allocation API.

Acked-by: Ezequiel Garcia <ezequiel@xxxxxxxxxxxxx>

Thanks a lot Matwey for your work.