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

[Tomasz Figa]

On Fri, Jul 20, 2018 at 8:23 PM Matwey V. Kornilov <matwey@xxxxxxxxxx> wrote:
>
> 2018-07-20 13:55 GMT+03:00 Tomasz Figa <tfiga@xxxxxxxxxxxx>:
> > On Wed, Jul 18, 2018 at 5:51 AM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> >>
> >> On Tue, 17 Jul 2018, Ezequiel Garcia wrote:
> >>
> >> > Hi Matwey,
> >> >
> >> > First of all, sorry for the delay.
> >> >
> >> > Adding Alan and Hans. Guys, do you have any feedback here?
> >>
> >> ...
> >>
> >> > > > So, what is the benefit of using consistent
> >> > > > for these URBs, as opposed to streaming?
> >> > >
> >> > > I don't know, I think there is no real benefit and all we see is a
> >> > > consequence of copy-pasta when some webcam drivers were inspired by
> >> > > others and development priparily was going at x86 platforms.
> >> >
> >> > You are probably right about the copy-pasta.
> >> >
> >> > >  It would
> >> > > be great if somebody corrected me here. DMA Coherence is quite strong
> >> > > property and I cannot figure out how can it help when streaming video.
> >> > > The CPU host always reads from the buffer and never writes to.
> >> > > Hardware perepherial always writes to and never reads from. Moreover,
> >> > > buffer access is mutually exclusive and separated in time by Interrupt
> >> > > fireing and URB starting (when we reuse existing URB for new request).
> >> > > Only single one memory barrier is really required here.
> >> > >
> >> >
> >> > Yeah, and not setting URB_NO_TRANSFER_DMA_MAP makes the USB core
> >> > create DMA mappings and use the streaming API. Which makes more
> >> > sense in hardware without hardware coherency.
> >>
> >> As far as I know, the _only_ advantage to using coherent DMA in this
> >> situation is that you then do not have to pay the overhead of
> >> constantly setting up and tearing down the streaming mappings.  So it
> >> depends very much on the platform: If coherent buffers are cached then
> >> it's a slight win and otherwise it's a big lose.
> >
> > Isn't it about usb_alloc_coherent() being backed by DMA coherent API
> > (dma_alloc_coherent/attrs()) and ending up allocating uncached (or
> > write-combine) memory for devices with non-coherent DMAs? I'm not sure
>
> Yes, this is what exactly happens at armv7l platforms.

Okay, thanks. So this seems to be exactly the same thing that is
happening in the UVC driver. There is quite a bit of random accesses
to extract some header fields and then a big memcpy into VB2 buffer to
assemble final frame.

If we don't want to pay the cost of creating and destroying the
streaming mapping, we could map (dma_map_single()) once, set
transfer_dma of URB and URB_NO_TRANSFER_DMA_MAP and then just
synchronize the caches (dma_sync_single()) before submitting/after
completing the URB.

Best regards,
Tomasz