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

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

2018-07-20 14:33 GMT+03:00 Tomasz Figa <tfiga@chromium.org>:
> On Fri, Jul 20, 2018 at 8:23 PM Matwey V. Kornilov <matwey@sai.msu.ru> wrote:
>>
>> 2018-07-20 13:55 GMT+03:00 Tomasz Figa <tfiga@chromium.org>:
>> > On Wed, Jul 18, 2018 at 5:51 AM Alan Stern <stern@rowland.harvard.edu> 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.

Thank you. Now it is becoming clearer to me. Please allow me some time
to try to sketch the implementation.

>
> Best regards,
> Tomasz
>



-- 
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119234, Moscow, Universitetsky pr-k 13, +7 (495) 9392382