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 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.

> how this memory is used by this driver, but if it reads from it using
> CPU, uncached mapping might be significantly slower than cached
> mapping of memory allocated with kmalloc().
>
> 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