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

Matwey V. Kornilov
2018-07-23T18:07:05Z

2018-07-20 14:57 GMT+03:00 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.

I've tried to strategies:

1) Use dma_unmap and dma_map inside the handler (I suppose this is
similar to how USB core does when there is no URB_NO_TRANSFER_DMA_MAP)

2) Use sync_cpu and sync_device inside the handler (and dma_map only
once at memory allocation)

It is interesting that dma_unmap/dma_map pair leads to the lower
overhead (+1us) than sync_cpu/sync_device (+2us) at x86_64 platform.
At armv7l platform using dma_unmap/dma_map leads to ~50 usec in the
handler, and sync_cpu/sync_device - ~65 usec.

However, I am not sure is it mandatory to call
dma_sync_single_for_device for FROM_DEVICE direction?

>
>>
>> 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

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