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
