2018-07-20 14:33 GMT+03:00 Tomasz Figa <tfiga@xxxxxxxxxxxx>: > 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. 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
