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. > 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
