On Sun, 31 Jul 2011, Laurent Pinchart wrote: > Hi Alan, > > On Saturday 30 July 2011 15:03:50 Alan Stern wrote: > > On Sat, 30 Jul 2011, Ming Lei wrote: > > > On Sat, Jul 30, 2011 at 7:15 AM, Alan Cooper <alcooperx@xxxxxxxxx> wrote: > > > > I'm seeing an interesting performance problem when using a USB HD > > > > Logitech video cam via UVC and I was hoping for some advice on the > > > > best way to resolve it. When I try to capture video data from the cam > > > > with a simple test app, I can't keep up with a 720x1440 data stream a > > > > 10 FPS. I traced the problem down to the fact that the UCV driver ends > > > > up allocating the USB transfer buffers via usb_alloc_coherent() which, > > > > on our MIPS system, returns buffers in uncached memory. UVC ends up > > > > doing a memcpy of the uncached data to the cached UVC buffers and this > > > > is where the performance is terrible. I did a quick benchmark and > > > > found that memcpy from uncached memory ran at about 3.5MB/s, instead > > > > of ~600MB for cached. > > > > > > Sounds like it is a arch dependent problem, and not much related with > > > usb. > > > > Yes indeed. Coherent buffers are allocated in uncached memory on some > > architectures, which makes them very expensive to access. But other > > architectures can use cached memory, which makes coherent buffers > > less expensive than non-coherent ones since no DMA mapping is required. > > > > This makes for a difficult tradeoff. Historically, I think the balance > > in USB has been tipped in favor of x86. > > > > For something as time-critical as uvcvideo, it might make sense for the > > driver to implement both options in arch-dependent code. > > Indeed, that would make sense. Alan (Cooper), what API should uvcvideo use to > allocate URB buffers on MIPS platform ? There are only two APIs. One is what uvcvideo is already using, the other is kmalloc. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
