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. One solution I was looking at would be to have the UVC driver use dma_alloc_noncoherent() instead of usb_alloc_coherent(). The problem then is that the UVC driver would have to invalidate the buffers before submission and this seems very platform specific. Any suggestions would be greatly appreciated. Thanks Al Cooper -- 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
