On Sun, 17 May 2015, Tom Mises wrote: > Hi, > > I posted this on linux-uvc-devel, but was told the data transfer part > is handled by the USB driver. > > I am running 3.17.8 kernel with DMA enabled on TI DM3730 (Gumstix > Overo Firestorm-P COM). When streaming video frames in mmap or userptr > mode, I am > seeing very high CPU usage, up to 70%. This CPU usage is reported to > be in software interrupts. The captured frames are NOT corrupted. > > Interestingly, when running 3.5 kernel, with DMA enabled as well, the > CPU usage is 30% and it is reported to be mostly in the user space. > So, it guess it suggests PIO not DMA is actually used here. EHCI always uses DMA. It cannot use PIO. > Neither of these results are satisfactory, I hoped DMA would take care > of the data transfer and the CPU resources would be available to run > data processing. > > I used the video capture example from the V4L2 API documentation. This > behavior was tested and observed for two different cameras, one of > them working in isochronous mode, the other working in bulk mode. > > Interrupts showing activity during the streaming are: > omap-dma-engine, > ehci_hcd:usb1. > > I also ran another test with very low resolution and fps > settings, 160x90 5ps YUV422. In that case, on 3.5 kernel, CPU usage was > very low. However, on 3.17.8 kernel, it was still similar as before, > around 60% and all in software interrupts; it was dropping to zero > intermittently this time. > > Perhaps, the problem is somewhere in the interaction between the UVC > driver and the DMA or USB driver. As far as I understand, the UVC > driver uses vmalloc(), which, again AFAIU, is not ideal for DMA > transfers. Is this known to work on other similar platforms? I haven't heard complaints about the UVC driver. > I am aware this is a somewhat vague > description unfortunately, but I am not sure what other detail would > be helpful. UVC debugging messages do not show anything unusual. > Beyond that, debugging these drivers on my own exceeds my expertise, > so I will welcome any suggestions. You might be able to learn more from ftrace. See the instructions in Documentation/trace/ftrace.txt. The "irqsoff" tracer may be the best one to try. > There is also the following error message from EDMA, I do not think it > is related though. > [ 0.557342] edma-dma-engine edma-dma-engine.0: Can't allocate PaRAM > dummy slot > [ 0.557434] edma-dma-engine: probe of edma-dma-engine.0 failed with error -22 It seems unlikely to be related. 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
