Re: pwc over musb: 100% frame drop (lost) on high resolution stream

[Bin Liu <b-liu@ti.com>]

Hi,

On Wed, Jul 20, 2016 at 12:25:44AM +0300, Matwey V. Kornilov wrote:
> 2016-07-19 23:56 GMT+03:00 Bin Liu <b-liu@ti.com>:
> > Hi,
> >
> > On Tue, Jul 19, 2016 at 11:21:17PM +0300, matwey@sai.msu.ru wrote:
> >> Hello,
> >>
> >> I have Philips SPC 900 camera (0471:0329) connected to my AM335x based BeagleBoneBlack SBC.
> >> I am sure that both of them are fine and work properly.
> >> I am running Linux 4.6.4 (my kernel config is available at https://clck.ru/A2kQs ) and I've just discovered, that there is an issue with frame transfer when high resolution formats are used.
> >>
> >> The issue is the following. I use simple v4l2 example tool (taken from API docs), which source code is available at http://pastebin.com/grcNXxfe
> >>
> >> When I use (see line 488) 640x480 frames
> >>
> >>                 fmt.fmt.pix.width       = 640;
> >>                 fmt.fmt.pix.height      = 480;
> >>
> >> then I get "select timeout" and don't get any frames.
> >>
> >> When I use 320x240 frames
> >>
> >>                 fmt.fmt.pix.width       = 320;
> >>                 fmt.fmt.pix.height      = 240;
> >>
> >> then about 60% frames are missed. An example outpout of ./a.out -f is available at https://yadi.sk/d/aRka8xWPtSc4y
> >> It looks like there are pauses between bulks of frames (frame counter and timestamp as returned from v4l2 API):
> >>
> >> 3 3705.142553
> >> 8 3705.342533
> >> 13 3705.542517
> >> 110 3708.776208
> >> 115 3708.976190
> >> 120 3709.176169
> >> 125 3709.376152
> >> 130 3709.576144
> >> 226 3712.807848
> >>
> >> When I use tiny 160x120 frames
> >>
> >>                 fmt.fmt.pix.width       = 160;
> >>                 fmt.fmt.pix.height      = 120;
> >>
> >> then more frames are received. See output example at https://yadi.sk/d/DedBmH6ftSc9t
> >> That is why I thought that everything was fine in May when used tiny xawtv window to check kernel OOPS presence (see http://www.spinics.net/lists/linux-usb/msg141188.html for reference)
> >>
> >> Even more. When I introduce USB hub between the host and the webcam, I can not receive even any 320x240 frames.
> >>
> >> I've managed to use ftrace to see what is going on when no frames are received.
> >> I've found that pwc_isoc_handler is called frequently as the following:
> >>
> >>  0)               |  pwc_isoc_handler [pwc]() {
> >>  0)               |    usb_submit_urb [usbcore]() {
> >>  0)               |      usb_submit_urb.part.3 [usbcore]() {
> >>  0)               |        usb_hcd_submit_urb [usbcore]() {
> >>  0)   0.834 us    |          usb_get_urb [usbcore]();
> >>  0)               |          musb_map_urb_for_dma [musb_hdrc]() {
> >>  0)   0.792 us    |            usb_hcd_map_urb_for_dma [usbcore]();
> >>  0)   5.750 us    |          }
> >>  0)               |          musb_urb_enqueue [musb_hdrc]() {
> >>  0)   0.750 us    |            _raw_spin_lock_irqsave();
> >>  0)               |            usb_hcd_link_urb_to_ep [usbcore]() {
> >>  0)   0.792 us    |              _raw_spin_lock();
> >>  0)   0.791 us    |              _raw_spin_unlock();
> >>  0) + 10.500 us   |            }
> >>  0)   0.791 us    |            _raw_spin_unlock_irqrestore();
> >>  0) + 25.375 us   |          }
> >>  0) + 45.208 us   |        }
> >>  0) + 51.042 us   |      }
> >>  0) + 56.084 us   |    }
> >>  0) + 61.292 us   |  }
> >>
> >> However, pwc_isoc_handler never calls vb2_buffer_done() that is why I get "select timeout" in userspace.
> >> Unfortunately, my kernel is not compiled with CONFIG_USB_PWC_DEBUG=y but I can recompile it, if you think that it could provide more information. I am also ready to perform additional tests (use usbmon maybe?).
> >>
> >> How could this issue be resolved?
> >>
> >> Thank you.
> >
> > Do you have CPPI DMA enabled? If so I think you might hit on a known
> > issue in CPPI Isoch transfer, in which the MUSB controller only sends IN
> > tokens in every other SOF, so only half of the bus bandwidth is
> > utilized, which causes video frame drops in higher resolution.
> >
> 
> Yes, I do use DMA:
> 
> CONFIG_USB_TI_CPPI41_DMA=y

Okay.

> 
> > To confirm this, use a bus analyzer to capture a bus trace, you would
> > see no IN tokens in every other SOF while transfering Isoch packets.
> >
> 
> I am sorry, I am new to USB debugging. Do you mean I need to use
> usbmon or some external hardware device?

I barely use usbmon, and not sure if it gives the information I am
looking for. But I meant the external test equipment - USB bus protocol
analyzer - a bus packet sniffer.

Regards,
-Bin,

> 
> > Regards,
> > -Bin.
> >
> 
> 
> 
> -- 
> With best regards,
> Matwey V. Kornilov.
> Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
> 119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382