How to debug video in the Linux Kernel?

[John Whitmore]

Hello all,

I'm running OpenSUSE Tumbleweed so I'm just using their stock
4.16.12-1-default kernel at the moment. I'm not sure what has changed recently
in their system but I'm having problems with video devices.

This is a laptop with a single built in USB Camera, but for some reason two
devices are created for that device:

$ ls /dev/vid*
/dev/video0  /dev/video1

I don't really use that camera but I then have a HDMI to USB capture device
which I do use. When that's plugged in again I get two devices created, which
I could live with, but the bigger problem is that the video being captured is
not the full resolution of the camera connected.

I'm displaying the camera's video with vlc and it's only displaying about 1/2
of the screen and totally distorted. The capture device is a MAGEWELL:

[ 9033.836808] usb 2-4: new SuperSpeed USB device number 2 using xhci_hcd
[ 9033.857976] usb 2-4: New USB device found, idVendor=2935, idProduct=0001
[ 9033.857983] usb 2-4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 9033.857988] usb 2-4: Product: XI100DUSB-HDMI
[ 9033.857991] usb 2-4: Manufacturer: Magewell
[ 9033.857995] usb 2-4: SerialNumber: A201161110153
[ 9033.859190] uvcvideo: Found UVC 1.00 device XI100DUSB-HDMI (2935:0001)
[ 9033.860466] input: XI100DUSB-HDMI: XI100DUSB-HDMI  as /devices/pci0000:00/0000:00:14.0/usb2/2-4/2-4:1.0/input/input26
[ 9033.862538] hid-generic 0003:2935:0001.0005: hiddev97,hidraw2: USB HID v1.01 Device [Magewell XI100DUSB-HDMI] on usb-0000:00:14.0-4/input4
[ 9034.432710] usbcore: registered new interface driver snd-usb-audio

$ lsusb
Bus 002 Device 002: ID 2935:0001  
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 006: ID 0951:1643 Kingston Technology DataTraveler G3
Bus 001 Device 005: ID 046d:c52b Logitech, Inc. Unifying Receiver
Bus 001 Device 002: ID 06cb:0078 Synaptics, Inc. 
Bus 001 Device 004: ID 8087:0aa7 Intel Corp. 
Bus 001 Device 003: ID 04f2:b5a7 Chicony Electronics Co., Ltd 
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

*MOUSE*

But now four video devices show up in /dev:

$ ls /dev/vid*
/dev/video0  /dev/video1  /dev/video2  /dev/video3

I've tried plugging both a digital camera into the HDMI capture device and the
HDMI output of the laptop but in both cases I'm getting the same result a
fraction of the screen and distorted. I've taken a screenshot and uploaded it
so it might better describe what I'm seeing. So up above in this email I've
added the text "*MOUSE*" my mouse if positioned on that word in the screen
shot, but it doesn't come out in the screen capture as it didn't capture my
mouse pointer. So in the screen capture you can see where my mouse is
positioned over that text about half way down the screen of the laptop. If you
then look at the VLC window which is displaying this captured video you can
see the mouse pointer is right at the bottom of the captured video.

https://pasteboard.co/HoMHNqa.png

Maybe that problem is not in the kernel at all but vlc does display videos
correctly. I've also been through all the vlc settings and can't find anything
that makes a different.

So given all that I'm wondering how I get to the problem with this? Searching
for the words linux and video hasn't given me any insight as to how to locate
the source of the problem.

I'll revert to OpenSUSE Leap and see if I can't get back to having this
working. Still I'd love to know how to resolve this issue, or at least shed
some light on the source of the problem.

John