Comment 0 for bug 1932367

The Elgato Cam Link 4K HDMI video capture card reports to support three
different pixel formats, where the first format depends on the connected
HDMI device.

```
$ v4l2-ctl -d /dev/video0 --list-formats-ext
ioctl: VIDIOC_ENUM_FMT
 Type: Video Capture

 [0]: 'NV12' (Y/CbCr 4:2:0)
  Size: Discrete 3840x2160
   Interval: Discrete 0.033s (29.970 fps)
 [1]: 'NV12' (Y/CbCr 4:2:0)
  Size: Discrete 3840x2160
   Interval: Discrete 0.033s (29.970 fps)
 [2]: 'YU12' (Planar YUV 4:2:0)
  Size: Discrete 3840x2160
   Interval: Discrete 0.033s (29.970 fps)
```

Changing the pixel format to anything besides the first pixel format
does not work:

```
$ v4l2-ctl -d /dev/video0 --try-fmt-video pixelformat=YU12
Format Video Capture:
 Width/Height : 3840/2160
 Pixel Format : 'NV12' (Y/CbCr 4:2:0)
 Field : None
 Bytes per Line : 3840
 Size Image : 12441600
 Colorspace : sRGB
 Transfer Function : Rec. 709
 YCbCr/HSV Encoding: Rec. 709
 Quantization : Default (maps to Limited Range)
 Flags :
```

User space applications like VLC might show an error message on the
terminal in that case:

```
libv4l2: error set_fmt gave us a different result than try_fmt!
```

Depending on the error handling of the user space applications, they
might display a distorted video, because they use the wrong pixel format
for decoding the stream.

The Elgato Cam Link 4K responds to the USB video probe
VS_PROBE_CONTROL/VS_COMMIT_CONTROL with a malformed data structure: The
second byte contains bFormatIndex (instead of being the second byte of
bmHint). The first byte is always zero. The third byte is always 1.

The firmware bug was reported to Elgato on 2020-12-01 and it was
forwarded by the support team to the developers as feature request.
There is no firmware update available since then. The latest firmware
for Elgato Cam Link 4K as of 2021-03-23 has MCU 20.02.19 and FPGA 67.

Therefore correct the malformed data structure for this device. The
change was successfully tested with VLC, OBS, and Chromium using
different pixel formats (YUYV, NV12, YU12), resolutions (3840x2160,
1920x1080), and frame rates (29.970 and 59.940 fps).

The fix was reviewed and accepted by the subsystem maintainer and will be included upstream: https://git.linuxtv.org/media_stage.git/commit/?id=4c6e0976295add7f0ed94d276c04a3d6f1ea8f83

Attached a backported patch for Ubuntu. I successfully tested it with Linux 5.11 on Ubuntu 21.04 and Linux 5.8 on Ubuntu 20.10. It should work on older kernel versions as well.