APPLIED (unstable/5.13): [PATCH 0/1][SRU][B][F][G][H][I](upstream) media: uvcvideo: Fix pixel format change for Elgato Cam Link 4K

[Andrea Righi]

On Wed, Jun 23, 2021 at 10:16:19PM +0000, Benjamin Drung wrote:
> BugLink: https://bugs.launchpad.net/bugs/1932367
> 
> [Impact]
> 
> 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.
> 
> [Fix]
> 
> 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).
> 
> [Test Case]
> 
> Connect an input device to the Cam Link 4K and try all three pixel
> formats with VLC:
> 
>   vlc v4l2:///dev/video0 --v4l2-chroma=NV12
>   vlc v4l2:///dev/video0 --v4l2-chroma=YU12
> 
> [Other Info]
> 
> 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
> 
> Benjamin Drung (1):
>   media: uvcvideo: Fix pixel format change for Elgato Cam Link 4K
> 
>  drivers/media/usb/uvc/uvc_video.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)

Applied to unstable/5.13.

Thanks,
-Andrea