Re: [PATCH] media: uvcvideo: Fix pixel format change for Elgato Cam Link 4K

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Mar 25, 2021 at 10:34:59PM +0100, Benjamin Drung wrote:
> 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 add a quirk to correct the malformed data structure.
> 
> The quirk 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).
> 
> Signed-off-by: Benjamin Drung <bdrung@posteo.de>
> ---
> 

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>