Re: [PATCH v2] media: uvcvideo: Fix pixel format change for Elgato Cam Link 4K

[Laurent Pinchart]

Hi Benjamin,

Thank you for the patch.

On Fri, Jun 04, 2021 at 05:19:42PM +0000, 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.

*sigh* :-( Same vendors are depressingly unable to perform even the most
basic conformance testing.

Thanks for all this analysis and bug reports.

> 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).
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Benjamin Drung <bdrung@xxxxxxxxx>
> ---
> 
> I am sending this patch a fourth time since I got no response and the
> last resend is over a month ago. This time I am including Linus Torvalds
> in the hope to get it reviewed.

The resend got to the top of my mailbox and I had time to review it
before it got burried again. Thanks for not giving up.

>  drivers/media/usb/uvc/uvc_driver.c | 13 +++++++++++++
>  drivers/media/usb/uvc/uvc_video.c  | 21 +++++++++++++++++++++
>  drivers/media/usb/uvc/uvcvideo.h   |  1 +
>  3 files changed, 35 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> index 9a791d8ef200..6ce58950d78b 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -3164,6 +3164,19 @@ static const struct usb_device_id uvc_ids[] = {
>  	  .bInterfaceSubClass	= 1,
>  	  .bInterfaceProtocol	= 0,
>  	  .driver_info		= UVC_INFO_META(V4L2_META_FMT_D4XX) },
> +	/*
> +	 * Elgato Cam Link 4K
> +	 * Latest firmware as of 2021-03-23 needs this quirk.
> +	 * MCU: 20.02.19, FPGA: 67
> +	 */
> +	{ .match_flags		= USB_DEVICE_ID_MATCH_DEVICE
> +				| USB_DEVICE_ID_MATCH_INT_INFO,
> +	  .idVendor		= 0x0fd9,
> +	  .idProduct		= 0x0066,
> +	  .bInterfaceClass	= USB_CLASS_VIDEO,
> +	  .bInterfaceSubClass	= 1,
> +	  .bInterfaceProtocol	= 0,
> +	  .driver_info		= UVC_INFO_QUIRK(UVC_QUIRK_FIX_FORMAT_INDEX) },
>  	/* Generic USB Video Class */
>  	{ USB_INTERFACE_INFO(USB_CLASS_VIDEO, 1, UVC_PC_PROTOCOL_UNDEFINED) },
>  	{ USB_INTERFACE_INFO(USB_CLASS_VIDEO, 1, UVC_PC_PROTOCOL_15) },
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index a777b389a66e..910d22233d74 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -131,6 +131,27 @@ static void uvc_fixup_video_ctrl(struct uvc_streaming *stream,
>  	struct uvc_frame *frame = NULL;
>  	unsigned int i;
>  
> +	/*
> +	 * The response of the Elgato Cam Link 4K is incorrect: 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 UVC 1.5 class specification defines the first five bits in the
> +	 * bmHint bitfield. The remaining bits are reserved and should be zero.
> +	 * Therefore a valid bmHint will be less than 32.
> +	 */
> +	if (stream->dev->quirks & UVC_QUIRK_FIX_FORMAT_INDEX && ctrl->bmHint > 255) {

Given that this is likely not going to affect other devices (at least in
the same way), I'd rather test the USB VID:PID that add a quirk.
Something along the lines of

	if (usb_match_one_id(stream->dev->intf, USB_DEVICE(0x0fd9, 0x0066)) {

> +		__u8 corrected_format_index;
> +
> +		corrected_format_index = ctrl->bmHint >> 8;
> +		uvc_dbg(stream->dev, CONTROL,
> +			"Correct USB video probe response from {bmHint: 0x%04x, bFormatIndex: 0x%02x} to {bmHint: 0x%04x, bFormatIndex: 0x%02x}.\n",
> +			ctrl->bmHint, ctrl->bFormatIndex,
> +			ctrl->bFormatIndex, corrected_format_index);
> +		ctrl->bmHint = ctrl->bFormatIndex;

According to your description above, this will always be 1. Is the third
byte always 1 because the driver always sets bmHint to 1, or would it
have a different value if we set bmHint to something else ? In the first
case I'd hardcode ctrl->bmHint to 1 here.

> +		ctrl->bFormatIndex = corrected_format_index;
> +	}
> +
>  	for (i = 0; i < stream->nformats; ++i) {
>  		if (stream->format[i].index == ctrl->bFormatIndex) {
>  			format = &stream->format[i];
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index cce5e38133cd..cbb4ef61a64d 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -209,6 +209,7 @@
>  #define UVC_QUIRK_RESTORE_CTRLS_ON_INIT	0x00000400
>  #define UVC_QUIRK_FORCE_Y8		0x00000800
>  #define UVC_QUIRK_FORCE_BPP		0x00001000
> +#define UVC_QUIRK_FIX_FORMAT_INDEX	0x00002000
>  
>  /* Format flags */
>  #define UVC_FMT_FLAG_COMPRESSED		0x00000001

-- 
Regards,

Laurent Pinchart