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Date:   Sun, 6 Jun 2021 00:56:06 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Benjamin Drung <bdrung@posteo.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Adam Goode <agoode@google.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [PATCH v4] media: uvcvideo: Fix pixel format change for Elgato
 Cam Link 4K
Message-ID: <YLvy9iE2Z+DpMvmg@pendragon.ideasonboard.com>
References: <YLqnU+FYSAcWwaAZ@pendragon.ideasonboard.com>
 <20210605201534.53114-1-bdrung@posteo.de>
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Hi Benjamin,

Thank you for the patch.

On Sat, Jun 05, 2021 at 08:15:36PM +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.
> 
> 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@vger.kernel.org
> Signed-off-by: Benjamin Drung <bdrung@posteo.de>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> (now sending this patch with v4 in the subject instead of falsely v3)
> 
> v2: enhanced the comment describing the quirk
> 
> v3:
> * hardcode ctrl->bmHint to 1
> * Use UVC_DBG_VIDEO instead of UVC_DBG_CONTROL (to match the rest of the
>   file)
> 
> v4:
> * Replace quirk bit by specific check for USB VID:PID test
> 
> I tried setting different values for bmHint, but the response from the
> Cam Link was always 1. So this patch hardcodes ctrl->bmHint to 1 as
> suggested.
> 
> Patch version 4 implements the recommendation of Laurent Pinchart. It
> requires defining the device ID as variable since usb_match_one_id takes
> an pointer to it. In case more Elgato products like Game Capture
> HD 60 S+ (0fd9:006a) are affected, this version is harder to extent.
> 
> Take patch version 3 or 4 depending on which version you prefer. Both
> work and are tested.
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index a777b389a66e..35c3ce0e0716 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -130,6 +130,31 @@ static void uvc_fixup_video_ctrl(struct uvc_streaming *stream,
>  	struct uvc_format *format = NULL;
>  	struct uvc_frame *frame = NULL;
>  	unsigned int i;
> +	static const struct usb_device_id elgato_cam_link_4k = { USB_DEVICE(0x0fd9, 0x0066) };

Let's avoid long line

	static const struct usb_device_id elgato_cam_link_4k = {
		USB_DEVICE(0x0fd9, 0x0066)
	};

> +
> +	/*
> +	 * 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.
> +	 *
> +	 * Latest Elgato Cam Link 4K firmware as of 2021-03-23 needs this quirk.
> +	 * MCU: 20.02.19, FPGA: 67
> +	 */
> +	if (usb_match_one_id(stream->dev->intf, &elgato_cam_link_4k) && ctrl->bmHint > 255) {

Similarly, I'd break this as

	if (usb_match_one_id(stream->dev->intf, &elgato_cam_link_4k) &&
	    ctrl->bmHint > 255) {

> +		__u8 corrected_format_index;

You can use u8 within the kernel.

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

I'd print bFormatIndex with %u as it's an index and thus more readable
as a decimal integer.

If you agree with those small changes, there's no need to resubmit, I
can fold them in when applying the patch.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

> +			ctrl->bmHint, ctrl->bFormatIndex,
> +			1, corrected_format_index);
> +		ctrl->bmHint = 1;
> +		ctrl->bFormatIndex = corrected_format_index;
> +	}
>  
>  	for (i = 0; i < stream->nformats; ++i) {
>  		if (stream->format[i].index == ctrl->bFormatIndex) {

-- 
Regards,

Laurent Pinchart