[PATCH v3] media: uvcvideo: Fix pixel format change for Elgato Cam Link 4K

[Benjamin Drung]

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@xxxxxxxxxxxxxxx
Signed-off-by: Benjamin Drung <bdrung@xxxxxxxxx>
---
 drivers/media/usb/uvc/uvc_video.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

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) };
+
+	/*
+	 * 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) {
+		__u8 corrected_format_index;
+
+		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",
+			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) {
-- 
2.27.0