[PATCH v5] media: uvcvideo: Fix pixel format change for Elgato Cam Link 4K - Linux-stable-mirror

6 Jun 2021

From: Benjamin Drung bdrung@posteo.de
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 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).
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Drung bdrung@posteo.de
Reviewed-by: Laurent Pinchart laurent.pinchart@ideasonboard.com
Signed-off-by: Laurent Pinchart laurent.pinchart@ideasonboard.com
---
Benjamin, could you double-check the adjustments ? I've also updated the
commit messages to not mention a quirk anymore.
---
 drivers/media/usb/uvc/uvc_video.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
index a777b389a66e..e16464606b14 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -127,10 +127,37 @@ int uvc_query_ctrl(struct uvc_device *dev, u8 query, u8 unit,
 static void uvc_fixup_video_ctrl(struct uvc_streaming *stream,
    struct uvc_streaming_control *ctrl)
 {
+	static const struct usb_device_id elgato_cam_link_4k = {
+		USB_DEVICE(0x0fd9, 0x0066)
+	};
    struct uvc_format *format = NULL;
    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.
+	 *
+	 * Latest Elgato Cam Link 4K firmware as of 2021-03-23 needs this fix.
+	 * 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 = ctrl->bmHint >> 8;
+
+		uvc_dbg(stream->dev, VIDEO,
+			"Correct USB video probe response from {bmHint: 0x%04x, bFormatIndex: %u} to {bmHint: 0x%04x, bFormatIndex: %u}\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) {
    		format = &stream->format[i];
-- 
Regards,

Laurent Pinchart


    