uvcvideo: logitech C920 resets controls during VIDIOC_STREAMON

Hi All

I've been attempting to use the Logitech C920 with the uvcvideo driver.
I set the controls with v4l2-ctl but some of them change during
VIDIOC_STREAMON. My understanding is that the values of controls should be preserved.

Minimal test case:

    #include <linux/videodev2.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <fcntl.h>
    #include <errno.h>
    #include <string.h>
    #include <unistd.h>
    #include <sys/ioctl.h>

    #define DEVICE "/dev/video2"

    int main()
    {
            int fd, type;

            fd = open(DEVICE, O_RDWR | O_CLOEXEC);
            if (fd < 0) {
                    perror("Failed to open " DEVICE "\n");
                    return 1;
            }

            struct v4l2_requestbuffers reqbuf = {
                    .type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
                    .memory = V4L2_MEMORY_MMAP,
                    .count = 1,
            };

            if (-1 == ioctl (fd, VIDIOC_REQBUFS, &reqbuf)) {
                    perror("VIDIOC_REQBUFS");
                    return 1;
            }

            system("v4l2-ctl -d" DEVICE " -l | grep exposure_absolute");

            type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
            if (ioctl (fd, VIDIOC_STREAMON, &type) != 0) {
                    perror("VIDIOC_STREAMON");
                    return 1;
            }

            printf("VIDIOC_STREAMON\n");

            usleep(100000);
            system("v4l2-ctl -d" DEVICE " -l | grep exposure_absolute");

            return 0;
    }

None of the other controls seem to be affected. Note: to get the C920 to report exposure_absolute correctly I also had to make this change to the uvcvideo kernel driver:

drivers/media/usb/uvc/uvc_ctrl.c +2 -1

index a2f4501..e7c805b 100644
@@ -227,7 +227,8 @@ static struct uvc_control_info uvc_ctrls[] = {
                .size           = 4,
                .flags          = UVC_CTRL_FLAG_SET_CUR
                                | UVC_CTRL_FLAG_GET_RANGE
-                               | UVC_CTRL_FLAG_RESTORE,
+                               | UVC_CTRL_FLAG_RESTORE
+                               | UVC_CTRL_FLAG_AUTO_UPDATE,
        },
        {
                .entity         = UVC_GUID_UVC_CAMERA,

The variables seem to be changed when the URBs are Submitted. To investigate I made the following change to the uvc driver:

drivers/media/usb/uvc/uvc_video.c +6 -0

index 3394c34..f2f66f6 100644
@@ -1649,17 +1649,23 @@ static int uvc_init_video(struct uvc_streaming
 *stream, gfp_t gfp_flags)
        if (ret < 0)
                return ret;
 
+       /* No effect: */
+       uvc_ctrl_resume_device(stream->dev);
+
        /* Submit the URBs. */
        for (i = 0; i < UVC_URBS; ++i) {
                ret = usb_submit_urb(stream->urb[i], gfp_flags);
                if (ret < 0) {
                        uvc_printk(KERN_ERR, "Failed to submit URB %u "
                                        "(%d).\n", i, ret);
                        uvc_uninit_video(stream, 1);
                        return ret;
                }
        }
 
+       /* "Fixes" the issue: */
+       uvc_ctrl_resume_device(stream->dev);
+
        return 0;
 }

At this point the backtrace looks something like:

uvc_init_video
uvc_video_enable
uvc_v4l2_do_ioctl (in the case VIDIOC_STREAMON:)

The call to uvc_ctrl_resume_device() has the effect that the v4l2 ctrls which are cached in the kernel get resubmitted to the camera as if it were coming out of suspend/resume.

I've looked at the wireshark capture of USB traffic and I can't find anywhere where the host causes exposure_auto to change. The camera does have a mode where it would change by itself but that is disabled (exposure_auto=1).

I've uploaded the wireshark trace to:

http://williammanley.net/usb-wireshark-streamon.pcapng

I'm guessing that this is a hardware bug. One fix would be modify the driver to save all values at the beginning of STREAMON and then restore them at the end. What do you think?

Thanks

Will
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Hi,

2014-03-05 23:01 GMT+00:00 William Manley <[email protected]>:

Quoted 7 lines

Hi All

I've been attempting to use the Logitech C920 with the uvcvideo driver.
I set the controls with v4l2-ctl but some of them change during
VIDIOC_STREAMON. My understanding is that the values of controls should be preserved.

Quoted 3 lines

The camera does
have a mode where it would change by itself but that is disabled (exposure_auto=1).

This alone doesn't guarantee that exposure absolute is untouched. To do so you need to set V4L2_CID_EXPOSURE_AUTO_PRIORITY to 0 and V4L2_CID_EXPOSURE_AUTO to V4L2_EXPOSURE_MANUAL

Quoted 10 lines · Paulo Assis

Hi,

2014-03-05 23:01 GMT+00:00 William Manley <[email protected]>:

Quoted 7 lines

Hi All

I've been attempting to use the Logitech C920 with the uvcvideo driver.
I set the controls with v4l2-ctl but some of them change during
VIDIOC_STREAMON. My understanding is that the values of controls should be preserved.

[snip]

Quoted 7 lines

Quoted 3 lines

The camera does
have a mode where it would change by itself but that is disabled (exposure_auto=1).

This alone doesn't guarantee that exposure absolute is untouched. To do so you need to set V4L2_CID_EXPOSURE_AUTO_PRIORITY to 0 and V4L2_CID_EXPOSURE_AUTO to V4L2_EXPOSURE_MANUAL

The behaviour is the same whether exposure_auto_priority is set to 1 or
0. I don't think I've explained myself clearly - the exposure time does not change by itself, apart from during VIDIOC_STREAMON:

* When I'm streaming video from the camera it's constant.
* When no data is coming from the camera it's constant

It's only modified during STREAMON and when I explicitly set it with v4l2-ctl.

This doesn't seem like correct behaviour as it breaks the use-case of setting the controls as you want before starting streaming. My workaround is to reset all the controls after calling VIDIOC_STREAMON but this is ugly and you get a few frames at the beginning of the video stream where the settings are set correctly.