uvcvideo: logitech C920 resets controls during VIDIOC_STREAMON

[William Manley <will@williammanley.net>]

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:

diff --git a/drivers/media/usb/uvc/uvc_ctrl.c
b/drivers/media/usb/uvc/uvc_ctrl.c
index a2f4501..e7c805b 100644
--- a/drivers/media/usb/uvc/uvc_ctrl.c
+++ b/drivers/media/usb/uvc/uvc_ctrl.c
@@ -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:


diff --git a/drivers/media/usb/uvc/uvc_video.c
b/drivers/media/usb/uvc/uvc_video.c
index 3394c34..f2f66f6 100644
--- a/drivers/media/usb/uvc/uvc_video.c
+++ b/drivers/media/usb/uvc/uvc_video.c
@@ -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