Created attachment 373240 [details] [review]
Proposed solution

V4L2 devices may have different inputs, which can be selected with ioctl VIDIOC_S_INPUT.

I'm using v4l2src and I need to change the input which is being used. GStreamer doesn't allow to do it (it used to allow it with GST_TUNER, but not now).

What I've tried is to put the pipeline to null state, make an ioctl S_INPUT to select a different input, and put the pipeline to playing again. However, GStreamer selects again the original input that is started using the first time, undoing the input change I made.

This is what I want to do:

    int input;

    // pipeline to null
    gst_element_set_state(pipeline, GST_STATE_NULL);
    
    // open video file descriptor
    int fd = open("/dev/video1", O_RDWR);

    // check current input
    ioctl(fd, VIDIOC_G_INPUT, &input);
    std::cout << "Current input: " << input << std::endl;

    // set new input
    input = NEW_INPUT;
    ioctl(fd, VIDIOC_S_INPUT, &input);
    
    // check that change is applied 
    ioctl(fd, VIDIOC_G_INPUT, &input);
    std::cout << "Current input (pre-play): " << input << std::endl;
    
    // close file descriptor
    close(fd);
    
    // pipeline to playing and again to null
    gst_element_set_state (pipeline, GST_STATE_PLAYING);
    gst_element_set_state(pipeline, GST_STATE_NULL);

    // check if input is still the new one
    fd = open("/dev/video1", O_RDWR);
    ioctl(fd, VIDIOC_G_INPUT, &input);
    std::cout << "Current input (post-play): " << input << std::endl;
    close(fd);

Now I get this output:
    Current input: 0
    Current input (pre-play): 1
    Current input (post-play): 0


I attach a patch with my proposed solution. When v4l2src changes to NULL state, it forget the previous input, making that next time it changes to PLAYING state, it checks again for the current selected input.

I think this is the better behavior, considering that GStreamer doesn't allow to select an input and the first time it selects the currently selected one, I would expect the next times to do the same. Also, if not done like this, it is not possible for the user to select a different input reusing the same pipeline.

Different inputs of the same device might have different formats, framesizes, etc, so for example I have had to change my selected caps for v4l2src, but that's something that the user have to take care of it.

Also, it would be a good enhancement to allow selecting the input as a property of v4l2src (gst-launch-1.0 v4l2src device=/dev/video1 input=1 ! autovideosink). I haven't used v4l2sink, but I guess that the same could apply to its outputs.