kernel warning __vb2_queue_cancel when streaming is stopped

[k-a-z-u]

Every time I captured several images from the xtion and then closed the device, I got a kernel warning and stack-dump:

[...] WARNING: CPU: x PID: xxxxxx at drivers/media/v4l2-core/videobuf2-core.c:2135 __vb2_queue_cancel+0x117/0x180 [videobuf2_core]()

Following raspberrypi/linux#817 this warning happens from kernel 3.18 onwards (due to new sanity checks within v4l) when not returning all buffers to v4l using vb2_buffer_done().

Adding some debug-output to xtion_kill_urbs I noticed that only 7 of the 8 buffer (aquired by my application) were returned. I only have a very basic knowledge of kernel-modules so I looked into the code of some other webcam modules. Just guessing: The queue contains only the currently unused buffers for later use and we miss to return the currently active buffer? Thus I added the following line:

/* It's important to clear current buffer */
vb2_buffer_done(&endp->active_buffer->vb, VB2_BUF_STATE_ERROR); <<<<
endp->active_buffer = 0;

and the warning disappeared. Is this the correct fix for the issue?

[xqms]

Yes, this looks like a valid fix. I'll check it on Wednesday as well. Thanks for reporting!

[k-a-z-u]

ah.. haven't done my homework properly... forgot to check for active_buffer == 0 and got a kernel panic when: disconnecting the xtion or killing the application currently using it ;)

/* It's important to clear current buffer */
if (endp->active_buffer != 0) {
    vb2_buffer_done(&endp->active_buffer->vb, VB2_BUF_STATE_ERROR);
    endp->active_buffer = 0;
}