Re: uvcvideo logging kernel warnings on device disconnect

[Greg KH]

On Fri, Dec 09, 2016 at 10:59:24AM +0200, Laurent Pinchart wrote:
> Hi Greg,
> 
> On Friday 09 Dec 2016 08:25:52 Greg KH wrote:
> > On Fri, Dec 09, 2016 at 01:09:21AM +0200, Laurent Pinchart wrote:
> > > On Thursday 08 Dec 2016 12:31:55 Dave Stevenson wrote:
> > >> Hi All.
> > >> 
> > >> I'm working with a USB webcam which has been seen to spontaneously
> > >> disconnect when in use. That's a separate issue, but when it does it
> > >> throws a load of warnings into the kernel log if there is a file handle
> > >> on the device open at the time, even if not streaming.
> > >> 
> > >> I've reproduced this with a generic Logitech C270 webcam on:
> > >> - Ubuntu 16.04 (kernel 4.4.0-51) vanilla, and with the latest media tree
> > >> from linuxtv.org
> > >> - Ubuntu 14.04 (kernel 4.4.0-42) vanilla
> > >> - an old 3.10.x tree on an embedded device.
> > >> 
> > >> To reproduce:
> > >> - connect USB webcam.
> > >> - run a simple app that opens /dev/videoX, sleeps for a while, and then
> > >> closes the handle.
> > >> - disconnect the webcam whilst the app is running.
> > >> - read kernel logs - observe warnings. We get the disconnect logged as
> > >> it occurs, but the warnings all occur when the file descriptor is
> > >> closed. (A copy of the logs from my Ubuntu 14.04 machine are below).
> > >> 
> > >> I can fully appreciate that the open file descriptor is holding
> > >> references to a now invalid device, but is there a way to avoid them? Or
> > >> do we really not care and have to put up with the log noise when doing
> > >> such silly things?
> > > 
> > > This is a known problem, caused by the driver core trying to remove the
> > > same sysfs attributes group twice.
> > 
> > Ick, not good.
> > 
> > > The group is first removed when the USB device is disconnected. The input
> > > device and media device created by the uvcvideo driver are children of the
> > > USB interface device, which is deleted from the system when the camera is
> > > unplugged. Due to the parent-child relationship, all sysfs attribute
> > > groups of the children are removed.
> > 
> > Wait, why is the USB device being removed from sysfs at this point,
> > didn't the input and media subsystems grab a reference to it so that it
> > does not disappear just yet?
> 
> References are taken in uvc_prove():
> 
>         dev->udev = usb_get_dev(udev);
>         dev->intf = usb_get_intf(intf);

s/uvc_prove/uvc_probe/ ?  :)

> 
> and released in uvc_delete(), called when the last video device node is 
> closed. This prevents the device from being released (freed), but device_del() 
> is synchronous to device unplug as far as I understand.

Ok, good, that means the UVC driver is doing the right thing here.

But the sysfs files should only be attempted to be removed by the driver
core once, when the device is removed from sysfs, not twice, which is
really odd.

Is there a copy of the "simple app that grabs the device node" anywhere
so that I can test it out here with my USB camera device to try to track
down where the problem is?

thanks,

greg k-h