linux-kernel - Re: uvcvideo logging kernel warnings on device disconnect

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Date:   Fri, 9 Dec 2016 10:43:04 +0100
From:   Greg KH <greg@...ah.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Dave Stevenson <linux-media@...tevenson.freeserve.co.uk>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: uvcvideo logging kernel warnings on device disconnect

On Fri, Dec 09, 2016 at 11:14:41AM +0200, Laurent Pinchart wrote:
> Hi Greg,
> 
> On Friday 09 Dec 2016 10:11:13 Greg KH wrote:
> > On Fri, Dec 09, 2016 at 10:59:24AM +0200, Laurent Pinchart wrote:
> > > 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/ ?  :)
> 
> Oops :-)
> 
> > > 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?
> 
> Sure. The easiest way is to grab http://git.ideasonboard.org/yavta.git and run
> 
> yavta -c /dev/video0
> 
> (your mileage may vary if you have other video devices)

I'll point it at the correct device, /dev/video0 is built into this
laptop and can't be physically removed :)

> While the application is running, unplug the webcam, and then terminate the 
> application with ctrl-C.

Ok, will try this out this afternoon and let you know how it goes.

thanks,

greg k-h