Message-ID: <2080235.u14sVkzQLZ@avalon>
Date:   Fri, 09 Dec 2016 11:14:41 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Greg KH <greg@...ah.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

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)

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

-- 
Regards,

Laurent Pinchart

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