Subject: Re: uvcvideo logging kernel warnings on device disconnect
To: Greg KH <greg@kroah.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>
References: <ab3241e7-c525-d855-ecb6-ba04dbdb030f@destevenson.freeserve.co.uk>
 <3934137.UccFJV1Tl7@avalon> <20161209091113.GB27160@kroah.com>
 <2080235.u14sVkzQLZ@avalon> <20161209094304.GB4755@kroah.com>
Cc: linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
From: Dave Stevenson <linux-media@destevenson.freeserve.co.uk>
Message-ID: <01726e81-bbc2-b9a0-b2f0-045e3208f7b2@destevenson.freeserve.co.uk>
Date: Tue, 20 Dec 2016 11:19:23 +0000
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Hi Greg.

On 09/12/16 09:43, Greg KH wrote:
> 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.

I hate to pester, but wondered if you had found anything obvious.
I really do appreciate you taking the time to look.

Thanks.
   Dave