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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
