Hi all, The current API for managing kernel -> userspace is a bit rough around the edges, so I would like to discuss extending the API. First of all an example use case scenarios where the current API falls short. 1) Redirection of USB devices to a virtual machine, qemu, vbox, etc. all have the ability to redirect a USB device to the virtual machine, and they all use usbfs for this. The first thing which will happen here when the user selects a device to redirect is a IOCTL_USBFS_DISCONNECT. This causes the kernel driver to see a device unplug, with no chances for the kernel driver to do anything against this. Now lets say the user does the following: -write a file to a usb flash disk -redirect the flash disk to a vm Currently this will cause the usb mass storage driver to see a disconnect, and any possible still pending writes are lost ... This is IMHO unacceptable, but currently there is nothing we can do to avoid this. 2) So called dual mode cameras are (cheap) stillcams often even without an lcdscreen viewfinder, and battery backed sram instead of flash, which double as a webcam. We have drivers for both the stillcam function of these (in libgphoto2, so using usbfs) as well as for the webcam function (v4l2 kernel drivers). These drivers work well, and are mature. Yet the user experience is rather poor. Under gnome the still-cam contents will be automatically be made available as a "drive" using a gvfs-gphoto2 fuse mount. This however involves sending a disconnect to the v4l2 kernel driver, and thus the /dev/video# node disappearing. So if a user wants to use the device as a webcam he/she needs to first go to nautilus and unmount the gvfs mount. Until that is done the user will simply get a message from an app like cheese that he has no webcam, not even an ebusy error, just that he has no such device. Again not good. ### So what do we need to make this situation better: 1) A usb_driver callback alternative to the disconnect callback, I propose to call this soft_disconnect. This serves 2 purposes a) It will allow the driver to tell the caller that that is not a good idea by returning an error code (think usb mass storage driver and mounted filesystem b) It will allow for example a v4l2 driver to keep its /dev/video node around Note that b) means that the normal disconnect handler should still be called after a soft reconnect on a real disconnect. 2) A usb_driver soft_reconnect callback to match the soft_disconnect 3) A mechanism for a usb_driver to signal a usbfs fd owner of the device it would like the device back. So for example the gvfs mount can be automatically unmounted (if not busy). 4) A IOCTL_USBFS_SOFT_DISCONNECT ioctl which will call the drivers soft_disconnect if it has one, and otherwise fall back to the regular disconnect. 5) A method for a usbfs fd owning app to know the device driver would like the device back. I suggest using poll with POLLIN to signal this. Regards, Hans -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html