On Wed, 2 Oct 2013, Daniel Mack wrote: > On 02.10.2013 14:01, Sebastian Andrzej Siewior wrote: > > On 10/02/2013 01:14 PM, Daniel Mack wrote: > >> On 02.10.2013 12:49, Sebastian Andrzej Siewior wrote: > > >>> What happens if the device is unplugged while the host is suspended and > >>> not there on resume? > >> > >> That condition is detected and a full teardown of the connected drivers > >> is conducted. Try what happens on your notebook when you do that. > >> Embedded systems should behave just the same. > > > > I had the feeling that the USB device gets disconnected and > > re-enumerated on resume. > > Yes. The device looses its +5V power supply, so it has to be reset and > re-enumerated on resume of course. It's just that USB drivers know about > the state the device has to be put back into after resume, and don't > assume the device was disconnected and reconnected. The latter would > (for usb-storage) also cause paritions to be removed and added. This description may be slightly misleading. According to the USB spec, a device is not supposed to lose its +5V power supply during suspend. Therefore it does not need to be reset and re-enumerated upon resume. (Except that some devices don't handle suspend properly because of buggy firmware; they _do_ need to be reset.) Of course, some systems don't follow the spec. If they can't supply suspend power to the host controller and the USB bus, then the device _will_ need to be reset and re-enumerated. Some USB drivers (those which define a reset_resume method) are able to handle this -- they put the device back into the right state and then act as if nothing had happened. Others aren't; they get unbound and rebound just as though the device really had been disconnected and reconnected. Alan Stern -- 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
