Re: [Qemu-devel] usb: a problem of using libusb for usb pass through

[Gonglei (Arei)]

> -----Original Message-----
> From: Gerd Hoffmann [mailto:address@hidden
> Sent: Wednesday, July 31, 2013 5:47 PM
> To: Hans de Goede
> Cc: Gonglei (Arei); address@hidden; Luonengjun; Huangweidong
> (Hardware); Linqiangmin
> Subject: Re: usb: a problem of using libusb for usb pass through
> 
> On 07/30/13 19:13, Hans de Goede wrote:
> > Hi,
> >
> > On 07/22/2013 02:58 PM, Gerd Hoffmann wrote:
> >> On 07/16/13 10:45, Gonglei (Arei) wrote:
> >>> Hi, Gerd
> >>>     My Qemu version is 1.5.1, and use libusb for usb pass through.
> >>>     I pass through a host usb device to the guest by bus number and
> >>> physical port, when I unplug the usb device from
> >>> the host, and plug in on the same physical port immediately, but the
> >>> usb device don't show up in the guest.
> >>>     The qemu log show:
> >>> libusb_release_interface: -4 [NO_DEVICE]
> >>> libusbx: error [_get_usbfs_fd] libusbx couldn't open USB device
> >>> /dev/bus/usb/002/004: No such file or directory
> >>> libusbx: error [_get_usbfs_fd] libusbx couldn't open USB device
> >>> /dev/bus/usb/002/004: No such file or directory
> >>> libusbx: error [_get_usbfs_fd] libusbx couldn't open USB device
> >>> /dev/bus/usb/002/004: No such file or directory
> >>>
> >>>     I find the reason is that the global libusb_context has recorded
> >>> the usb devices' devnum, but when the host device was unplugged and
> >>> plugged
> >>> from the host, the devnum will change, finally cause the problem.
> >>>     Any ideas ? Thanks!
> >>
> >> Sounds like libusbx doesn't flush the cache on unplug even though it
> >> should.  Hans?
> >
> > This looks like qemu's host-libusb code is re-using the libusb_device
> > handle, what it should do after seeing an
> > unplug (ie ENODEV error), it should close both the handle, and free any
> > devices it may have, then redo
> > the libusb_get_device_list() and you should get a libusb_device with the
> > new address. This is what the
> > usb code in the spice-client does, and it has no problems with sc
> 
> host-libusb.c doesn't do that, and I also can't reproduce the issue.
> Running libusbx-1.0.14 at the moment.
> 

Hi, Gerd. I was using libusbx-1.0.16 when I reported that overhead problem.

-Gonglei

> I get slightly different errors:
> 
> libusb_release_interface: -4 [NO_DEVICE]
> libusbx: error [_open_sysfs_attr] open
> /sys/bus/usb/devices/10-1/bConfigurationValue failed ret=-1 errno=2
> 
> Which comes from host-libusb trying to release the (unplugged) device on
> closing.  They are harmless.
> 
> >> [ qemu uses one global libusb_context.  libusb_device and
> >>    libusb_device_handle are allocated and released on open/close ]
> >
> > That is fine, do you also free the device_list, and re-do the
> > libusb_get_device_list() ?
> 
> Sure.  Each time it looks for new devices it calls libusb_get_device_list().
> 
> cheers,
>   Gerd
>