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Date: Mon, 22 Jul 2013 14:58:59 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] usb: a problem of using libusb for usb pass through
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To: "Gonglei (Arei)" <arei.gonglei@huawei.com>
Cc: Linqiangmin <linqiangmin@huawei.com>, Hans de Goede <hdegoede@redhat.com>, Luonengjun <luonengjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Huangweidong (Hardware)" <huangweidong@huawei.com>

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?

[ qemu uses one global libusb_context.  libusb_device and
  libusb_device_handle are allocated and released on open/close ]

cheers,
  Gerd