usb: failure to obtain HID descriptor via usb_control_msg

[martin f krafft <madduck@madduck.net>]

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Hi there,

First off, please apologise the cross-post. I will make sure to send
the solution to both lists. Please, if you identify the problem to
be with either project, take the other list out of the reply
recipients.

I am trying to get the HID descriptor of a HID device with
usb_control_msg:

  udev = usb_open(dev);
  usb_set_altinterface(udev, 0);
  usb_claim_interface(udev, 0);
  usb_control_msg(udev, USB_ENDPOINT_IN+1, USB_REQ_GET_DESCRIPTOR,
                  USB_DT_HID, 0, buf, 0x9, USB_TIMEOUT);

However, this fails with the following debug messages (level 3),
even though it was invoked by root:

  Check that you have permissions to write to 002/004 and, if you
    don't, that you set up hotplug
    (http://linux-hotplug.sourceforge.net/) correctly.
  USB error: could not claim interface 0: Device or resource busy
  USB error: error sending control message: Device or resource busy

So I dived into hotplug and ended up adding a line to
/etc/hotplug/usb.usermap to effectively disable the device by
specifying a name unequal to all kernel module names, as advised on
the hotplug webpage:

  nonexistant 0x0 0x06c2 0x0038 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0

My reasoning is that I assume the device to be busy because the HID
kernel driver has assumed control over it (and it's in fact
accessible via /dev/usb/hiddev?). However, it may also be a libusb
problem that I am overseeing

However the result was not very pleasing:

  usb.agent[7712]: Setup hid for USB product 6c2/38/300
  usb.agent[7712]: kernel driver hid already loaded
  usb.agent[7712]: Setup nonexistant for USB product 6c2/38/300

effectively, my addition caused the 'nonexistant' pseudo-driver to
be set up in addition to hid.

How can I disable hotplug setting up 'hid' for this very device?

Thanks for any pointers or tips.

-- 
martin;              (greetings from the heart of the sun.)
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