I've written a patch Vojtech and I discussed for enhancing the
hiddev code to optionally provide more detailed output on read().
The old functionality is still supported by default, and in
situations where HID usage codes are unique across reports, the
old method is still preferable due to its terseness.
    
  The new method provides the ability to determine exactly which
value has changed, in cases where the HID usage codes are not  
unique.  It also provides a means to optionally receive notification
when input reports are received from the device, whether or not
any of the values in the report have changed.

  The details of the changes are as follows:
  
  - All current code behaves identically

  - A new ioctl pair HIDIOCGFLAG/HIDIOCSFLAG gets and clears
    flags on the hiddev device.                             

  - If you set the flag HIDDEV_FLAG_UREF, the read() call switches
    from reading hiddev_event structures to hiddev_usage_ref
    structures.  The change takes effect immediately, even to
    already queued events that haven't been read() yet.  Here's
    an example of enabling FLAG_UREF:                          

    {
        int flag = HIDDEV_FLAG_UREF;
        if (ioctl(fd, HIDIOCSFLAG, &flag) != 0) {
                perror("ioctl");
                exit(1);
        }
    }
  
  - With the HIDDEV_FLAG_REPORT set (which is only allowed if
    HIDDEV_FLAG_UREF is also set), there is a special uref that
    will be read() in addition to the ones corresponding to
    changes in the device state: when uref.field_index is set to
    HID_FIELD_INDEX_NONE, this uref is a notification that the
    report referred to by report_type and report_id has been
    received from the device.  This can be useful in situations
    when the notification of the arrival of a report is useful
    even if there is no change in state.