N-Trig touchscreen Driver
  -------------------------
  	Copyright (c) 2008-2010 Rafi Rubin <rafi@seas.upenn.edu>
  	Copyright (c) 2009-2010 Stephane Chatty
  
  This driver provides support for N-Trig pen and multi-touch sensors.  Single
  and multi-touch events are translated to the appropriate protocols for
  the hid and input systems.  Pen events are sufficiently hid compliant and
  are left to the hid core.  The driver also provides additional filtering
  and utility functions accessible with sysfs and module parameters.
  
  This driver has been reported to work properly with multiple N-Trig devices
  attached.
  
  
  Parameters
  ----------
  
  Note: values set at load time are global and will apply to all applicable
  devices.  Adjusting parameters with sysfs will override the load time values,
  but only for that one device.
  
  The following parameters are used to configure filters to reduce noise:
  
  activate_slack		number of fingers to ignore before processing events
  
  activation_height	size threshold to activate immediately
  activation_width
  
  min_height		size threshold bellow which fingers are ignored
  min_width		both to decide activation and during activity
  
  deactivate_slack	the number of "no contact" frames to ignore before
  			propagating the end of activity events
  
  When the last finger is removed from the device, it sends a number of empty
  frames.  By holding off on deactivation for a few frames we can tolerate false
  erroneous disconnects, where the sensor may mistakenly not detect a finger that
  is still present.  Thus deactivate_slack addresses problems where a users might
  see breaks in lines during drawing, or drop an object during a long drag.
  
  
  Additional sysfs items
  ----------------------
  
  These nodes just provide easy access to the ranges reported by the device.
  sensor_logical_height	the range for positions reported during activity
  sensor_logical_width
  
  sensor_physical_height	internal ranges not used for normal events but
  sensor_physical_width	useful for tuning
  
  All N-Trig devices with product id of 1 report events in the ranges of
  X: 0-9600
  Y: 0-7200
  However not all of these devices have the same physical dimensions.  Most
  seem to be 12" sensors (Dell Latitude XT and XT2 and the HP TX2), and
  at least one model (Dell Studio 17) has a 17" sensor.  The ratio of physical
  to logical sizes is used to adjust the size based filter parameters.
  
  
  Filtering
  ---------
  
  With the release of the early multi-touch firmwares it became increasingly
  obvious that these sensors were prone to erroneous events.  Users reported
  seeing both inappropriately dropped contact and ghosts, contacts reported
  where no finger was actually touching the screen.
  
  Deactivation slack helps prevent dropped contact for single touch use, but does
  not address the problem of dropping one of more contacts while other contacts
  are still active.  Drops in the multi-touch context require additional
  processing and should be handled in tandem with tacking.
  
  As observed ghost contacts are similar to actual use of the sensor, but they
  seem to have different profiles.  Ghost activity typically shows up as small
  short lived touches.  As such, I assume that the longer the continuous stream
  of events the more likely those events are from a real contact, and that the
  larger the size of each contact the more likely it is real.  Balancing the
  goals of preventing ghosts and accepting real events quickly (to minimize
  user observable latency), the filter accumulates confidence for incoming
  events until it hits thresholds and begins propagating.  In the interest in
  minimizing stored state as well as the cost of operations to make a decision,
  I've kept that decision simple.
  
  Time is measured in terms of the number of fingers reported, not frames since
  the probability of multiple simultaneous ghosts is expected to drop off
  dramatically with increasing numbers.  Rather than accumulate weight as a
  function of size, I just use it as a binary threshold.  A sufficiently large
  contact immediately overrides the waiting period and leads to activation.
  
  Setting the activation size thresholds to large values will result in deciding
  primarily on activation slack.  If you see longer lived ghosts, turning up the
  activation slack while reducing the size thresholds may suffice to eliminate
  the ghosts while keeping the screen quite responsive to firm taps.
  
  Contacts continue to be filtered with min_height and min_width even after
  the initial activation filter is satisfied.  The intent is to provide
  a mechanism for filtering out ghosts in the form of an extra finger while
  you actually are using the screen.  In practice this sort of ghost has
  been far less problematic or relatively rare and I've left the defaults
  set to 0 for both parameters, effectively turning off that filter.
  
  I don't know what the optimal values are for these filters.  If the defaults
  don't work for you, please play with the parameters.  If you do find other
  values more comfortable, I would appreciate feedback.
  
  The calibration of these devices does drift over time.  If ghosts or contact
  dropping worsen and interfere with the normal usage of your device, try
  recalibrating it.
  
  
  Calibration
  -----------
  
  The N-Trig windows tools provide calibration and testing routines.  Also an
  unofficial unsupported set of user space tools including a calibrator is
  available at:
  http://code.launchpad.net/~rafi-seas/+junk/ntrig_calib
  
  
  Tracking
  --------
  
  As of yet, all tested N-Trig firmwares do not track fingers.  When multiple
  contacts are active they seem to be sorted primarily by Y position.