HID: ntrig: add documention

The doctumentation includes a brief introduction to the driver and
explanations of the filtering parameters as well as a discussion
of the need for and working of the filters.
Signed-off-by: Rafi Rubin <rafi@seas.upenn.edu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/input/ntrig.txt

+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.