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+ALPS Touchpad Protocol
+----------------------
+
+Introduction
+------------
+
+Currently the ALPS touchpad driver supports four protocol versions in use by
+ALPS touchpads, called versions 1, 2, 3, and 4. Information about the various
+protocol versions is contained in the following sections.
+
+Detection
+---------
+
+All ALPS touchpads should respond to the "E6 report" command sequence:
+E8-E6-E6-E6-E9. An ALPS touchpad should respond with either 00-00-0A or
+00-00-64.
+
+If the E6 report is successful, the touchpad model is identified using the "E7
+report" sequence: E8-E7-E7-E7-E9. The response is the model signature and is
+matched against known models in the alps_model_data_array.
+
+With protocol versions 3 and 4, the E7 report model signature is always
+73-02-64. To differentiate between these versions, the response from the
+"Enter Command Mode" sequence must be inspected as described below.
+
+Command Mode
+------------
+
+Protocol versions 3 and 4 have a command mode that is used to read and write
+one-byte device registers in a 16-bit address space. The command sequence
+EC-EC-EC-E9 places the device in command mode, and the device will respond
+with 88-07 followed by a third byte. This third byte can be used to determine
+whether the devices uses the version 3 or 4 protocol.
+
+To exit command mode, PSMOUSE_CMD_SETSTREAM (EA) is sent to the touchpad.
+
+While in command mode, register addresses can be set by first sending a
+specific command, either EC for v3 devices or F5 for v4 devices. Then the
+address is sent one nibble at a time, where each nibble is encoded as a
+command with optional data. This enoding differs slightly between the v3 and
+v4 protocols.
+
+Once an address has been set, the addressed register can be read by sending
+PSMOUSE_CMD_GETINFO (E9). The first two bytes of the response contains the
+address of the register being read, and the third contains the value of the
+register. Registers are written by writing the value one nibble at a time
+using the same encoding used for addresses.
+
+Packet Format
+-------------
+
+In the following tables, the following notation is used.
+
+ CAPITALS = stick, miniscules = touchpad
+
+?'s can have different meanings on different models, such as wheel rotation,
+extra buttons, stick buttons on a dualpoint, etc.
+
+PS/2 packet format
+------------------
+
+ byte 0:  0    0 YSGN XSGN    1    M    R    L
+ byte 1: X7   X6   X5   X4   X3   X2   X1   X0
+ byte 2: Y7   Y6   Y5   Y4   Y3   Y2   Y1   Y0
+
+Note that the device never signals overflow condition.
+
+ALPS Absolute Mode - Protocol Verion 1
+--------------------------------------
+
+ byte 0:  1    0    0    0    1   x9   x8   x7
+ byte 1:  0   x6   x5   x4   x3   x2   x1   x0
+ byte 2:  0    ?    ?    l    r    ?  fin  ges
+ byte 3:  0    ?    ?    ?    ?   y9   y8   y7
+ byte 4:  0   y6   y5   y4   y3   y2   y1   y0
+ byte 5:  0   z6   z5   z4   z3   z2   z1   z0
+
+ALPS Absolute Mode - Protocol Version 2
+---------------------------------------
+
+ byte 0:  1    ?    ?    ?    1    ?    ?    ?
+ byte 1:  0   x6   x5   x4   x3   x2   x1   x0
+ byte 2:  0  x10   x9   x8   x7    ?  fin  ges
+ byte 3:  0   y9   y8   y7    1    M    R    L
+ byte 4:  0   y6   y5   y4   y3   y2   y1   y0
+ byte 5:  0   z6   z5   z4   z3   z2   z1   z0
+
+Dualpoint device -- interleaved packet format
+---------------------------------------------
+
+ byte 0:    1    1    0    0    1    1    1    1
+ byte 1:    0   x6   x5   x4   x3   x2   x1   x0
+ byte 2:    0  x10   x9   x8   x7    0  fin  ges
+ byte 3:    0    0 YSGN XSGN    1    1    1    1
+ byte 4:   X7   X6   X5   X4   X3   X2   X1   X0
+ byte 5:   Y7   Y6   Y5   Y4   Y3   Y2   Y1   Y0
+ byte 6:    0   y9   y8   y7    1    m    r    l
+ byte 7:    0   y6   y5   y4   y3   y2   y1   y0
+ byte 8:    0   z6   z5   z4   z3   z2   z1   z0
+
+ALPS Absolute Mode - Protocol Version 3
+---------------------------------------
+
+ALPS protocol version 3 has three different packet formats. The first two are
+associated with touchpad events, and the third is associatd with trackstick
+events.
+
+The first type is the touchpad position packet.
+
+ byte 0:    1    ?   x1   x0    1    1    1    1
+ byte 1:    0  x10   x9   x8   x7   x6   x5   x4
+ byte 2:    0  y10   y9   y8   y7   y6   y5   y4
+ byte 3:    0    M    R    L    1    m    r    l
+ byte 4:    0   mt   x3   x2   y3   y2   y1   y0
+ byte 5:    0   z6   z5   z4   z3   z2   z1   z0
+
+Note that for some devices the trackstick buttons are reported in this packet,
+and on others it is reported in the trackstick packets.
+
+The second packet type contains bitmaps representing the x and y axes. In the
+bitmaps a given bit is set if there is a finger covering that position on the
+given axis. Thus the bitmap packet can be used for low-resolution multi-touch
+data, although finger tracking is not possible.  This packet also encodes the
+number of contacts (f1 and f0 in the table below).
+
+ byte 0:    1    1   x1   x0    1    1    1    1
+ byte 1:    0   x8   x7   x6   x5   x4   x3   x2
+ byte 2:    0   y7   y6   y5   y4   y3   y2   y1
+ byte 3:    0  y10   y9   y8    1    1    1    1
+ byte 4:    0  x14  x13  x12  x11  x10   x9   y0
+ byte 5:    0    1    ?    ?    ?    ?   f1   f0
+
+This packet only appears after a position packet with the mt bit set, and
+ususally only appears when there are two or more contacts (although
+ocassionally it's seen with only a single contact).
+
+The final v3 packet type is the trackstick packet.
+
+ byte 0:    1    1   x7   y7    1    1    1    1
+ byte 1:    0   x6   x5   x4   x3   x2   x1   x0
+ byte 2:    0   y6   y5   y4   y3   y2   y1   y0
+ byte 3:    0    1    0    0    1    0    0    0
+ byte 4:    0   z4   z3   z2   z1   z0    ?    ?
+ byte 5:    0    0    1    1    1    1    1    1
+
+ALPS Absolute Mode - Protocol Version 4
+---------------------------------------
+
+Protocol version 4 has an 8-byte packet format.
+
+ byte 0:    1    ?   x1   x0    1    1    1    1
+ byte 1:    0  x10   x9   x8   x7   x6   x5   x4
+ byte 2:    0  y10   y9   y8   y7   y6   y5   y4
+ byte 3:    0    1   x3   x2   y3   y2   y1   y0
+ byte 4:    0    ?    ?    ?    1    ?    r    l
+ byte 5:    0   z6   z5   z4   z3   z2   z1   z0
+ byte 6:    bitmap data (described below)
+ byte 7:    bitmap data (described below)
+
+The last two bytes represent a partial bitmap packet, with 3 full packets
+required to construct a complete bitmap packet.  Once assembled, the 6-byte
+bitmap packet has the following format:
+
+ byte 0:    0    1   x7   x6   x5   x4   x3   x2
+ byte 1:    0   x1   x0   y4   y3   y2   y1   y0
+ byte 2:    0    0    ?  x14  x13  x12  x11  x10
+ byte 3:    0   x9   x8   y9   y8   y7   y6   y5
+ byte 4:    0    0    0    0    0    0    0    0
+ byte 5:    0    0    0    0    0    0    0  y10
+
+There are several things worth noting here.
+
+ 1) In the bitmap data, bit 6 of byte 0 serves as a sync byte to
+    identify the first fragment of a bitmap packet.
+
+ 2) The bitmaps represent the same data as in the v3 bitmap packets, although
+    the packet layout is different.
+
+ 3) There doesn't seem to be a count of the contact points anywhere in the v4
+    protocol packets. Deriving a count of contact points must be done by
+    analyzing the bitmaps.
+
+ 4) There is a 3 to 1 ratio of position packets to bitmap packets. Therefore
+    MT position can only be updated for every third ST position update, and
+    the count of contact points can only be updated every third packet as
+    well.
+
+So far no v4 devices with tracksticks have been encountered.