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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.