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.