INSN_CONFIG:

The first data element (data[0]) is an ID.

Generic return values:

  Bad ID: EINVAL  (should be EIO?)
  Length is wrong: EINVAL

  EAGAIN: changed arguments, retry (like command_test)


Notes:
  - Should we use the length as an ID? (no, cuz its dumb)
  - Should there be a generic flags parameter? (no)
  - Should trigger source configs return a token? (no)
  - Would it be better to have a special subdevice for trigger
    configuration? (no, because it goes against the "subdevices
    are independent" design.)
  - Should this configure generic triggers and return a token
    to be put into a command, or configure specific triggers
    that are tied to start_src, convert_src, etc.?


ID=0: COMEDI_INPUT
ID=1: COMEDI_OUTPUT
ID=2: open drain input (?)
ID=3: open drain output

  length=1
  chanspec is used to specify channel
  type: SUBD_DIO

  These are used to configure the direction of digital I/O lines.

  Modifications: none

  Errors: none

  Notes:
    - does "open drain input" make sense? (no)


ID=xxx:  analog output waveform generation

  Specify the number of samples in buffer.


ID=xxx:  analog output deglitch configuration
ID=xxx:  analog input dithering configuration
  
  Also could put AO range configuration here.  (ds: that would be
  dumb.)


ID=xxx:  analog triggering

  length=N
  chanspec is ignored
  type: any with cmd

  [0] - ID
  [1] - analog trigger channel
  [2] - flags
          trigger
          slope/level
	  hysteresis
  [3] - high
  [4] - low

  Modifications: ?
  Errors:
    - bad channel
    - bad flags
    - 
  Notes:
    - are there devices that can specify debouncing?
    - are there devices that can specify a time value after
      which the analog trigger resets?
    - has to account for hysteresis


ID=xxx: simple digital triggering

  length =
  chanspec is ignored
  type: any with cmd

  [0] - ID
  [1] - flags
          trigger 
  	  edge/level
	  rising/falling
  [2] - divider (?)

  Notes:
    - should this configure pins, or configure triggers?
      (ds: triggers, see independent subdevice design rule)
      (ds: pins, because it makes other things easier)
    - divider?  (ds: too complicated.  This is supposed to
      be simple.)
    - should this be encoded into the TRIG_EXT argument?
      

ID=xxx: extended digital triggering

  length =
  chanspec: ignored
  type: any with cmd

  [0] - ID
  [1] - flags
          trigger

  Notes:
    - should this be the same as triggering for counter/timers?


ID=xxx: counter configuration

  The subdevice has an accumulator which changes based on
  triggers.

  [0] - ID
  [ ] - flags
          generate command trigger when?
  [1] - mode
  [2] - source1_src
  [3] - source1_arg
  [4] - source2_src
  [5] - source2_arg
  [6] - gate_src
  [7] - gate_arg

  Modes:
    UP_COUNTER:
      acc++ on source1

    DOWN_COUNTER:
      acc-- on source1
    
    QUADRATURE:
      acc+=atan(source1,source2)

  Notes:
    - configuring pins vs. configuring triggers would make this
      easier


ID=xxx: counter configuration, up counter

  chanspec: used to determine particular counter channel

  [0] - ID
  [1] - flags
          up/down
  [2] - src (like command trigger)
  [3] - arg

  Notes:
    - too simple?  no gating

ID=xxx: counter configuration, up counter with gating
ID=xxx: counter configuration, quadrature


ID=xxx: command trigger, pattern matching

  [0] - ID
  [1] - bit mask (1 - watch, 0 - don't watch)
  [2] - bit pattern


ID=xxx: set software gate
  [0] - ID
  [1] - set/clear gate




Applications:

 - Pulse counting:
   (STC: simple event counting)

    mode - up counter
    primary source - external line (+/-)

 - Pulse counting with gate:
   (STC: simple gated event counting)
    
    mode - up counter
    primary source - external line
    gate source - external line

 - Pulse length measurement:
    (STC: single pulse-width measurement)

    mode - up counter with gate
    primary source - timer
    gate source - external line (+/-)

    latch and reset on gate (-) transition

 - Timestamping
    (STC: buffered [non-]cumulative event counting,
          single period measurement)

    mode - up counter
    primary source - timer
    secondary source - external line

    latch on secondary source (+) transition

 - Up/Down counter:
    (STC: relative position sensing)

    mode - up/down counter
    primary source - external line
    secondary source - external line

    when(P)acc+=S?+1:-1

 - Quadrature counting:
    
    mode - quadrature
    primary source - external line 1
    secondary source - external line 2

 - Frequency generation:

    mode - up counter
    primary source - timer
    
    invert output on primary source trigger

 - Pulse generation
    
    (XXX needs work)
    mode - dual up counter
    primary source - timer
    secondary source - timer

    set output on primary source trigger
    clear output on secondary source trigger

  - Pulse Width Modulation

    same as above
    secondary source - timer (latch arg)

  - Frequency Shift Keying:

    mode - double up counter
    primary source - timer
    secondary source - timer
    gate - soft, or external line

    invert output on gate?primary:secondary source trigger

  -