.. SPDX-License-Identifier: GPL-2.0
  
  =========================
  Generic Counter Interface
  =========================
  
  Introduction
  ============
  
  Counter devices are prevalent within a diverse spectrum of industries.
  The ubiquitous presence of these devices necessitates a common interface
  and standard of interaction and exposure. This driver API attempts to
  resolve the issue of duplicate code found among existing counter device
  drivers by introducing a generic counter interface for consumption. The
  Generic Counter interface enables drivers to support and expose a common
  set of components and functionality present in counter devices.
  
  Theory
  ======
  
  Counter devices can vary greatly in design, but regardless of whether
  some devices are quadrature encoder counters or tally counters, all
  counter devices consist of a core set of components. This core set of
  components, shared by all counter devices, is what forms the essence of
  the Generic Counter interface.
  
  There are three core components to a counter:
  
  * Count:
    Count data for a set of Signals.
  
  * Signal:
    Input data that is evaluated by the counter to determine the count
    data.
  
  * Synapse:
    The association of a Signal with a respective Count.
  
  COUNT
  -----
  A Count represents the count data for a set of Signals. The Generic
  Counter interface provides the following available count data types:
  
  * COUNT_POSITION:
    Unsigned integer value representing position.
  
  A Count has a count function mode which represents the update behavior
  for the count data. The Generic Counter interface provides the following
  available count function modes:
  
  * Increase:
    Accumulated count is incremented.
  
  * Decrease:
    Accumulated count is decremented.
  
  * Pulse-Direction:
    Rising edges on signal A updates the respective count. The input level
    of signal B determines direction.
  
  * Quadrature:
    A pair of quadrature encoding signals are evaluated to determine
    position and direction. The following Quadrature modes are available:
  
    - x1 A:
      If direction is forward, rising edges on quadrature pair signal A
      updates the respective count; if the direction is backward, falling
      edges on quadrature pair signal A updates the respective count.
      Quadrature encoding determines the direction.
  
    - x1 B:
      If direction is forward, rising edges on quadrature pair signal B
      updates the respective count; if the direction is backward, falling
      edges on quadrature pair signal B updates the respective count.
      Quadrature encoding determines the direction.
  
    - x2 A:
      Any state transition on quadrature pair signal A updates the
      respective count. Quadrature encoding determines the direction.
  
    - x2 B:
      Any state transition on quadrature pair signal B updates the
      respective count. Quadrature encoding determines the direction.
  
    - x4:
      Any state transition on either quadrature pair signals updates the
      respective count. Quadrature encoding determines the direction.
  
  A Count has a set of one or more associated Signals.
  
  SIGNAL
  ------
  A Signal represents a counter input data; this is the input data that is
  evaluated by the counter to determine the count data; e.g. a quadrature
  signal output line of a rotary encoder. Not all counter devices provide
  user access to the Signal data.
  
  The Generic Counter interface provides the following available signal
  data types for when the Signal data is available for user access:
  
  * SIGNAL_LEVEL:
    Signal line state level. The following states are possible:
  
    - SIGNAL_LEVEL_LOW:
      Signal line is in a low state.
  
    - SIGNAL_LEVEL_HIGH:
      Signal line is in a high state.
  
  A Signal may be associated with one or more Counts.
  
  SYNAPSE
  -------
  A Synapse represents the association of a Signal with a respective
  Count. Signal data affects respective Count data, and the Synapse
  represents this relationship.
  
  The Synapse action mode specifies the Signal data condition which
  triggers the respective Count's count function evaluation to update the
  count data. The Generic Counter interface provides the following
  available action modes:
  
  * None:
    Signal does not trigger the count function. In Pulse-Direction count
    function mode, this Signal is evaluated as Direction.
  
  * Rising Edge:
    Low state transitions to high state.
  
  * Falling Edge:
    High state transitions to low state.
  
  * Both Edges:
    Any state transition.
  
  A counter is defined as a set of input signals associated with count
  data that are generated by the evaluation of the state of the associated
  input signals as defined by the respective count functions. Within the
  context of the Generic Counter interface, a counter consists of Counts
  each associated with a set of Signals, whose respective Synapse
  instances represent the count function update conditions for the
  associated Counts.
  
  Paradigm
  ========
  
  The most basic counter device may be expressed as a single Count
  associated with a single Signal via a single Synapse. Take for example
  a counter device which simply accumulates a count of rising edges on a
  source input line::
  
                  Count                Synapse        Signal
                  -----                -------        ------
          +---------------------+
          | Data: Count         |    Rising Edge     ________
          | Function: Increase  |  <-------------   / Source \
          |                     |                  ____________
          +---------------------+
  
  In this example, the Signal is a source input line with a pulsing
  voltage, while the Count is a persistent count value which is repeatedly
  incremented. The Signal is associated with the respective Count via a
  Synapse. The increase function is triggered by the Signal data condition
  specified by the Synapse -- in this case a rising edge condition on the
  voltage input line. In summary, the counter device existence and
  behavior is aptly represented by respective Count, Signal, and Synapse
  components: a rising edge condition triggers an increase function on an
  accumulating count datum.
  
  A counter device is not limited to a single Signal; in fact, in theory
  many Signals may be associated with even a single Count. For example, a
  quadrature encoder counter device can keep track of position based on
  the states of two input lines::
  
                     Count                 Synapse     Signal
                     -----                 -------     ------
          +-------------------------+
          | Data: Position          |    Both Edges     ___
          | Function: Quadrature x4 |  <------------   / A \
          |                         |                 _______
          |                         |
          |                         |    Both Edges     ___
          |                         |  <------------   / B \
          |                         |                 _______
          +-------------------------+
  
  In this example, two Signals (quadrature encoder lines A and B) are
  associated with a single Count: a rising or falling edge on either A or
  B triggers the "Quadrature x4" function which determines the direction
  of movement and updates the respective position data. The "Quadrature
  x4" function is likely implemented in the hardware of the quadrature
  encoder counter device; the Count, Signals, and Synapses simply
  represent this hardware behavior and functionality.
  
  Signals associated with the same Count can have differing Synapse action
  mode conditions. For example, a quadrature encoder counter device
  operating in a non-quadrature Pulse-Direction mode could have one input
  line dedicated for movement and a second input line dedicated for
  direction::
  
                     Count                   Synapse      Signal
                     -----                   -------      ------
          +---------------------------+
          | Data: Position            |    Rising Edge     ___
          | Function: Pulse-Direction |  <-------------   / A \ (Movement)
          |                           |                  _______
          |                           |
          |                           |       None         ___
          |                           |  <-------------   / B \ (Direction)
          |                           |                  _______
          +---------------------------+
  
  Only Signal A triggers the "Pulse-Direction" update function, but the
  instantaneous state of Signal B is still required in order to know the
  direction so that the position data may be properly updated. Ultimately,
  both Signals are associated with the same Count via two respective
  Synapses, but only one Synapse has an active action mode condition which
  triggers the respective count function while the other is left with a
  "None" condition action mode to indicate its respective Signal's
  availability for state evaluation despite its non-triggering mode.
  
  Keep in mind that the Signal, Synapse, and Count are abstract
  representations which do not need to be closely married to their
  respective physical sources. This allows the user of a counter to
  divorce themselves from the nuances of physical components (such as
  whether an input line is differential or single-ended) and instead focus
  on the core idea of what the data and process represent (e.g. position
  as interpreted from quadrature encoding data).
  
  Userspace Interface
  ===================
  
  Several sysfs attributes are generated by the Generic Counter interface,
  and reside under the /sys/bus/counter/devices/counterX directory, where
  counterX refers to the respective counter device. Please see

  Documentation/ABI/testing/sysfs-bus-counter for detailed

  information on each Generic Counter interface sysfs attribute.
  
  Through these sysfs attributes, programs and scripts may interact with
  the Generic Counter paradigm Counts, Signals, and Synapses of respective
  counter devices.
  
  Driver API
  ==========
  
  Driver authors may utilize the Generic Counter interface in their code
  by including the include/linux/counter.h header file. This header file
  provides several core data structures, function prototypes, and macros
  for defining a counter device.
  
  .. kernel-doc:: include/linux/counter.h
     :internal:

  .. kernel-doc:: drivers/counter/counter.c

     :export:
  
  Implementation
  ==============
  
  To support a counter device, a driver must first allocate the available
  Counter Signals via counter_signal structures. These Signals should
  be stored as an array and set to the signals array member of an
  allocated counter_device structure before the Counter is registered to
  the system.
  
  Counter Counts may be allocated via counter_count structures, and
  respective Counter Signal associations (Synapses) made via
  counter_synapse structures. Associated counter_synapse structures are
  stored as an array and set to the the synapses array member of the
  respective counter_count structure. These counter_count structures are
  set to the counts array member of an allocated counter_device structure
  before the Counter is registered to the system.
  
  Driver callbacks should be provided to the counter_device structure via
  a constant counter_ops structure in order to communicate with the
  device: to read and write various Signals and Counts, and to set and get
  the "action mode" and "function mode" for various Synapses and Counts
  respectively.
  
  A defined counter_device structure may be registered to the system by
  passing it to the counter_register function, and unregistered by passing
  it to the counter_unregister function. Similarly, the
  devm_counter_register and devm_counter_unregister functions may be used
  if device memory-managed registration is desired.
  
  Extension sysfs attributes can be created for auxiliary functionality
  and data by passing in defined counter_device_ext, counter_count_ext,
  and counter_signal_ext structures. In these cases, the
  counter_device_ext structure is used for global configuration of the
  respective Counter device, while the counter_count_ext and
  counter_signal_ext structures allow for auxiliary exposure and
  configuration of a specific Count or Signal respectively.
  
  Architecture
  ============
  
  When the Generic Counter interface counter module is loaded, the
  counter_init function is called which registers a bus_type named
  "counter" to the system. Subsequently, when the module is unloaded, the
  counter_exit function is called which unregisters the bus_type named
  "counter" from the system.
  
  Counter devices are registered to the system via the counter_register
  function, and later removed via the counter_unregister function. The
  counter_register function establishes a unique ID for the Counter
  device and creates a respective sysfs directory, where X is the
  mentioned unique ID:
  
      /sys/bus/counter/devices/counterX
  
  Sysfs attributes are created within the counterX directory to expose
  functionality, configurations, and data relating to the Counts, Signals,
  and Synapses of the Counter device, as well as options and information
  for the Counter device itself.
  
  Each Signal has a directory created to house its relevant sysfs
  attributes, where Y is the unique ID of the respective Signal:
  
      /sys/bus/counter/devices/counterX/signalY
  
  Similarly, each Count has a directory created to house its relevant
  sysfs attributes, where Y is the unique ID of the respective Count:
  
      /sys/bus/counter/devices/counterX/countY
  
  For a more detailed breakdown of the available Generic Counter interface
  sysfs attributes, please refer to the

  Documentation/ABI/testing/sysfs-bus-counter file.

  
  The Signals and Counts associated with the Counter device are registered
  to the system as well by the counter_register function. The
  signal_read/signal_write driver callbacks are associated with their
  respective Signal attributes, while the count_read/count_write and
  function_get/function_set driver callbacks are associated with their
  respective Count attributes; similarly, the same is true for the
  action_get/action_set driver callbacks and their respective Synapse
  attributes. If a driver callback is left undefined, then the respective
  read/write permission is left disabled for the relevant attributes.
  
  Similarly, extension sysfs attributes are created for the defined
  counter_device_ext, counter_count_ext, and counter_signal_ext
  structures that are passed in.