.. SPDX-License-Identifier: GPL-2.0

=========================
Generic Counter Interface
=========================

Introduction
============

Counter devices are prevalent among 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:

* Signal:
  Stream of data to be evaluated by the counter.

* Synapse:
  Association of a Signal, and evaluation trigger, with a Count.

* Count:
  Accumulation of the effects of connected Synapses.

SIGNAL
------
A Signal represents a stream of 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, so exposure is optional for drivers.

When the Signal data is available for user access, the Generic Counter
interface provides the following available signal values:

* SIGNAL_LOW:
  Signal line is in a low state.

* SIGNAL_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 Count. Signal
data affects respective Count data, and the Synapse represents this
relationship.

The Synapse action mode specifies the Signal data condition that
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.

A Synapse associates one Signal with one Count.

COUNT
-----
A Count represents the accumulation of the effects of connected
Synapses; i.e. the count data for a set of Signals. The Generic
Counter interface represents the count data as a natural number.

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 Synapses.

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-core.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 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/miscellaneous exposure
and 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.

Determining the type of extension to create is a matter of scope.

* Signal extensions are attributes that expose information/control
  specific to a Signal. These types of attributes will exist under a
  Signal's directory in sysfs.

  For example, if you have an invert feature for a Signal, you can have
  a Signal extension called "invert" that toggles that feature:
  /sys/bus/counter/devices/counterX/signalY/invert

* Count extensions are attributes that expose information/control
  specific to a Count. These type of attributes will exist under a
  Count's directory in sysfs.

  For example, if you want to pause/unpause a Count from updating, you
  can have a Count extension called "enable" that toggles such:
  /sys/bus/counter/devices/counterX/countY/enable

* Device extensions are attributes that expose information/control
  non-specific to a particular Count or Signal. This is where you would
  put your global features or other miscellaneous functionality.

  For example, if your device has an overtemp sensor, you can report the
  chip overheated via a device extension called "error_overtemp":
  /sys/bus/counter/devices/counterX/error_overtemp

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.