.. 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).

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:

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

Driver 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 must be provided to the counter_device 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 counter_device structure is allocated using counter_alloc() and then
registered to the system by passing it to the counter_add() function, and
unregistered by passing it to the counter_unregister function. There are
device managed variants of these functions: devm_counter_alloc() and
devm_counter_add().

The struct counter_comp structure is used to define counter extensions
for Signals, Synapses, and Counts.

The "type" member specifies the type of high-level data (e.g. BOOL,
COUNT_DIRECTION, etc.) handled by this extension. The "``*_read``" and
"``*_write``" members can then be set by the counter device driver with
callbacks to handle that data using native C data types (i.e. u8, u64,
etc.).

Convenience macros such as ``COUNTER_COMP_COUNT_U64`` are provided for
use by driver authors. In particular, driver authors are expected to use
the provided macros for standard Counter subsystem attributes in order
to maintain a consistent interface for userspace. For example, a counter
device driver may define several standard attributes like so::

        struct counter_comp count_ext[] = {
                COUNTER_COMP_DIRECTION(count_direction_read),
                COUNTER_COMP_ENABLE(count_enable_read, count_enable_write),
                COUNTER_COMP_CEILING(count_ceiling_read, count_ceiling_write),
        };

This makes it simple to see, add, and modify the attributes that are
supported by this driver ("direction", "enable", and "ceiling") and to
maintain this code without getting lost in a web of struct braces.

Callbacks must match the function type expected for the respective
component or extension. These function types are defined in the struct
counter_comp structure as the "``*_read``" and "``*_write``" union
members.

The corresponding callback prototypes for the extensions mentioned in
the previous example above would be::

        int count_direction_read(struct counter_device *counter,
                                 struct counter_count *count,
                                 enum counter_count_direction *direction);
        int count_enable_read(struct counter_device *counter,
                              struct counter_count *count, u8 *enable);
        int count_enable_write(struct counter_device *counter,
                               struct counter_count *count, u8 enable);
        int count_ceiling_read(struct counter_device *counter,
                               struct counter_count *count, u64 *ceiling);
        int count_ceiling_write(struct counter_device *counter,
                                struct counter_count *count, u64 ceiling);

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

Subsystem Architecture
======================

Counter drivers pass and take data natively (i.e. ``u8``, ``u64``, etc.)
and the shared counter module handles the translation between the sysfs
interface. This guarantees a standard userspace interface for all
counter drivers, and enables a Generic Counter chrdev interface via a
generalized device driver ABI.

A high-level view of how a count value is passed down from a counter
driver is exemplified by the following. The driver callbacks are first
registered to the Counter core component for use by the Counter
userspace interface components::

        Driver callbacks registration:
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                        +----------------------------+
                        | Counter device driver      |
                        +----------------------------+
                        | Processes data from device |
                        +----------------------------+
                                |
                         -------------------
                        / driver callbacks /
                        -------------------
                                |
                                V
                        +----------------------+
                        | Counter core         |
                        +----------------------+
                        | Routes device driver |
                        | callbacks to the     |
                        | userspace interfaces |
                        +----------------------+
                                |
                         -------------------
                        / driver callbacks /
                        -------------------
                                |
                +---------------+---------------+
                |                               |
                V                               V
        +--------------------+          +---------------------+
        | Counter sysfs      |          | Counter chrdev      |
        +--------------------+          +---------------------+
        | Translates to the  |          | Translates to the   |
        | standard Counter   |          | standard Counter    |
        | sysfs output       |          | character device    |
        +--------------------+          +---------------------+

Thereafter, data can be transferred directly between the Counter device
driver and Counter userspace interface::

        Count data request:
        ~~~~~~~~~~~~~~~~~~~
                         ----------------------
                        / Counter device       \
                        +----------------------+
                        | Count register: 0x28 |
                        +----------------------+
                                |
                         -----------------
                        / raw count data /
                        -----------------
                                |
                                V
                        +----------------------------+
                        | Counter device driver      |
                        +----------------------------+
                        | Processes data from device |
                        |----------------------------|
                        | Type: u64                  |
                        | Value: 42                  |
                        +----------------------------+
                                |
                         ----------
                        / u64     /
                        ----------
                                |
                +---------------+---------------+
                |                               |
                V                               V
        +--------------------+          +---------------------+
        | Counter sysfs      |          | Counter chrdev      |
        +--------------------+          +---------------------+
        | Translates to the  |          | Translates to the   |
        | standard Counter   |          | standard Counter    |
        | sysfs output       |          | character device    |
        |--------------------|          |---------------------|
        | Type: const char * |          | Type: u64           |
        | Value: "42"        |          | Value: 42           |
        +--------------------+          +---------------------+
                |                               |
         ---------------                 -----------------------
        / const char * /                / struct counter_event /
        ---------------                 -----------------------
                |                               |
                |                               V
                |                       +-----------+
                |                       | read      |
                |                       +-----------+
                |                       \ Count: 42 /
                |                        -----------
                |
                V
        +--------------------------------------------------+
        | `/sys/bus/counter/devices/counterX/countY/count` |
        +--------------------------------------------------+
        \ Count: "42"                                      /
         --------------------------------------------------

There are four primary components involved:

Counter device driver
---------------------
Communicates with the hardware device to read/write data; e.g. counter
drivers for quadrature encoders, timers, etc.

Counter core
------------
Registers the counter device driver to the system so that the respective
callbacks are called during userspace interaction.

Counter sysfs
-------------
Translates counter data to the standard Counter sysfs interface format
and vice versa.

Please refer to the ``Documentation/ABI/testing/sysfs-bus-counter`` file
for a detailed breakdown of the available Generic Counter interface
sysfs attributes.

Counter chrdev
--------------
Translates Counter events to the standard Counter character device; data
is transferred via standard character device read calls, while Counter
events are configured via ioctl calls.

Sysfs Interface
===============

Several sysfs attributes are generated by the Generic Counter interface,
and reside under the ``/sys/bus/counter/devices/counterX`` directory,
where ``X`` is to the respective counter device id. 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.

Counter Character Device
========================

Counter character device nodes are created under the ``/dev`` directory
as ``counterX``, where ``X`` is the respective counter device id.
Defines for the standard Counter data types are exposed via the
userspace ``include/uapi/linux/counter.h`` file.

Counter events
--------------
Counter device drivers can support Counter events by utilizing the
``counter_push_event`` function::

        void counter_push_event(struct counter_device *const counter, const u8 event,
                                const u8 channel);

The event id is specified by the ``event`` parameter; the event channel
id is specified by the ``channel`` parameter. When this function is
called, the Counter data associated with the respective event is
gathered, and a ``struct counter_event`` is generated for each datum and
pushed to userspace.

Counter events can be configured by users to report various Counter
data of interest. This can be conceptualized as a list of Counter
component read calls to perform. For example:

        +------------------------+------------------------+
        | COUNTER_EVENT_OVERFLOW | COUNTER_EVENT_INDEX    |
        +========================+========================+
        | Channel 0              | Channel 0              |
        +------------------------+------------------------+
        | * Count 0              | * Signal 0             |
        | * Count 1              | * Signal 0 Extension 0 |
        | * Signal 3             | * Extension 4          |
        | * Count 4 Extension 2  +------------------------+
        | * Signal 5 Extension 0 | Channel 1              |
        |                        +------------------------+
        |                        | * Signal 4             |
        |                        | * Signal 4 Extension 0 |
        |                        | * Count 7              |
        +------------------------+------------------------+

When ``counter_push_event(counter, COUNTER_EVENT_INDEX, 1)`` is called
for example, it will go down the list for the ``COUNTER_EVENT_INDEX``
event channel 1 and execute the read callbacks for Signal 4, Signal 4
Extension 0, and Count 7 -- the data returned for each is pushed to a
kfifo as a ``struct counter_event``, which userspace can retrieve via a
standard read operation on the respective character device node.

Userspace
---------
Userspace applications can configure Counter events via ioctl operations
on the Counter character device node. There following ioctl codes are
supported and provided by the ``linux/counter.h`` userspace header file:

* :c:macro:`COUNTER_ADD_WATCH_IOCTL`

* :c:macro:`COUNTER_ENABLE_EVENTS_IOCTL`

* :c:macro:`COUNTER_DISABLE_EVENTS_IOCTL`

To configure events to gather Counter data, users first populate a
``struct counter_watch`` with the relevant event id, event channel id,
and the information for the desired Counter component from which to
read, and then pass it via the ``COUNTER_ADD_WATCH_IOCTL`` ioctl
command.

Note that an event can be watched without gathering Counter data by
setting the ``component.type`` member equal to
``COUNTER_COMPONENT_NONE``. With this configuration the Counter
character device will simply populate the event timestamps for those
respective ``struct counter_event`` elements and ignore the component
value.

The ``COUNTER_ADD_WATCH_IOCTL`` command will buffer these Counter
watches. When ready, the ``COUNTER_ENABLE_EVENTS_IOCTL`` ioctl command
may be used to activate these Counter watches.

Userspace applications can then execute a ``read`` operation (optionally
calling ``poll`` first) on the Counter character device node to retrieve
``struct counter_event`` elements with the desired data.