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Date:   Fri, 22 Jun 2018 17:51:46 +0100
From:   Jonathan Cameron <jonathan.cameron@huawei.com>
To:     William Breathitt Gray <vilhelm.gray@gmail.com>
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Subject: Re: [PATCH v7 03/10] docs: Add Generic Counter interface
 documentation
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On Thu, 21 Jun 2018 17:07:30 -0400
William Breathitt Gray <vilhelm.gray@gmail.com> wrote:

> This patch adds high-level documentation about the Generic Counter
> interface.
> 
> Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

> ---
>  Documentation/driver-api/generic-counter.rst | 342 +++++++++++++++++++
>  Documentation/driver-api/index.rst           |   1 +
>  MAINTAINERS                                  |   1 +
>  3 files changed, 344 insertions(+)
>  create mode 100644 Documentation/driver-api/generic-counter.rst
> 
> diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst
> new file mode 100644
> index 000000000000..f51db893f595
> --- /dev/null
> +++ b/Documentation/driver-api/generic-counter.rst
> @@ -0,0 +1,342 @@
> +.. 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/sys-bus-counter-generic-sysfs 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/generic-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/sys-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.
> diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
> index f4180e7c7ed5..e39a9fd3d8c9 100644
> --- a/Documentation/driver-api/index.rst
> +++ b/Documentation/driver-api/index.rst
> @@ -52,6 +52,7 @@ available subsections can be seen below.
>     slimbus
>     soundwire/index
>     fpga/index
> +   generic-counter
>  
>  .. only::  subproject and html
>  
> diff --git a/MAINTAINERS b/MAINTAINERS
> index f8a47fd197a1..c7fd36500635 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -3693,6 +3693,7 @@ M:	William Breathitt Gray <vilhelm.gray@gmail.com>
>  L:	linux-iio@vger.kernel.org
>  S:	Maintained
>  F:	Documentation/ABI/testing/sysfs-bus-counter*
> +F:	Documentation/driver-api/generic-counter.rst
>  F:	drivers/counter/
>  F:	include/linux/counter.h
>