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- .. 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.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 miscellanous 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.
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