.. 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.