574 lines
24 KiB
ReStructuredText
574 lines
24 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=========================
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Generic Counter Interface
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=========================
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Introduction
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============
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Counter devices are prevalent among a diverse spectrum of industries.
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The ubiquitous presence of these devices necessitates a common interface
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and standard of interaction and exposure. This driver API attempts to
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resolve the issue of duplicate code found among existing counter device
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drivers by introducing a generic counter interface for consumption. The
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Generic Counter interface enables drivers to support and expose a common
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set of components and functionality present in counter devices.
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Theory
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======
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Counter devices can vary greatly in design, but regardless of whether
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some devices are quadrature encoder counters or tally counters, all
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counter devices consist of a core set of components. This core set of
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components, shared by all counter devices, is what forms the essence of
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the Generic Counter interface.
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There are three core components to a counter:
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* Signal:
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Stream of data to be evaluated by the counter.
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* Synapse:
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Association of a Signal, and evaluation trigger, with a Count.
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* Count:
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Accumulation of the effects of connected Synapses.
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SIGNAL
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------
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A Signal represents a stream of data. This is the input data that is
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evaluated by the counter to determine the count data; e.g. a quadrature
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signal output line of a rotary encoder. Not all counter devices provide
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user access to the Signal data, so exposure is optional for drivers.
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When the Signal data is available for user access, the Generic Counter
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interface provides the following available signal values:
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* SIGNAL_LOW:
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Signal line is in a low state.
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* SIGNAL_HIGH:
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Signal line is in a high state.
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A Signal may be associated with one or more Counts.
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SYNAPSE
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-------
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A Synapse represents the association of a Signal with a Count. Signal
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data affects respective Count data, and the Synapse represents this
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relationship.
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The Synapse action mode specifies the Signal data condition that
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triggers the respective Count's count function evaluation to update the
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count data. The Generic Counter interface provides the following
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available action modes:
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* None:
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Signal does not trigger the count function. In Pulse-Direction count
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function mode, this Signal is evaluated as Direction.
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* Rising Edge:
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Low state transitions to high state.
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* Falling Edge:
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High state transitions to low state.
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* Both Edges:
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Any state transition.
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A counter is defined as a set of input signals associated with count
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data that are generated by the evaluation of the state of the associated
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input signals as defined by the respective count functions. Within the
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context of the Generic Counter interface, a counter consists of Counts
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each associated with a set of Signals, whose respective Synapse
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instances represent the count function update conditions for the
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associated Counts.
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A Synapse associates one Signal with one Count.
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COUNT
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-----
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A Count represents the accumulation of the effects of connected
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Synapses; i.e. the count data for a set of Signals. The Generic
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Counter interface represents the count data as a natural number.
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A Count has a count function mode which represents the update behavior
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for the count data. The Generic Counter interface provides the following
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available count function modes:
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* Increase:
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Accumulated count is incremented.
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* Decrease:
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Accumulated count is decremented.
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* Pulse-Direction:
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Rising edges on signal A updates the respective count. The input level
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of signal B determines direction.
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* Quadrature:
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A pair of quadrature encoding signals are evaluated to determine
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position and direction. The following Quadrature modes are available:
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- x1 A:
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If direction is forward, rising edges on quadrature pair signal A
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updates the respective count; if the direction is backward, falling
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edges on quadrature pair signal A updates the respective count.
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Quadrature encoding determines the direction.
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- x1 B:
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If direction is forward, rising edges on quadrature pair signal B
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updates the respective count; if the direction is backward, falling
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edges on quadrature pair signal B updates the respective count.
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Quadrature encoding determines the direction.
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- x2 A:
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Any state transition on quadrature pair signal A updates the
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respective count. Quadrature encoding determines the direction.
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- x2 B:
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Any state transition on quadrature pair signal B updates the
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respective count. Quadrature encoding determines the direction.
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- x4:
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Any state transition on either quadrature pair signals updates the
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respective count. Quadrature encoding determines the direction.
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A Count has a set of one or more associated Synapses.
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Paradigm
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========
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The most basic counter device may be expressed as a single Count
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associated with a single Signal via a single Synapse. Take for example
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a counter device which simply accumulates a count of rising edges on a
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source input line::
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Count Synapse Signal
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----- ------- ------
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+---------------------+
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| Data: Count | Rising Edge ________
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| Function: Increase | <------------- / Source \
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| | ____________
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+---------------------+
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In this example, the Signal is a source input line with a pulsing
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voltage, while the Count is a persistent count value which is repeatedly
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incremented. The Signal is associated with the respective Count via a
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Synapse. The increase function is triggered by the Signal data condition
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specified by the Synapse -- in this case a rising edge condition on the
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voltage input line. In summary, the counter device existence and
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behavior is aptly represented by respective Count, Signal, and Synapse
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components: a rising edge condition triggers an increase function on an
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accumulating count datum.
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A counter device is not limited to a single Signal; in fact, in theory
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many Signals may be associated with even a single Count. For example, a
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quadrature encoder counter device can keep track of position based on
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the states of two input lines::
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Count Synapse Signal
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----- ------- ------
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+-------------------------+
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| Data: Position | Both Edges ___
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| Function: Quadrature x4 | <------------ / A \
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| | _______
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| | Both Edges ___
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| | <------------ / B \
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| | _______
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+-------------------------+
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In this example, two Signals (quadrature encoder lines A and B) are
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associated with a single Count: a rising or falling edge on either A or
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B triggers the "Quadrature x4" function which determines the direction
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of movement and updates the respective position data. The "Quadrature
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x4" function is likely implemented in the hardware of the quadrature
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encoder counter device; the Count, Signals, and Synapses simply
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represent this hardware behavior and functionality.
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Signals associated with the same Count can have differing Synapse action
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mode conditions. For example, a quadrature encoder counter device
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operating in a non-quadrature Pulse-Direction mode could have one input
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line dedicated for movement and a second input line dedicated for
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direction::
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Count Synapse Signal
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----- ------- ------
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+---------------------------+
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| Data: Position | Rising Edge ___
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| Function: Pulse-Direction | <------------- / A \ (Movement)
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| | _______
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| | None ___
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| | <------------- / B \ (Direction)
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| | _______
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+---------------------------+
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Only Signal A triggers the "Pulse-Direction" update function, but the
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instantaneous state of Signal B is still required in order to know the
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direction so that the position data may be properly updated. Ultimately,
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both Signals are associated with the same Count via two respective
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Synapses, but only one Synapse has an active action mode condition which
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triggers the respective count function while the other is left with a
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"None" condition action mode to indicate its respective Signal's
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availability for state evaluation despite its non-triggering mode.
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Keep in mind that the Signal, Synapse, and Count are abstract
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representations which do not need to be closely married to their
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respective physical sources. This allows the user of a counter to
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divorce themselves from the nuances of physical components (such as
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whether an input line is differential or single-ended) and instead focus
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on the core idea of what the data and process represent (e.g. position
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as interpreted from quadrature encoding data).
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Driver API
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==========
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Driver authors may utilize the Generic Counter interface in their code
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by including the include/linux/counter.h header file. This header file
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provides several core data structures, function prototypes, and macros
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for defining a counter device.
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.. kernel-doc:: include/linux/counter.h
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:internal:
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.. kernel-doc:: drivers/counter/counter-core.c
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:export:
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.. kernel-doc:: drivers/counter/counter-chrdev.c
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:export:
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Driver Implementation
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=====================
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To support a counter device, a driver must first allocate the available
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Counter Signals via counter_signal structures. These Signals should
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be stored as an array and set to the signals array member of an
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allocated counter_device structure before the Counter is registered to
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the system.
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Counter Counts may be allocated via counter_count structures, and
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respective Counter Signal associations (Synapses) made via
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counter_synapse structures. Associated counter_synapse structures are
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stored as an array and set to the synapses array member of the
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respective counter_count structure. These counter_count structures are
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set to the counts array member of an allocated counter_device structure
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before the Counter is registered to the system.
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Driver callbacks must be provided to the counter_device structure in
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order to communicate with the device: to read and write various Signals
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and Counts, and to set and get the "action mode" and "function mode" for
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various Synapses and Counts respectively.
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A counter_device structure is allocated using counter_alloc() and then
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registered to the system by passing it to the counter_add() function, and
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unregistered by passing it to the counter_unregister function. There are
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device managed variants of these functions: devm_counter_alloc() and
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devm_counter_add().
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The struct counter_comp structure is used to define counter extensions
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for Signals, Synapses, and Counts.
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The "type" member specifies the type of high-level data (e.g. BOOL,
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COUNT_DIRECTION, etc.) handled by this extension. The "``*_read``" and
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"``*_write``" members can then be set by the counter device driver with
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callbacks to handle that data using native C data types (i.e. u8, u64,
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etc.).
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Convenience macros such as ``COUNTER_COMP_COUNT_U64`` are provided for
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use by driver authors. In particular, driver authors are expected to use
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the provided macros for standard Counter subsystem attributes in order
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to maintain a consistent interface for userspace. For example, a counter
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device driver may define several standard attributes like so::
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struct counter_comp count_ext[] = {
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COUNTER_COMP_DIRECTION(count_direction_read),
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COUNTER_COMP_ENABLE(count_enable_read, count_enable_write),
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COUNTER_COMP_CEILING(count_ceiling_read, count_ceiling_write),
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};
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This makes it simple to see, add, and modify the attributes that are
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supported by this driver ("direction", "enable", and "ceiling") and to
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maintain this code without getting lost in a web of struct braces.
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Callbacks must match the function type expected for the respective
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component or extension. These function types are defined in the struct
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counter_comp structure as the "``*_read``" and "``*_write``" union
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members.
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The corresponding callback prototypes for the extensions mentioned in
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the previous example above would be::
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int count_direction_read(struct counter_device *counter,
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struct counter_count *count,
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enum counter_count_direction *direction);
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int count_enable_read(struct counter_device *counter,
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struct counter_count *count, u8 *enable);
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int count_enable_write(struct counter_device *counter,
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struct counter_count *count, u8 enable);
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int count_ceiling_read(struct counter_device *counter,
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struct counter_count *count, u64 *ceiling);
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int count_ceiling_write(struct counter_device *counter,
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struct counter_count *count, u64 ceiling);
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Determining the type of extension to create is a matter of scope.
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* Signal extensions are attributes that expose information/control
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specific to a Signal. These types of attributes will exist under a
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Signal's directory in sysfs.
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For example, if you have an invert feature for a Signal, you can have
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a Signal extension called "invert" that toggles that feature:
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/sys/bus/counter/devices/counterX/signalY/invert
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* Count extensions are attributes that expose information/control
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specific to a Count. These type of attributes will exist under a
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Count's directory in sysfs.
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For example, if you want to pause/unpause a Count from updating, you
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can have a Count extension called "enable" that toggles such:
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/sys/bus/counter/devices/counterX/countY/enable
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* Device extensions are attributes that expose information/control
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non-specific to a particular Count or Signal. This is where you would
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put your global features or other miscellaneous functionality.
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For example, if your device has an overtemp sensor, you can report the
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chip overheated via a device extension called "error_overtemp":
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/sys/bus/counter/devices/counterX/error_overtemp
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Subsystem Architecture
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======================
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Counter drivers pass and take data natively (i.e. ``u8``, ``u64``, etc.)
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and the shared counter module handles the translation between the sysfs
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interface. This guarantees a standard userspace interface for all
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counter drivers, and enables a Generic Counter chrdev interface via a
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generalized device driver ABI.
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A high-level view of how a count value is passed down from a counter
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driver is exemplified by the following. The driver callbacks are first
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registered to the Counter core component for use by the Counter
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userspace interface components::
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Driver callbacks registration:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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+----------------------------+
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| Counter device driver |
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+----------------------------+
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| Processes data from device |
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+----------------------------+
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-------------------
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/ driver callbacks /
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-------------------
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V
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+----------------------+
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| Counter core |
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+----------------------+
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| Routes device driver |
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| callbacks to the |
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| userspace interfaces |
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+----------------------+
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-------------------
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/ driver callbacks /
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-------------------
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+---------------+---------------+
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V V
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+--------------------+ +---------------------+
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| Counter sysfs | | Counter chrdev |
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+--------------------+ +---------------------+
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| Translates to the | | Translates to the |
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| standard Counter | | standard Counter |
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| sysfs output | | character device |
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+--------------------+ +---------------------+
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Thereafter, data can be transferred directly between the Counter device
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driver and Counter userspace interface::
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Count data request:
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~~~~~~~~~~~~~~~~~~~
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----------------------
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/ Counter device \
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+----------------------+
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| Count register: 0x28 |
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+----------------------+
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-----------------
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/ raw count data /
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-----------------
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V
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+----------------------------+
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| Counter device driver |
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+----------------------------+
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| Processes data from device |
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|----------------------------|
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| Type: u64 |
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| Value: 42 |
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+----------------------------+
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----------
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/ u64 /
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----------
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+---------------+---------------+
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V V
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+--------------------+ +---------------------+
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| Counter sysfs | | Counter chrdev |
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+--------------------+ +---------------------+
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| Translates to the | | Translates to the |
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| standard Counter | | standard Counter |
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| sysfs output | | character device |
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|--------------------| |---------------------|
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| Type: const char * | | Type: u64 |
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| Value: "42" | | Value: 42 |
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+--------------------+ +---------------------+
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--------------- -----------------------
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/ const char * / / struct counter_event /
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--------------- -----------------------
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| V
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| +-----------+
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| | read |
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| +-----------+
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| \ Count: 42 /
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| -----------
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V
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+--------------------------------------------------+
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| `/sys/bus/counter/devices/counterX/countY/count` |
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+--------------------------------------------------+
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\ Count: "42" /
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--------------------------------------------------
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There are four primary components involved:
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Counter device driver
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---------------------
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Communicates with the hardware device to read/write data; e.g. counter
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drivers for quadrature encoders, timers, etc.
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Counter core
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------------
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Registers the counter device driver to the system so that the respective
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callbacks are called during userspace interaction.
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Counter sysfs
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-------------
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Translates counter data to the standard Counter sysfs interface format
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and vice versa.
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Please refer to the ``Documentation/ABI/testing/sysfs-bus-counter`` file
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for a detailed breakdown of the available Generic Counter interface
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sysfs attributes.
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Counter chrdev
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--------------
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Translates Counter events to the standard Counter character device; data
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is transferred via standard character device read calls, while Counter
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events are configured via ioctl calls.
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Sysfs Interface
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===============
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Several sysfs attributes are generated by the Generic Counter interface,
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and reside under the ``/sys/bus/counter/devices/counterX`` directory,
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where ``X`` is to the respective counter device id. Please see
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``Documentation/ABI/testing/sysfs-bus-counter`` for detailed information
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on each Generic Counter interface sysfs attribute.
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Through these sysfs attributes, programs and scripts may interact with
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the Generic Counter paradigm Counts, Signals, and Synapses of respective
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counter devices.
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Counter Character Device
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========================
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Counter character device nodes are created under the ``/dev`` directory
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as ``counterX``, where ``X`` is the respective counter device id.
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Defines for the standard Counter data types are exposed via the
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userspace ``include/uapi/linux/counter.h`` file.
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Counter events
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--------------
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Counter device drivers can support Counter events by utilizing the
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``counter_push_event`` function::
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void counter_push_event(struct counter_device *const counter, const u8 event,
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const u8 channel);
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The event id is specified by the ``event`` parameter; the event channel
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id is specified by the ``channel`` parameter. When this function is
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called, the Counter data associated with the respective event is
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gathered, and a ``struct counter_event`` is generated for each datum and
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pushed to userspace.
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Counter events can be configured by users to report various Counter
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data of interest. This can be conceptualized as a list of Counter
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component read calls to perform. For example:
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+------------------------+------------------------+
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| COUNTER_EVENT_OVERFLOW | COUNTER_EVENT_INDEX |
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+========================+========================+
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| Channel 0 | Channel 0 |
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+------------------------+------------------------+
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| * Count 0 | * Signal 0 |
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| * Count 1 | * Signal 0 Extension 0 |
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| * Signal 3 | * Extension 4 |
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| * Count 4 Extension 2 +------------------------+
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| * Signal 5 Extension 0 | Channel 1 |
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| +------------------------+
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| | * Signal 4 |
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| | * Signal 4 Extension 0 |
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| | * Count 7 |
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+------------------------+------------------------+
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When ``counter_push_event(counter, COUNTER_EVENT_INDEX, 1)`` is called
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for example, it will go down the list for the ``COUNTER_EVENT_INDEX``
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event channel 1 and execute the read callbacks for Signal 4, Signal 4
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Extension 0, and Count 7 -- the data returned for each is pushed to a
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kfifo as a ``struct counter_event``, which userspace can retrieve via a
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standard read operation on the respective character device node.
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Userspace
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---------
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Userspace applications can configure Counter events via ioctl operations
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on the Counter character device node. There following ioctl codes are
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supported and provided by the ``linux/counter.h`` userspace header file:
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* :c:macro:`COUNTER_ADD_WATCH_IOCTL`
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* :c:macro:`COUNTER_ENABLE_EVENTS_IOCTL`
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* :c:macro:`COUNTER_DISABLE_EVENTS_IOCTL`
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To configure events to gather Counter data, users first populate a
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``struct counter_watch`` with the relevant event id, event channel id,
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and the information for the desired Counter component from which to
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read, and then pass it via the ``COUNTER_ADD_WATCH_IOCTL`` ioctl
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command.
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Note that an event can be watched without gathering Counter data by
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setting the ``component.type`` member equal to
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``COUNTER_COMPONENT_NONE``. With this configuration the Counter
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character device will simply populate the event timestamps for those
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respective ``struct counter_event`` elements and ignore the component
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value.
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The ``COUNTER_ADD_WATCH_IOCTL`` command will buffer these Counter
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watches. When ready, the ``COUNTER_ENABLE_EVENTS_IOCTL`` ioctl command
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may be used to activate these Counter watches.
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Userspace applications can then execute a ``read`` operation (optionally
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calling ``poll`` first) on the Counter character device node to retrieve
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``struct counter_event`` elements with the desired data.
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