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Subject[PATCH v5 3/8] docs: Add Generic Counter interface documentation
This patch adds high-level documentation about the Generic Counter

Signed-off-by: William Breathitt Gray <>
Documentation/driver-api/generic-counter.rst | 321 +++++++++++++++++++++++++++
Documentation/driver-api/index.rst | 1 +
3 files changed, 323 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..bce0cbc31963
--- /dev/null
+++ b/Documentation/driver-api/generic-counter.rst
@@ -0,0 +1,321 @@
+Generic Counter Interface
+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.
+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:
+ -----
+ A Count represents the count data for a set of Signals. The
+ Generic Counter interface provides the following available count
+ data types:
+ Unsigned integer value representing position.
+ Signed 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 quadrature pair signal A updates
+ the respective count. The input level of
+ quadrature pair signal B determines direction.
+ * Quadrature x1:
+ 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.
+ * Quadrature x2:
+ Any state transition on quadrature pair signal A
+ updates the respective count. Quadrature
+ encoding determines the direction.
+ * Quadrature 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.
+ ------
+ A Signal represents a counter input data; this is the input data
+ that is analyzed 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 line is in a low state.
+ Signal line is in a high state.
+ A Signal may be associated to one or more Counts.
+ -------
+ 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 to 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 to a set of Signals, whose respective Synapse instances
+represent the count function update conditions for the associated
+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 to 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 to 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
+ 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 to 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/iio/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:
+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 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.
+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 to their
+respective Signal attributes, while the count_read/count_write and
+function_get/function_set driver callbacks are associated to 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 e9b41b1634f3..57e6e2c1d063 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -25,6 +25,7 @@ available subsections can be seen below.
+ generic-counter
index 2a7bf2f84272..a71dff6eae87 100644
@@ -3673,6 +3673,7 @@ M: William Breathitt Gray <>
S: Maintained
F: Documentation/ABI/testing/sysfs-bus-counter*
+F: Documentation/driver-api/generic-counter.rst
F: drivers/counter/
F: include/linux/counter.h

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