1.. SPDX-License-Identifier: GPL-2.0
2
3=========================
4Generic Counter Interface
5=========================
6
7Introduction
8============
9
10Counter devices are prevalent within a diverse spectrum of industries.
11The ubiquitous presence of these devices necessitates a common interface
12and standard of interaction and exposure. This driver API attempts to
13resolve the issue of duplicate code found among existing counter device
14drivers by introducing a generic counter interface for consumption. The
15Generic Counter interface enables drivers to support and expose a common
16set of components and functionality present in counter devices.
17
18Theory
19======
20
21Counter devices can vary greatly in design, but regardless of whether
22some devices are quadrature encoder counters or tally counters, all
23counter devices consist of a core set of components. This core set of
24components, shared by all counter devices, is what forms the essence of
25the Generic Counter interface.
26
27There are three core components to a counter:
28
29* Count:
30  Count data for a set of Signals.
31
32* Signal:
33  Input data that is evaluated by the counter to determine the count
34  data.
35
36* Synapse:
37  The association of a Signal with a respective Count.
38
39COUNT
40-----
41A Count represents the count data for a set of Signals. The Generic
42Counter interface provides the following available count data types:
43
44* COUNT_POSITION:
45  Unsigned integer value representing position.
46
47A Count has a count function mode which represents the update behavior
48for the count data. The Generic Counter interface provides the following
49available count function modes:
50
51* Increase:
52  Accumulated count is incremented.
53
54* Decrease:
55  Accumulated count is decremented.
56
57* Pulse-Direction:
58  Rising edges on signal A updates the respective count. The input level
59  of signal B determines direction.
60
61* Quadrature:
62  A pair of quadrature encoding signals are evaluated to determine
63  position and direction. The following Quadrature modes are available:
64
65  - x1 A:
66    If direction is forward, rising edges on quadrature pair signal A
67    updates the respective count; if the direction is backward, falling
68    edges on quadrature pair signal A updates the respective count.
69    Quadrature encoding determines the direction.
70
71  - x1 B:
72    If direction is forward, rising edges on quadrature pair signal B
73    updates the respective count; if the direction is backward, falling
74    edges on quadrature pair signal B updates the respective count.
75    Quadrature encoding determines the direction.
76
77  - x2 A:
78    Any state transition on quadrature pair signal A updates the
79    respective count. Quadrature encoding determines the direction.
80
81  - x2 B:
82    Any state transition on quadrature pair signal B updates the
83    respective count. Quadrature encoding determines the direction.
84
85  - x4:
86    Any state transition on either quadrature pair signals updates the
87    respective count. Quadrature encoding determines the direction.
88
89A Count has a set of one or more associated Signals.
90
91SIGNAL
92------
93A Signal represents a counter input data; this is the input data that is
94evaluated by the counter to determine the count data; e.g. a quadrature
95signal output line of a rotary encoder. Not all counter devices provide
96user access to the Signal data.
97
98The Generic Counter interface provides the following available signal
99data types for when the Signal data is available for user access:
100
101* SIGNAL_LEVEL:
102  Signal line state level. The following states are possible:
103
104  - SIGNAL_LEVEL_LOW:
105    Signal line is in a low state.
106
107  - SIGNAL_LEVEL_HIGH:
108    Signal line is in a high state.
109
110A Signal may be associated with one or more Counts.
111
112SYNAPSE
113-------
114A Synapse represents the association of a Signal with a respective
115Count. Signal data affects respective Count data, and the Synapse
116represents this relationship.
117
118The Synapse action mode specifies the Signal data condition which
119triggers the respective Count's count function evaluation to update the
120count data. The Generic Counter interface provides the following
121available action modes:
122
123* None:
124  Signal does not trigger the count function. In Pulse-Direction count
125  function mode, this Signal is evaluated as Direction.
126
127* Rising Edge:
128  Low state transitions to high state.
129
130* Falling Edge:
131  High state transitions to low state.
132
133* Both Edges:
134  Any state transition.
135
136A counter is defined as a set of input signals associated with count
137data that are generated by the evaluation of the state of the associated
138input signals as defined by the respective count functions. Within the
139context of the Generic Counter interface, a counter consists of Counts
140each associated with a set of Signals, whose respective Synapse
141instances represent the count function update conditions for the
142associated Counts.
143
144Paradigm
145========
146
147The most basic counter device may be expressed as a single Count
148associated with a single Signal via a single Synapse. Take for example
149a counter device which simply accumulates a count of rising edges on a
150source input line::
151
152                Count                Synapse        Signal
153                -----                -------        ------
154        +---------------------+
155        | Data: Count         |    Rising Edge     ________
156        | Function: Increase  |  <-------------   / Source \
157        |                     |                  ____________
158        +---------------------+
159
160In this example, the Signal is a source input line with a pulsing
161voltage, while the Count is a persistent count value which is repeatedly
162incremented. The Signal is associated with the respective Count via a
163Synapse. The increase function is triggered by the Signal data condition
164specified by the Synapse -- in this case a rising edge condition on the
165voltage input line. In summary, the counter device existence and
166behavior is aptly represented by respective Count, Signal, and Synapse
167components: a rising edge condition triggers an increase function on an
168accumulating count datum.
169
170A counter device is not limited to a single Signal; in fact, in theory
171many Signals may be associated with even a single Count. For example, a
172quadrature encoder counter device can keep track of position based on
173the states of two input lines::
174
175                   Count                 Synapse     Signal
176                   -----                 -------     ------
177        +-------------------------+
178        | Data: Position          |    Both Edges     ___
179        | Function: Quadrature x4 |  <------------   / A \
180        |                         |                 _______
181        |                         |
182        |                         |    Both Edges     ___
183        |                         |  <------------   / B \
184        |                         |                 _______
185        +-------------------------+
186
187In this example, two Signals (quadrature encoder lines A and B) are
188associated with a single Count: a rising or falling edge on either A or
189B triggers the "Quadrature x4" function which determines the direction
190of movement and updates the respective position data. The "Quadrature
191x4" function is likely implemented in the hardware of the quadrature
192encoder counter device; the Count, Signals, and Synapses simply
193represent this hardware behavior and functionality.
194
195Signals associated with the same Count can have differing Synapse action
196mode conditions. For example, a quadrature encoder counter device
197operating in a non-quadrature Pulse-Direction mode could have one input
198line dedicated for movement and a second input line dedicated for
199direction::
200
201                   Count                   Synapse      Signal
202                   -----                   -------      ------
203        +---------------------------+
204        | Data: Position            |    Rising Edge     ___
205        | Function: Pulse-Direction |  <-------------   / A \ (Movement)
206        |                           |                  _______
207        |                           |
208        |                           |       None         ___
209        |                           |  <-------------   / B \ (Direction)
210        |                           |                  _______
211        +---------------------------+
212
213Only Signal A triggers the "Pulse-Direction" update function, but the
214instantaneous state of Signal B is still required in order to know the
215direction so that the position data may be properly updated. Ultimately,
216both Signals are associated with the same Count via two respective
217Synapses, but only one Synapse has an active action mode condition which
218triggers the respective count function while the other is left with a
219"None" condition action mode to indicate its respective Signal's
220availability for state evaluation despite its non-triggering mode.
221
222Keep in mind that the Signal, Synapse, and Count are abstract
223representations which do not need to be closely married to their
224respective physical sources. This allows the user of a counter to
225divorce themselves from the nuances of physical components (such as
226whether an input line is differential or single-ended) and instead focus
227on the core idea of what the data and process represent (e.g. position
228as interpreted from quadrature encoding data).
229
230Userspace Interface
231===================
232
233Several sysfs attributes are generated by the Generic Counter interface,
234and reside under the /sys/bus/counter/devices/counterX directory, where
235counterX refers to the respective counter device. Please see
236Documentation/ABI/testing/sysfs-bus-counter for detailed
237information on each Generic Counter interface sysfs attribute.
238
239Through these sysfs attributes, programs and scripts may interact with
240the Generic Counter paradigm Counts, Signals, and Synapses of respective
241counter devices.
242
243Driver API
244==========
245
246Driver authors may utilize the Generic Counter interface in their code
247by including the include/linux/counter.h header file. This header file
248provides several core data structures, function prototypes, and macros
249for defining a counter device.
250
251.. kernel-doc:: include/linux/counter.h
252   :internal:
253
254.. kernel-doc:: drivers/counter/counter.c
255   :export:
256
257Implementation
258==============
259
260To support a counter device, a driver must first allocate the available
261Counter Signals via counter_signal structures. These Signals should
262be stored as an array and set to the signals array member of an
263allocated counter_device structure before the Counter is registered to
264the system.
265
266Counter Counts may be allocated via counter_count structures, and
267respective Counter Signal associations (Synapses) made via
268counter_synapse structures. Associated counter_synapse structures are
269stored as an array and set to the the synapses array member of the
270respective counter_count structure. These counter_count structures are
271set to the counts array member of an allocated counter_device structure
272before the Counter is registered to the system.
273
274Driver callbacks should be provided to the counter_device structure via
275a constant counter_ops structure in order to communicate with the
276device: to read and write various Signals and Counts, and to set and get
277the "action mode" and "function mode" for various Synapses and Counts
278respectively.
279
280A defined counter_device structure may be registered to the system by
281passing it to the counter_register function, and unregistered by passing
282it to the counter_unregister function. Similarly, the
283devm_counter_register and devm_counter_unregister functions may be used
284if device memory-managed registration is desired.
285
286Extension sysfs attributes can be created for auxiliary functionality
287and data by passing in defined counter_device_ext, counter_count_ext,
288and counter_signal_ext structures. In these cases, the
289counter_device_ext structure is used for global configuration of the
290respective Counter device, while the counter_count_ext and
291counter_signal_ext structures allow for auxiliary exposure and
292configuration of a specific Count or Signal respectively.
293
294Architecture
295============
296
297When the Generic Counter interface counter module is loaded, the
298counter_init function is called which registers a bus_type named
299"counter" to the system. Subsequently, when the module is unloaded, the
300counter_exit function is called which unregisters the bus_type named
301"counter" from the system.
302
303Counter devices are registered to the system via the counter_register
304function, and later removed via the counter_unregister function. The
305counter_register function establishes a unique ID for the Counter
306device and creates a respective sysfs directory, where X is the
307mentioned unique ID:
308
309    /sys/bus/counter/devices/counterX
310
311Sysfs attributes are created within the counterX directory to expose
312functionality, configurations, and data relating to the Counts, Signals,
313and Synapses of the Counter device, as well as options and information
314for the Counter device itself.
315
316Each Signal has a directory created to house its relevant sysfs
317attributes, where Y is the unique ID of the respective Signal:
318
319    /sys/bus/counter/devices/counterX/signalY
320
321Similarly, each Count has a directory created to house its relevant
322sysfs attributes, where Y is the unique ID of the respective Count:
323
324    /sys/bus/counter/devices/counterX/countY
325
326For a more detailed breakdown of the available Generic Counter interface
327sysfs attributes, please refer to the
328Documentation/ABI/testing/sysfs-bus-counter file.
329
330The Signals and Counts associated with the Counter device are registered
331to the system as well by the counter_register function. The
332signal_read/signal_write driver callbacks are associated with their
333respective Signal attributes, while the count_read/count_write and
334function_get/function_set driver callbacks are associated with their
335respective Count attributes; similarly, the same is true for the
336action_get/action_set driver callbacks and their respective Synapse
337attributes. If a driver callback is left undefined, then the respective
338read/write permission is left disabled for the relevant attributes.
339
340Similarly, extension sysfs attributes are created for the defined
341counter_device_ext, counter_count_ext, and counter_signal_ext
342structures that are passed in.
343