1.. SPDX-License-Identifier: GPL-2.0
2
3=========================
4Generic Counter Interface
5=========================
6
7Introduction
8============
9
10Counter devices are prevalent among 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* Signal:
30  Stream of data to be evaluated by the counter.
31
32* Synapse:
33  Association of a Signal, and evaluation trigger, with a Count.
34
35* Count:
36  Accumulation of the effects of connected Synapses.
37
38SIGNAL
39------
40A Signal represents a stream of data. This is the input data that is
41evaluated by the counter to determine the count data; e.g. a quadrature
42signal output line of a rotary encoder. Not all counter devices provide
43user access to the Signal data, so exposure is optional for drivers.
44
45When the Signal data is available for user access, the Generic Counter
46interface provides the following available signal values:
47
48* SIGNAL_LOW:
49  Signal line is in a low state.
50
51* SIGNAL_HIGH:
52  Signal line is in a high state.
53
54A Signal may be associated with one or more Counts.
55
56SYNAPSE
57-------
58A Synapse represents the association of a Signal with a Count. Signal
59data affects respective Count data, and the Synapse represents this
60relationship.
61
62The Synapse action mode specifies the Signal data condition that
63triggers the respective Count's count function evaluation to update the
64count data. The Generic Counter interface provides the following
65available action modes:
66
67* None:
68  Signal does not trigger the count function. In Pulse-Direction count
69  function mode, this Signal is evaluated as Direction.
70
71* Rising Edge:
72  Low state transitions to high state.
73
74* Falling Edge:
75  High state transitions to low state.
76
77* Both Edges:
78  Any state transition.
79
80A counter is defined as a set of input signals associated with count
81data that are generated by the evaluation of the state of the associated
82input signals as defined by the respective count functions. Within the
83context of the Generic Counter interface, a counter consists of Counts
84each associated with a set of Signals, whose respective Synapse
85instances represent the count function update conditions for the
86associated Counts.
87
88A Synapse associates one Signal with one Count.
89
90COUNT
91-----
92A Count represents the accumulation of the effects of connected
93Synapses; i.e. the count data for a set of Signals. The Generic
94Counter interface represents the count data as a natural number.
95
96A Count has a count function mode which represents the update behavior
97for the count data. The Generic Counter interface provides the following
98available count function modes:
99
100* Increase:
101  Accumulated count is incremented.
102
103* Decrease:
104  Accumulated count is decremented.
105
106* Pulse-Direction:
107  Rising edges on signal A updates the respective count. The input level
108  of signal B determines direction.
109
110* Quadrature:
111  A pair of quadrature encoding signals are evaluated to determine
112  position and direction. The following Quadrature modes are available:
113
114  - x1 A:
115    If direction is forward, rising edges on quadrature pair signal A
116    updates the respective count; if the direction is backward, falling
117    edges on quadrature pair signal A updates the respective count.
118    Quadrature encoding determines the direction.
119
120  - x1 B:
121    If direction is forward, rising edges on quadrature pair signal B
122    updates the respective count; if the direction is backward, falling
123    edges on quadrature pair signal B updates the respective count.
124    Quadrature encoding determines the direction.
125
126  - x2 A:
127    Any state transition on quadrature pair signal A updates the
128    respective count. Quadrature encoding determines the direction.
129
130  - x2 B:
131    Any state transition on quadrature pair signal B updates the
132    respective count. Quadrature encoding determines the direction.
133
134  - x4:
135    Any state transition on either quadrature pair signals updates the
136    respective count. Quadrature encoding determines the direction.
137
138A Count has a set of one or more associated Synapses.
139
140Paradigm
141========
142
143The most basic counter device may be expressed as a single Count
144associated with a single Signal via a single Synapse. Take for example
145a counter device which simply accumulates a count of rising edges on a
146source input line::
147
148                Count                Synapse        Signal
149                -----                -------        ------
150        +---------------------+
151        | Data: Count         |    Rising Edge     ________
152        | Function: Increase  |  <-------------   / Source \
153        |                     |                  ____________
154        +---------------------+
155
156In this example, the Signal is a source input line with a pulsing
157voltage, while the Count is a persistent count value which is repeatedly
158incremented. The Signal is associated with the respective Count via a
159Synapse. The increase function is triggered by the Signal data condition
160specified by the Synapse -- in this case a rising edge condition on the
161voltage input line. In summary, the counter device existence and
162behavior is aptly represented by respective Count, Signal, and Synapse
163components: a rising edge condition triggers an increase function on an
164accumulating count datum.
165
166A counter device is not limited to a single Signal; in fact, in theory
167many Signals may be associated with even a single Count. For example, a
168quadrature encoder counter device can keep track of position based on
169the states of two input lines::
170
171                   Count                 Synapse     Signal
172                   -----                 -------     ------
173        +-------------------------+
174        | Data: Position          |    Both Edges     ___
175        | Function: Quadrature x4 |  <------------   / A \
176        |                         |                 _______
177        |                         |
178        |                         |    Both Edges     ___
179        |                         |  <------------   / B \
180        |                         |                 _______
181        +-------------------------+
182
183In this example, two Signals (quadrature encoder lines A and B) are
184associated with a single Count: a rising or falling edge on either A or
185B triggers the "Quadrature x4" function which determines the direction
186of movement and updates the respective position data. The "Quadrature
187x4" function is likely implemented in the hardware of the quadrature
188encoder counter device; the Count, Signals, and Synapses simply
189represent this hardware behavior and functionality.
190
191Signals associated with the same Count can have differing Synapse action
192mode conditions. For example, a quadrature encoder counter device
193operating in a non-quadrature Pulse-Direction mode could have one input
194line dedicated for movement and a second input line dedicated for
195direction::
196
197                   Count                   Synapse      Signal
198                   -----                   -------      ------
199        +---------------------------+
200        | Data: Position            |    Rising Edge     ___
201        | Function: Pulse-Direction |  <-------------   / A \ (Movement)
202        |                           |                  _______
203        |                           |
204        |                           |       None         ___
205        |                           |  <-------------   / B \ (Direction)
206        |                           |                  _______
207        +---------------------------+
208
209Only Signal A triggers the "Pulse-Direction" update function, but the
210instantaneous state of Signal B is still required in order to know the
211direction so that the position data may be properly updated. Ultimately,
212both Signals are associated with the same Count via two respective
213Synapses, but only one Synapse has an active action mode condition which
214triggers the respective count function while the other is left with a
215"None" condition action mode to indicate its respective Signal's
216availability for state evaluation despite its non-triggering mode.
217
218Keep in mind that the Signal, Synapse, and Count are abstract
219representations which do not need to be closely married to their
220respective physical sources. This allows the user of a counter to
221divorce themselves from the nuances of physical components (such as
222whether an input line is differential or single-ended) and instead focus
223on the core idea of what the data and process represent (e.g. position
224as interpreted from quadrature encoding data).
225
226Userspace Interface
227===================
228
229Several sysfs attributes are generated by the Generic Counter interface,
230and reside under the /sys/bus/counter/devices/counterX directory, where
231counterX refers to the respective counter device. Please see
232Documentation/ABI/testing/sysfs-bus-counter for detailed
233information on each Generic Counter interface sysfs attribute.
234
235Through these sysfs attributes, programs and scripts may interact with
236the Generic Counter paradigm Counts, Signals, and Synapses of respective
237counter devices.
238
239Driver API
240==========
241
242Driver authors may utilize the Generic Counter interface in their code
243by including the include/linux/counter.h header file. This header file
244provides several core data structures, function prototypes, and macros
245for defining a counter device.
246
247.. kernel-doc:: include/linux/counter.h
248   :internal:
249
250.. kernel-doc:: drivers/counter/counter.c
251   :export:
252
253Implementation
254==============
255
256To support a counter device, a driver must first allocate the available
257Counter Signals via counter_signal structures. These Signals should
258be stored as an array and set to the signals array member of an
259allocated counter_device structure before the Counter is registered to
260the system.
261
262Counter Counts may be allocated via counter_count structures, and
263respective Counter Signal associations (Synapses) made via
264counter_synapse structures. Associated counter_synapse structures are
265stored as an array and set to the synapses array member of the
266respective counter_count structure. These counter_count structures are
267set to the counts array member of an allocated counter_device structure
268before the Counter is registered to the system.
269
270Driver callbacks should be provided to the counter_device structure via
271a constant counter_ops structure in order to communicate with the
272device: to read and write various Signals and Counts, and to set and get
273the "action mode" and "function mode" for various Synapses and Counts
274respectively.
275
276A defined counter_device structure may be registered to the system by
277passing it to the counter_register function, and unregistered by passing
278it to the counter_unregister function. Similarly, the
279devm_counter_register and devm_counter_unregister functions may be used
280if device memory-managed registration is desired.
281
282Extension sysfs attributes can be created for auxiliary functionality
283and data by passing in defined counter_device_ext, counter_count_ext,
284and counter_signal_ext structures. In these cases, the
285counter_device_ext structure is used for global/miscellaneous exposure
286and configuration of the respective Counter device, while the
287counter_count_ext and counter_signal_ext structures allow for auxiliary
288exposure and configuration of a specific Count or Signal respectively.
289
290Determining the type of extension to create is a matter of scope.
291
292* Signal extensions are attributes that expose information/control
293  specific to a Signal. These types of attributes will exist under a
294  Signal's directory in sysfs.
295
296  For example, if you have an invert feature for a Signal, you can have
297  a Signal extension called "invert" that toggles that feature:
298  /sys/bus/counter/devices/counterX/signalY/invert
299
300* Count extensions are attributes that expose information/control
301  specific to a Count. These type of attributes will exist under a
302  Count's directory in sysfs.
303
304  For example, if you want to pause/unpause a Count from updating, you
305  can have a Count extension called "enable" that toggles such:
306  /sys/bus/counter/devices/counterX/countY/enable
307
308* Device extensions are attributes that expose information/control
309  non-specific to a particular Count or Signal. This is where you would
310  put your global features or other miscellaneous functionality.
311
312  For example, if your device has an overtemp sensor, you can report the
313  chip overheated via a device extension called "error_overtemp":
314  /sys/bus/counter/devices/counterX/error_overtemp
315
316Architecture
317============
318
319When the Generic Counter interface counter module is loaded, the
320counter_init function is called which registers a bus_type named
321"counter" to the system. Subsequently, when the module is unloaded, the
322counter_exit function is called which unregisters the bus_type named
323"counter" from the system.
324
325Counter devices are registered to the system via the counter_register
326function, and later removed via the counter_unregister function. The
327counter_register function establishes a unique ID for the Counter
328device and creates a respective sysfs directory, where X is the
329mentioned unique ID:
330
331    /sys/bus/counter/devices/counterX
332
333Sysfs attributes are created within the counterX directory to expose
334functionality, configurations, and data relating to the Counts, Signals,
335and Synapses of the Counter device, as well as options and information
336for the Counter device itself.
337
338Each Signal has a directory created to house its relevant sysfs
339attributes, where Y is the unique ID of the respective Signal:
340
341    /sys/bus/counter/devices/counterX/signalY
342
343Similarly, each Count has a directory created to house its relevant
344sysfs attributes, where Y is the unique ID of the respective Count:
345
346    /sys/bus/counter/devices/counterX/countY
347
348For a more detailed breakdown of the available Generic Counter interface
349sysfs attributes, please refer to the
350Documentation/ABI/testing/sysfs-bus-counter file.
351
352The Signals and Counts associated with the Counter device are registered
353to the system as well by the counter_register function. The
354signal_read/signal_write driver callbacks are associated with their
355respective Signal attributes, while the count_read/count_write and
356function_get/function_set driver callbacks are associated with their
357respective Count attributes; similarly, the same is true for the
358action_get/action_set driver callbacks and their respective Synapse
359attributes. If a driver callback is left undefined, then the respective
360read/write permission is left disabled for the relevant attributes.
361
362Similarly, extension sysfs attributes are created for the defined
363counter_device_ext, counter_count_ext, and counter_signal_ext
364structures that are passed in.
365