Generic Counter Interface

Introduction

Counter devices are prevalent among 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:

  • Signal: Stream of data to be evaluated by the counter.
  • Synapse: Association of a Signal, and evaluation trigger, with a Count.
  • Count: Accumulation of the effects of connected Synapses.

SIGNAL

A Signal represents a stream of 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, so exposure is optional for drivers.

When the Signal data is available for user access, the Generic Counter interface provides the following available signal values:

  • SIGNAL_LOW: Signal line is in a low state.
  • SIGNAL_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 Count. Signal data affects respective Count data, and the Synapse represents this relationship.

The Synapse action mode specifies the Signal data condition that 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.

A Synapse associates one Signal with one Count.

COUNT

A Count represents the accumulation of the effects of connected Synapses; i.e. the count data for a set of Signals. The Generic Counter interface represents the count data as a natural number.

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 Synapses.

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.

struct counter_comp

Counter component node

Definition

struct counter_comp {
  enum counter_comp_type type;
  const char *name;
  void *priv;
  union {
    int (*action_read)(struct counter_device *counter,struct counter_count *count,struct counter_synapse *synapse, enum counter_synapse_action *action);
    int (*device_u8_read)(struct counter_device *counter, u8 *val);
    int (*count_u8_read)(struct counter_device *counter, struct counter_count *count, u8 *val);
    int (*signal_u8_read)(struct counter_device *counter, struct counter_signal *signal, u8 *val);
    int (*device_u32_read)(struct counter_device *counter, u32 *val);
    int (*count_u32_read)(struct counter_device *counter, struct counter_count *count, u32 *val);
    int (*signal_u32_read)(struct counter_device *counter, struct counter_signal *signal, u32 *val);
    int (*device_u64_read)(struct counter_device *counter, u64 *val);
    int (*count_u64_read)(struct counter_device *counter, struct counter_count *count, u64 *val);
    int (*signal_u64_read)(struct counter_device *counter, struct counter_signal *signal, u64 *val);
  };
  union {
    int (*action_write)(struct counter_device *counter,struct counter_count *count,struct counter_synapse *synapse, enum counter_synapse_action action);
    int (*device_u8_write)(struct counter_device *counter, u8 val);
    int (*count_u8_write)(struct counter_device *counter, struct counter_count *count, u8 val);
    int (*signal_u8_write)(struct counter_device *counter, struct counter_signal *signal, u8 val);
    int (*device_u32_write)(struct counter_device *counter, u32 val);
    int (*count_u32_write)(struct counter_device *counter, struct counter_count *count, u32 val);
    int (*signal_u32_write)(struct counter_device *counter, struct counter_signal *signal, u32 val);
    int (*device_u64_write)(struct counter_device *counter, u64 val);
    int (*count_u64_write)(struct counter_device *counter, struct counter_count *count, u64 val);
    int (*signal_u64_write)(struct counter_device *counter, struct counter_signal *signal, u64 val);
  };
};

Members

type
Counter component data type
name
device-specific component name
priv
component-relevant data
{unnamed_union}
anonymous
action_read
Synapse action mode read callback. The read value of the respective Synapse action mode should be passed back via the action parameter.
device_u8_read
Device u8 component read callback. The read value of the respective Device u8 component should be passed back via the val parameter.
count_u8_read
Count u8 component read callback. The read value of the respective Count u8 component should be passed back via the val parameter.
signal_u8_read
Signal u8 component read callback. The read value of the respective Signal u8 component should be passed back via the val parameter.
device_u32_read
Device u32 component read callback. The read value of the respective Device u32 component should be passed back via the val parameter.
count_u32_read
Count u32 component read callback. The read value of the respective Count u32 component should be passed back via the val parameter.
signal_u32_read
Signal u32 component read callback. The read value of the respective Signal u32 component should be passed back via the val parameter.
device_u64_read
Device u64 component read callback. The read value of the respective Device u64 component should be passed back via the val parameter.
count_u64_read
Count u64 component read callback. The read value of the respective Count u64 component should be passed back via the val parameter.
signal_u64_read
Signal u64 component read callback. The read value of the respective Signal u64 component should be passed back via the val parameter.
{unnamed_union}
anonymous
action_write
Synapse action mode write callback. The write value of the respective Synapse action mode is passed via the action parameter.
device_u8_write
Device u8 component write callback. The write value of the respective Device u8 component is passed via the val parameter.
count_u8_write
Count u8 component write callback. The write value of the respective Count u8 component is passed via the val parameter.
signal_u8_write
Signal u8 component write callback. The write value of the respective Signal u8 component is passed via the val parameter.
device_u32_write
Device u32 component write callback. The write value of the respective Device u32 component is passed via the val parameter.
count_u32_write
Count u32 component write callback. The write value of the respective Count u32 component is passed via the val parameter.
signal_u32_write
Signal u32 component write callback. The write value of the respective Signal u32 component is passed via the val parameter.
device_u64_write
Device u64 component write callback. The write value of the respective Device u64 component is passed via the val parameter.
count_u64_write
Count u64 component write callback. The write value of the respective Count u64 component is passed via the val parameter.
signal_u64_write
Signal u64 component write callback. The write value of the respective Signal u64 component is passed via the val parameter.
struct counter_signal

Counter Signal node

Definition

struct counter_signal {
  int id;
  const char *name;
  struct counter_comp *ext;
  size_t num_ext;
};

Members

id
unique ID used to identify signal
name
device-specific Signal name; ideally, this should match the name as it appears in the datasheet documentation
ext
optional array of Counter Signal extensions
num_ext
number of Counter Signal extensions specified in ext
struct counter_synapse

Counter Synapse node

Definition

struct counter_synapse {
  const enum counter_synapse_action *actions_list;
  size_t num_actions;
  struct counter_signal *signal;
};

Members

actions_list
array of available action modes
num_actions
number of action modes specified in actions_list
signal
pointer to associated signal
struct counter_count

Counter Count node

Definition

struct counter_count {
  int id;
  const char *name;
  const enum counter_function *functions_list;
  size_t num_functions;
  struct counter_synapse *synapses;
  size_t num_synapses;
  struct counter_comp *ext;
  size_t num_ext;
};

Members

id
unique ID used to identify Count
name
device-specific Count name; ideally, this should match the name as it appears in the datasheet documentation
functions_list
array available function modes
num_functions
number of function modes specified in functions_list
synapses
array of synapses for initialization
num_synapses
number of synapses specified in synapses
ext
optional array of Counter Count extensions
num_ext
number of Counter Count extensions specified in ext
struct counter_ops

Callbacks from driver

Definition

struct counter_ops {
  int (*signal_read)(struct counter_device *counter,struct counter_signal *signal, enum counter_signal_level *level);
  int (*count_read)(struct counter_device *counter, struct counter_count *count, u64 *value);
  int (*count_write)(struct counter_device *counter, struct counter_count *count, u64 value);
  int (*function_read)(struct counter_device *counter,struct counter_count *count, enum counter_function *function);
  int (*function_write)(struct counter_device *counter,struct counter_count *count, enum counter_function function);
  int (*action_read)(struct counter_device *counter,struct counter_count *count,struct counter_synapse *synapse, enum counter_synapse_action *action);
  int (*action_write)(struct counter_device *counter,struct counter_count *count,struct counter_synapse *synapse, enum counter_synapse_action action);
};

Members

signal_read
optional read callback for Signal attribute. The read level of the respective Signal should be passed back via the level parameter.
count_read
optional read callback for Count attribute. The read value of the respective Count should be passed back via the val parameter.
count_write
optional write callback for Count attribute. The write value for the respective Count is passed in via the val parameter.
function_read
read callback the Count function modes. The read function mode of the respective Count should be passed back via the function parameter.
function_write
write callback for Count function modes. The function mode to write for the respective Count is passed in via the function parameter.
action_read
read callback the Synapse action modes. The read action mode of the respective Synapse should be passed back via the action parameter.
action_write
write callback for Synapse action modes. The action mode to write for the respective Synapse is passed in via the action parameter.
struct counter_device

Counter data structure

Definition

struct counter_device {
  const char *name;
  struct device *parent;
  const struct counter_ops *ops;
  struct counter_signal *signals;
  size_t num_signals;
  struct counter_count *counts;
  size_t num_counts;
  struct counter_comp *ext;
  size_t num_ext;
  void *priv;
  struct device dev;
};

Members

name
name of the device as it appears in the datasheet
parent
optional parent device providing the counters
ops
callbacks from driver
signals
array of Signals
num_signals
number of Signals specified in signals
counts
array of Counts
num_counts
number of Counts specified in counts
ext
optional array of Counter device extensions
num_ext
number of Counter device extensions specified in ext
priv
optional private data supplied by driver
dev
internal device structure
int counter_register(struct counter_device *const counter)

register Counter to the system

Parameters

struct counter_device *const counter
pointer to Counter to register

Description

This function registers a Counter to the system. A sysfs “counter” directory will be created and populated with sysfs attributes correlating with the Counter Signals, Synapses, and Counts respectively.

void counter_unregister(struct counter_device *const counter)

unregister Counter from the system

Parameters

struct counter_device *const counter
pointer to Counter to unregister

Description

The Counter is unregistered from the system.

int devm_counter_register(struct device *dev, struct counter_device *const counter)

Resource-managed counter_register

Parameters

struct device *dev
device to allocate counter_device for
struct counter_device *const counter
pointer to Counter to register

Description

Managed counter_register. The Counter registered with this function is automatically unregistered on driver detach. This function calls counter_register internally. Refer to that function for more information.

Return

0 on success, negative error number on failure.

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 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/miscellaneous exposure and 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.

Determining the type of extension to create is a matter of scope.

  • Signal extensions are attributes that expose information/control specific to a Signal. These types of attributes will exist under a Signal’s directory in sysfs.

    For example, if you have an invert feature for a Signal, you can have a Signal extension called “invert” that toggles that feature: /sys/bus/counter/devices/counterX/signalY/invert

  • Count extensions are attributes that expose information/control specific to a Count. These type of attributes will exist under a Count’s directory in sysfs.

    For example, if you want to pause/unpause a Count from updating, you can have a Count extension called “enable” that toggles such: /sys/bus/counter/devices/counterX/countY/enable

  • Device extensions are attributes that expose information/control non-specific to a particular Count or Signal. This is where you would put your global features or other miscellaneous functionality.

    For example, if your device has an overtemp sensor, you can report the chip overheated via a device extension called “error_overtemp”: /sys/bus/counter/devices/counterX/error_overtemp

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.