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).
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
typeCounter component data type
namedevice-specific component name
privcomponent-relevant data 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.
- 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.
{unnamed_union}anonymous
{unnamed_union}anonymous
-
struct counter_signal¶
Counter Signal node
Definition
struct counter_signal {
int id;
const char *name;
struct counter_comp *ext;
size_t num_ext;
};
Members
idunique ID used to identify the Signal
namedevice-specific Signal name
extoptional array of Signal extensions
num_extnumber of 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_listarray of available action modes
num_actionsnumber of action modes specified in actions_list
signalpointer to the 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
idunique ID used to identify the Count
namedevice-specific Count name
functions_listarray of available function modes
num_functionsnumber of function modes specified in functions_list
synapsesarray of Synapses for initialization
num_synapsesnumber of Synapses specified in synapses
extoptional array of Count extensions
num_extnumber of Count extensions specified in ext
-
struct counter_event_node¶
Counter Event node
Definition
struct counter_event_node {
struct list_head l;
u8 event;
u8 channel;
struct list_head comp_list;
};
Members
llist of current watching Counter events
eventevent that triggers
channelevent channel
comp_listlist of components to watch when event triggers
-
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);
int (*events_configure)(struct counter_device *counter);
int (*watch_validate)(struct counter_device *counter, const struct counter_watch *watch);
};
Members
signal_readoptional read callback for Signals. The read level of the respective Signal should be passed back via the level parameter.
count_readread callback for Counts. The read value of the respective Count should be passed back via the value parameter.
count_writeoptional write callback for Counts. The write value for the respective Count is passed in via the value parameter.
function_readread callback the Count function modes. The read function mode of the respective Count should be passed back via the function parameter.
function_writeoptional write callback for Count function modes. The function mode to write for the respective Count is passed in via the function parameter.
action_readoptional read callback the Synapse action modes. The read action mode of the respective Synapse should be passed back via the action parameter.
action_writeoptional write callback for Synapse action modes. The action mode to write for the respective Synapse is passed in via the action parameter.
events_configureoptional write callback to configure events. The list of
struct counter_event_nodemay be accessed via the events_list member of the counter parameter.watch_validateoptional callback to validate a watch. The Counter component watch configuration is passed in via the watch parameter. A return value of 0 indicates a valid Counter component watch configuration.
-
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;
struct cdev chrdev;
struct list_head events_list;
spinlock_t events_list_lock;
struct list_head next_events_list;
struct mutex n_events_list_lock;
struct counter_event *events;
wait_queue_head_t events_wait;
spinlock_t events_in_lock;
struct mutex events_out_lock;
struct mutex ops_exist_lock;
};
Members
namename of the device
parentoptional parent device providing the counters
opscallbacks from driver
signalsarray of Signals
num_signalsnumber of Signals specified in signals
countsarray of Counts
num_countsnumber of Counts specified in counts
extoptional array of Counter device extensions
num_extnumber of Counter device extensions specified in ext
privoptional private data supplied by driver
devinternal device structure
chrdevinternal character device structure
events_listlist of current watching Counter events
events_list_locklock to protect Counter events list operations
next_events_listlist of next watching Counter events
n_events_list_locklock to protect Counter next events list operations
eventsqueue of detected Counter events
events_waitwait queue to allow blocking reads of Counter events
events_in_locklock to protect Counter events queue in operations
events_out_locklock to protect Counter events queue out operations
ops_exist_locklock to prevent use during removal
-
int counter_register(struct counter_device *const counter)¶
register Counter to the system
Parameters
struct counter_device *const counterpointer 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.
Return
0 on success, negative error number on failure.
-
void counter_unregister(struct counter_device *const counter)¶
unregister Counter from the system
Parameters
struct counter_device *const counterpointer 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 *devdevice to allocate counter_device for
struct counter_device *const counterpointer 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.
-
void counter_push_event(struct counter_device *const counter, const u8 event, const u8 channel)¶
queue event for userspace reading
Parameters
struct counter_device *const counterpointer to Counter structure
const u8 eventtriggered event
const u8 channelevent channel
Note
If no one is watching for the respective event, it is silently discarded.
Driver 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 must 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 function may be used if device memory-managed registration is desired.
The struct counter_comp structure is used to define counter extensions
for Signals, Synapses, and Counts.
The “type” member specifies the type of high-level data (e.g. BOOL,
COUNT_DIRECTION, etc.) handled by this extension. The “*_read” and
“*_write” members can then be set by the counter device driver with
callbacks to handle that data using native C data types (i.e. u8, u64,
etc.).
Convenience macros such as COUNTER_COMP_COUNT_U64 are provided for
use by driver authors. In particular, driver authors are expected to use
the provided macros for standard Counter subsystem attributes in order
to maintain a consistent interface for userspace. For example, a counter
device driver may define several standard attributes like so:
struct counter_comp count_ext[] = {
COUNTER_COMP_DIRECTION(count_direction_read),
COUNTER_COMP_ENABLE(count_enable_read, count_enable_write),
COUNTER_COMP_CEILING(count_ceiling_read, count_ceiling_write),
};
This makes it simple to see, add, and modify the attributes that are supported by this driver (“direction”, “enable”, and “ceiling”) and to maintain this code without getting lost in a web of struct braces.
Callbacks must match the function type expected for the respective
component or extension. These function types are defined in the struct
counter_comp structure as the “*_read” and “*_write” union
members.
The corresponding callback prototypes for the extensions mentioned in the previous example above would be:
int count_direction_read(struct counter_device *counter,
struct counter_count *count,
enum counter_count_direction *direction);
int count_enable_read(struct counter_device *counter,
struct counter_count *count, u8 *enable);
int count_enable_write(struct counter_device *counter,
struct counter_count *count, u8 enable);
int count_ceiling_read(struct counter_device *counter,
struct counter_count *count, u64 *ceiling);
int count_ceiling_write(struct counter_device *counter,
struct counter_count *count, u64 ceiling);
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
Subsystem Architecture¶
Counter drivers pass and take data natively (i.e. u8, u64, etc.)
and the shared counter module handles the translation between the sysfs
interface. This guarantees a standard userspace interface for all
counter drivers, and enables a Generic Counter chrdev interface via a
generalized device driver ABI.
A high-level view of how a count value is passed down from a counter driver is exemplified by the following. The driver callbacks are first registered to the Counter core component for use by the Counter userspace interface components:
Driver callbacks registration:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+----------------------------+
| Counter device driver |
+----------------------------+
| Processes data from device |
+----------------------------+
|
-------------------
/ driver callbacks /
-------------------
|
V
+----------------------+
| Counter core |
+----------------------+
| Routes device driver |
| callbacks to the |
| userspace interfaces |
+----------------------+
|
-------------------
/ driver callbacks /
-------------------
|
+---------------+---------------+
| |
V V
+--------------------+ +---------------------+
| Counter sysfs | | Counter chrdev |
+--------------------+ +---------------------+
| Translates to the | | Translates to the |
| standard Counter | | standard Counter |
| sysfs output | | character device |
+--------------------+ +---------------------+
Thereafter, data can be transferred directly between the Counter device driver and Counter userspace interface:
Count data request:
~~~~~~~~~~~~~~~~~~~
----------------------
/ Counter device \
+----------------------+
| Count register: 0x28 |
+----------------------+
|
-----------------
/ raw count data /
-----------------
|
V
+----------------------------+
| Counter device driver |
+----------------------------+
| Processes data from device |
|----------------------------|
| Type: u64 |
| Value: 42 |
+----------------------------+
|
----------
/ u64 /
----------
|
+---------------+---------------+
| |
V V
+--------------------+ +---------------------+
| Counter sysfs | | Counter chrdev |
+--------------------+ +---------------------+
| Translates to the | | Translates to the |
| standard Counter | | standard Counter |
| sysfs output | | character device |
|--------------------| |---------------------|
| Type: const char * | | Type: u64 |
| Value: "42" | | Value: 42 |
+--------------------+ +---------------------+
| |
--------------- -----------------------
/ const char * / / struct counter_event /
--------------- -----------------------
| |
| V
| +-----------+
| | read |
| +-----------+
| \ Count: 42 /
| -----------
|
V
+--------------------------------------------------+
| `/sys/bus/counter/devices/counterX/countY/count` |
+--------------------------------------------------+
\ Count: "42" /
--------------------------------------------------
There are four primary components involved:
Counter device driver¶
Communicates with the hardware device to read/write data; e.g. counter drivers for quadrature encoders, timers, etc.
Counter core¶
Registers the counter device driver to the system so that the respective callbacks are called during userspace interaction.
Counter sysfs¶
Translates counter data to the standard Counter sysfs interface format and vice versa.
Please refer to the Documentation/ABI/testing/sysfs-bus-counter file
for a detailed breakdown of the available Generic Counter interface
sysfs attributes.
Counter chrdev¶
Translates Counter events to the standard Counter character device; data is transferred via standard character device read calls, while Counter events are configured via ioctl calls.
Sysfs Interface¶
Several sysfs attributes are generated by the Generic Counter interface,
and reside under the /sys/bus/counter/devices/counterX directory,
where X is to the respective counter device id. 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.
Counter Character Device¶
Counter character device nodes are created under the /dev directory
as counterX, where X is the respective counter device id.
Defines for the standard Counter data types are exposed via the
userspace include/uapi/linux/counter.h file.
Counter events¶
Counter device drivers can support Counter events by utilizing the
counter_push_event function:
void counter_push_event(struct counter_device *const counter, const u8 event,
const u8 channel);
The event id is specified by the event parameter; the event channel
id is specified by the channel parameter. When this function is
called, the Counter data associated with the respective event is
gathered, and a struct counter_event is generated for each datum and
pushed to userspace.
Counter events can be configured by users to report various Counter data of interest. This can be conceptualized as a list of Counter component read calls to perform. For example:
COUNTER_EVENT_OVERFLOW
COUNTER_EVENT_INDEX
Channel 0
Channel 0
Count 0
Count 1
Signal 3
Count 4 Extension 2
Signal 5 Extension 0
Signal 0
Signal 0 Extension 0
Extension 4
Channel 1
Signal 4
Signal 4 Extension 0
Count 7
When counter_push_event(counter, COUNTER_EVENT_INDEX, 1) is called
for example, it will go down the list for the COUNTER_EVENT_INDEX
event channel 1 and execute the read callbacks for Signal 4, Signal 4
Extension 0, and Count 7 – the data returned for each is pushed to a
kfifo as a struct counter_event, which userspace can retrieve via a
standard read operation on the respective character device node.
Userspace¶
Userspace applications can configure Counter events via ioctl operations
on the Counter character device node. There following ioctl codes are
supported and provided by the linux/counter.h userspace header file:
COUNTER_ADD_WATCH_IOCTLCOUNTER_ENABLE_EVENTS_IOCTLCOUNTER_DISABLE_EVENTS_IOCTL
To configure events to gather Counter data, users first populate a
struct counter_watch with the relevant event id, event channel id,
and the information for the desired Counter component from which to
read, and then pass it via the COUNTER_ADD_WATCH_IOCTL ioctl
command.
Note that an event can be watched without gathering Counter data by
setting the component.type member equal to
COUNTER_COMPONENT_NONE. With this configuration the Counter
character device will simply populate the event timestamps for those
respective struct counter_event elements and ignore the component
value.
The COUNTER_ADD_WATCH_IOCTL command will buffer these Counter
watches. When ready, the COUNTER_ENABLE_EVENTS_IOCTL ioctl command
may be used to activate these Counter watches.
Userspace applications can then execute a read operation (optionally
calling poll first) on the Counter character device node to retrieve
struct counter_event elements with the desired data.