| 1 | |
| 2 |
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| 3 | =========================
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| 4 | Generic Counter Interface
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| 5 | =========================
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| 6 |
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| 7 | Introduction
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| 8 | ============
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| 9 |
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| 10 | Counter devices are prevalent among a diverse spectrum of industries.
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| 11 | The ubiquitous presence of these devices necessitates a common interface
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| 12 | and standard of interaction and exposure. This driver API attempts to
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| 13 | resolve the issue of duplicate code found among existing counter device
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| 14 | drivers by introducing a generic counter interface for consumption. The
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| 15 | Generic Counter interface enables drivers to support and expose a common
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| 16 | set of components and functionality present in counter devices.
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| 17 |
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| 18 | Theory
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| 19 | ======
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| 20 |
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| 21 | Counter devices can vary greatly in design, but regardless of whether
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| 22 | some devices are quadrature encoder counters or tally counters, all
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| 23 | counter devices consist of a core set of components. This core set of
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| 24 | components, shared by all counter devices, is what forms the essence of
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| 25 | the Generic Counter interface.
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| 26 |
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| 27 | There are three core components to a counter:
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| 28 |
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| 29 | * Signal:
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| 30 | Stream of data to be evaluated by the counter.
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| 31 |
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| 32 | * Synapse:
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| 33 | Association of a Signal, and evaluation trigger, with a Count.
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| 34 |
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| 35 | * Count:
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| 36 | Accumulation of the effects of connected Synapses.
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| 37 |
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| 38 | SIGNAL
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| 39 | ------
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| 40 | A Signal represents a stream of data. This is the input data that is
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| 41 | evaluated by the counter to determine the count data; e.g. a quadrature
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| 42 | signal output line of a rotary encoder. Not all counter devices provide
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| 43 | user access to the Signal data, so exposure is optional for drivers.
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| 44 |
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| 45 | When the Signal data is available for user access, the Generic Counter
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| 46 | interface provides the following available signal values:
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| 47 |
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| 48 | * SIGNAL_LOW:
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| 49 | Signal line is in a low state.
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| 50 |
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| 51 | * SIGNAL_HIGH:
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| 52 | Signal line is in a high state.
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| 53 |
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| 54 | A Signal may be associated with one or more Counts.
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| 55 |
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| 56 | SYNAPSE
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| 57 | -------
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| 58 | A Synapse represents the association of a Signal with a Count. Signal
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| 59 | data affects respective Count data, and the Synapse represents this
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| 60 | relationship.
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| 61 |
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| 62 | The Synapse action mode specifies the Signal data condition that
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| 63 | triggers the respective Count's count function evaluation to update the
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| 64 | count data. The Generic Counter interface provides the following
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| 65 | available action modes:
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| 66 |
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| 67 | * None:
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| 68 | Signal does not trigger the count function. In Pulse-Direction count
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| 69 | function mode, this Signal is evaluated as Direction.
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| 70 |
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| 71 | * Rising Edge:
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| 72 | Low state transitions to high state.
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| 73 |
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| 74 | * Falling Edge:
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| 75 | High state transitions to low state.
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| 76 |
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| 77 | * Both Edges:
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| 78 | Any state transition.
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| 79 |
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| 80 | A counter is defined as a set of input signals associated with count
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| 81 | data that are generated by the evaluation of the state of the associated
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| 82 | input signals as defined by the respective count functions. Within the
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| 83 | context of the Generic Counter interface, a counter consists of Counts
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| 84 | each associated with a set of Signals, whose respective Synapse
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| 85 | instances represent the count function update conditions for the
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| 86 | associated Counts.
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| 87 |
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| 88 | A Synapse associates one Signal with one Count.
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| 89 |
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| 90 | COUNT
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| 91 | -----
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| 92 | A Count represents the accumulation of the effects of connected
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| 93 | Synapses; i.e. the count data for a set of Signals. The Generic
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| 94 | Counter interface represents the count data as a natural number.
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| 95 |
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| 96 | A Count has a count function mode which represents the update behavior
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| 97 | for the count data. The Generic Counter interface provides the following
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| 98 | available count function modes:
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| 99 |
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| 100 | * Increase:
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| 101 | Accumulated count is incremented.
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| 102 |
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| 103 | * Decrease:
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| 104 | Accumulated count is decremented.
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| 105 |
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| 106 | * Pulse-Direction:
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| 107 | Rising edges on signal A updates the respective count. The input level
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| 108 | of signal B determines direction.
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| 109 |
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| 110 | * Quadrature:
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| 111 | A pair of quadrature encoding signals are evaluated to determine
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| 112 | position and direction. The following Quadrature modes are available:
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| 113 |
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| 114 | - x1 A:
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| 115 | If direction is forward, rising edges on quadrature pair signal A
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| 116 | updates the respective count; if the direction is backward, falling
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| 117 | edges on quadrature pair signal A updates the respective count.
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| 118 | Quadrature encoding determines the direction.
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| 119 |
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| 120 | - x1 B:
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| 121 | If direction is forward, rising edges on quadrature pair signal B
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| 122 | updates the respective count; if the direction is backward, falling
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| 123 | edges on quadrature pair signal B updates the respective count.
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| 124 | Quadrature encoding determines the direction.
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| 125 |
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| 126 | - x2 A:
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| 127 | Any state transition on quadrature pair signal A updates the
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| 128 | respective count. Quadrature encoding determines the direction.
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| 129 |
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| 130 | - x2 B:
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| 131 | Any state transition on quadrature pair signal B updates the
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| 132 | respective count. Quadrature encoding determines the direction.
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| 133 |
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| 134 | - x4:
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| 135 | Any state transition on either quadrature pair signals updates the
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| 136 | respective count. Quadrature encoding determines the direction.
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| 137 |
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| 138 | A Count has a set of one or more associated Synapses.
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| 139 |
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| 140 | Paradigm
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| 141 | ========
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| 142 |
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| 143 | The most basic counter device may be expressed as a single Count
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| 144 | associated with a single Signal via a single Synapse. Take for example
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| 145 | a counter device which simply accumulates a count of rising edges on a
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| 146 | source input line::
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| 147 |
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| 148 | Count Synapse Signal
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| 149 | ----- ------- ------
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| 150 | +---------------------+
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| 151 | | Data: Count | Rising Edge ________
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| 152 | | Function: Increase | <------------- / Source \
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| 153 | | | ____________
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| 154 | +---------------------+
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| 155 |
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| 156 | In this example, the Signal is a source input line with a pulsing
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| 157 | voltage, while the Count is a persistent count value which is repeatedly
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| 158 | incremented. The Signal is associated with the respective Count via a
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| 159 | Synapse. The increase function is triggered by the Signal data condition
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| 160 | specified by the Synapse -- in this case a rising edge condition on the
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| 161 | voltage input line. In summary, the counter device existence and
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| 162 | behavior is aptly represented by respective Count, Signal, and Synapse
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| 163 | components: a rising edge condition triggers an increase function on an
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| 164 | accumulating count datum.
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| 165 |
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| 166 | A counter device is not limited to a single Signal; in fact, in theory
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| 167 | many Signals may be associated with even a single Count. For example, a
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| 168 | quadrature encoder counter device can keep track of position based on
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| 169 | the states of two input lines::
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| 170 |
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| 171 | Count Synapse Signal
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| 172 | ----- ------- ------
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| 173 | +-------------------------+
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| 174 | | Data: Position | Both Edges ___
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| 175 | | Function: Quadrature x4 | <------------ / A \
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| 176 | | | _______
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| 177 | | |
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| 178 | | | Both Edges ___
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| 179 | | | <------------ / B \
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| 180 | | | _______
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| 181 | +-------------------------+
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| 182 |
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| 183 | In this example, two Signals (quadrature encoder lines A and B) are
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| 184 | associated with a single Count: a rising or falling edge on either A or
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| 185 | B triggers the "Quadrature x4" function which determines the direction
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| 186 | of movement and updates the respective position data. The "Quadrature
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| 187 | x4" function is likely implemented in the hardware of the quadrature
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| 188 | encoder counter device; the Count, Signals, and Synapses simply
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| 189 | represent this hardware behavior and functionality.
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| 190 |
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| 191 | Signals associated with the same Count can have differing Synapse action
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| 192 | mode conditions. For example, a quadrature encoder counter device
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| 193 | operating in a non-quadrature Pulse-Direction mode could have one input
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| 194 | line dedicated for movement and a second input line dedicated for
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| 195 | direction::
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| 196 |
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| 197 | Count Synapse Signal
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| 198 | ----- ------- ------
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| 199 | +---------------------------+
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| 200 | | Data: Position | Rising Edge ___
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| 201 | | Function: Pulse-Direction | <------------- / A \ (Movement)
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| 202 | | | _______
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| 203 | | |
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| 204 | | | None ___
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| 205 | | | <------------- / B \ (Direction)
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| 206 | | | _______
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| 207 | +---------------------------+
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| 208 |
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| 209 | Only Signal A triggers the "Pulse-Direction" update function, but the
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| 210 | instantaneous state of Signal B is still required in order to know the
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| 211 | direction so that the position data may be properly updated. Ultimately,
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| 212 | both Signals are associated with the same Count via two respective
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| 213 | Synapses, but only one Synapse has an active action mode condition which
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| 214 | triggers the respective count function while the other is left with a
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| 215 | "None" condition action mode to indicate its respective Signal's
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| 216 | availability for state evaluation despite its non-triggering mode.
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| 217 |
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| 218 | Keep in mind that the Signal, Synapse, and Count are abstract
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| 219 | representations which do not need to be closely married to their
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| 220 | respective physical sources. This allows the user of a counter to
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| 221 | divorce themselves from the nuances of physical components (such as
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| 222 | whether an input line is differential or single-ended) and instead focus
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| 223 | on the core idea of what the data and process represent (e.g. position
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| 224 | as interpreted from quadrature encoding data).
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| 225 |
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| 226 | Driver API
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| 227 | ==========
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| 228 |
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| 229 | Driver authors may utilize the Generic Counter interface in their code
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| 230 | by including the include/linux/counter.h header file. This header file
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| 231 | provides several core data structures, function prototypes, and macros
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| 232 | for defining a counter device.
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| 233 |
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| 234 | .. kernel-doc:: include/linux/counter.h
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| 235 | :internal:
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| 236 |
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| 237 | .. kernel-doc:: drivers/counter/counter-core.c
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| 238 | :export:
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| 239 |
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| 240 | .. kernel-doc:: drivers/counter/counter-chrdev.c
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| 241 | :export:
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| 242 |
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| 243 | Driver Implementation
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| 244 | =====================
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| 245 |
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| 246 | To support a counter device, a driver must first allocate the available
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| 247 | Counter Signals via counter_signal structures. These Signals should
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| 248 | be stored as an array and set to the signals array member of an
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| 249 | allocated counter_device structure before the Counter is registered to
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| 250 | the system.
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| 251 |
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| 252 | Counter Counts may be allocated via counter_count structures, and
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| 253 | respective Counter Signal associations (Synapses) made via
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| 254 | counter_synapse structures. Associated counter_synapse structures are
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| 255 | stored as an array and set to the synapses array member of the
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| 256 | respective counter_count structure. These counter_count structures are
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| 257 | set to the counts array member of an allocated counter_device structure
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| 258 | before the Counter is registered to the system.
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| 259 |
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| 260 | Driver callbacks must be provided to the counter_device structure in
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| 261 | order to communicate with the device: to read and write various Signals
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| 262 | and Counts, and to set and get the "action mode" and "function mode" for
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| 263 | various Synapses and Counts respectively.
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| 264 |
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| 265 | A counter_device structure is allocated using counter_alloc() and then
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| 266 | registered to the system by passing it to the counter_add() function, and
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| 267 | unregistered by passing it to the counter_unregister function. There are
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| 268 | device managed variants of these functions: devm_counter_alloc() and
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| 269 | devm_counter_add().
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| 270 |
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| 271 | The struct counter_comp structure is used to define counter extensions
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| 272 | for Signals, Synapses, and Counts.
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| 273 |
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| 274 | The "type" member specifies the type of high-level data (e.g. BOOL,
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| 275 | COUNT_DIRECTION, etc.) handled by this extension. The "``*_read``" and
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| 276 | "``*_write``" members can then be set by the counter device driver with
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| 277 | callbacks to handle that data using native C data types (i.e. u8, u64,
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| 278 | etc.).
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| 279 |
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| 280 | Convenience macros such as ``COUNTER_COMP_COUNT_U64`` are provided for
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| 281 | use by driver authors. In particular, driver authors are expected to use
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| 282 | the provided macros for standard Counter subsystem attributes in order
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| 283 | to maintain a consistent interface for userspace. For example, a counter
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| 284 | device driver may define several standard attributes like so::
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| 285 |
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| 286 | struct counter_comp count_ext[] = {
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| 287 | COUNTER_COMP_DIRECTION(count_direction_read),
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| 288 | COUNTER_COMP_ENABLE(count_enable_read, count_enable_write),
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| 289 | COUNTER_COMP_CEILING(count_ceiling_read, count_ceiling_write),
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| 290 | };
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| 291 |
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| 292 | This makes it simple to see, add, and modify the attributes that are
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| 293 | supported by this driver ("direction", "enable", and "ceiling") and to
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| 294 | maintain this code without getting lost in a web of struct braces.
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| 295 |
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| 296 | Callbacks must match the function type expected for the respective
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| 297 | component or extension. These function types are defined in the struct
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| 298 | counter_comp structure as the "``*_read``" and "``*_write``" union
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| 299 | members.
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| 300 |
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| 301 | The corresponding callback prototypes for the extensions mentioned in
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| 302 | the previous example above would be::
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| 303 |
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| 304 | int count_direction_read(struct counter_device *counter,
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| 305 | struct counter_count *count,
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| 306 | enum counter_count_direction *direction);
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| 307 | int count_enable_read(struct counter_device *counter,
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| 308 | struct counter_count *count, u8 *enable);
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| 309 | int count_enable_write(struct counter_device *counter,
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| 310 | struct counter_count *count, u8 enable);
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| 311 | int count_ceiling_read(struct counter_device *counter,
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| 312 | struct counter_count *count, u64 *ceiling);
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| 313 | int count_ceiling_write(struct counter_device *counter,
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| 314 | struct counter_count *count, u64 ceiling);
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| 315 |
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| 316 | Determining the type of extension to create is a matter of scope.
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| 317 |
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| 318 | * Signal extensions are attributes that expose information/control
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| 319 | specific to a Signal. These types of attributes will exist under a
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| 320 | Signal's directory in sysfs.
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| 321 |
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| 322 | For example, if you have an invert feature for a Signal, you can have
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| 323 | a Signal extension called "invert" that toggles that feature:
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| 324 | /sys/bus/counter/devices/counterX/signalY/invert
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| 325 |
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| 326 | * Count extensions are attributes that expose information/control
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| 327 | specific to a Count. These type of attributes will exist under a
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| 328 | Count's directory in sysfs.
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| 329 |
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| 330 | For example, if you want to pause/unpause a Count from updating, you
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| 331 | can have a Count extension called "enable" that toggles such:
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| 332 | /sys/bus/counter/devices/counterX/countY/enable
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| 333 |
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| 334 | * Device extensions are attributes that expose information/control
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| 335 | non-specific to a particular Count or Signal. This is where you would
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| 336 | put your global features or other miscellaneous functionality.
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| 337 |
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| 338 | For example, if your device has an overtemp sensor, you can report the
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| 339 | chip overheated via a device extension called "error_overtemp":
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| 340 | /sys/bus/counter/devices/counterX/error_overtemp
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| 341 |
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| 342 | Subsystem Architecture
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| 343 | ======================
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| 344 |
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| 345 | Counter drivers pass and take data natively (i.e. ``u8``, ``u64``, etc.)
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| 346 | and the shared counter module handles the translation between the sysfs
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| 347 | interface. This guarantees a standard userspace interface for all
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| 348 | counter drivers, and enables a Generic Counter chrdev interface via a
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| 349 | generalized device driver ABI.
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| 350 |
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| 351 | A high-level view of how a count value is passed down from a counter
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| 352 | driver is exemplified by the following. The driver callbacks are first
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| 353 | registered to the Counter core component for use by the Counter
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| 354 | userspace interface components::
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| 355 |
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| 356 | Driver callbacks registration:
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| 357 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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| 358 | +----------------------------+
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| 359 | | Counter device driver |
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| 360 | +----------------------------+
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| 361 | | Processes data from device |
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| 362 | +----------------------------+
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| 363 | |
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| 364 | -------------------
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| 365 | / driver callbacks /
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| 366 | -------------------
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| 367 | |
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| 368 | V
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| 369 | +----------------------+
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| 370 | | Counter core |
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| 371 | +----------------------+
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| 372 | | Routes device driver |
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| 373 | | callbacks to the |
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| 374 | | userspace interfaces |
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| 375 | +----------------------+
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| 376 | |
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| 377 | -------------------
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| 378 | / driver callbacks /
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| 379 | -------------------
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| 380 | |
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| 381 | +---------------+---------------+
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| 382 | | |
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| 383 | V V
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| 384 | +--------------------+ +---------------------+
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| 385 | | Counter sysfs | | Counter chrdev |
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| 386 | +--------------------+ +---------------------+
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| 387 | | Translates to the | | Translates to the |
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| 388 | | standard Counter | | standard Counter |
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| 389 | | sysfs output | | character device |
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| 390 | +--------------------+ +---------------------+
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| 391 |
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| 392 | Thereafter, data can be transferred directly between the Counter device
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| 393 | driver and Counter userspace interface::
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| 394 |
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| 395 | Count data request:
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| 396 | ~~~~~~~~~~~~~~~~~~~
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| 397 | ----------------------
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| 398 | / Counter device \
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| 399 | +----------------------+
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| 400 | | Count register: 0x28 |
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| 401 | +----------------------+
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| 402 | |
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| 403 | -----------------
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| 404 | / raw count data /
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| 405 | -----------------
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| 406 | |
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| 407 | V
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| 408 | +----------------------------+
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| 409 | | Counter device driver |
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| 410 | +----------------------------+
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| 411 | | Processes data from device |
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| 412 | |----------------------------|
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| 413 | | Type: u64 |
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| 414 | | Value: 42 |
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| 415 | +----------------------------+
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| 416 | |
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| 417 | ----------
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| 418 | / u64 /
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| 419 | ----------
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| 420 | |
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| 421 | +---------------+---------------+
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| 422 | | |
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| 423 | V V
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| 424 | +--------------------+ +---------------------+
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| 425 | | Counter sysfs | | Counter chrdev |
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| 426 | +--------------------+ +---------------------+
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| 427 | | Translates to the | | Translates to the |
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| 428 | | standard Counter | | standard Counter |
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| 429 | | sysfs output | | character device |
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| 430 | |--------------------| |---------------------|
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| 431 | | Type: const char * | | Type: u64 |
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| 432 | | Value: "42" | | Value: 42 |
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| 433 | +--------------------+ +---------------------+
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| 434 | | |
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| 435 | --------------- -----------------------
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| 436 | / const char * / / struct counter_event /
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| 437 | --------------- -----------------------
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| 438 | | |
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| 439 | | V
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| 440 | | +-----------+
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| 441 | | | read |
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| 442 | | +-----------+
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| 443 | | \ Count: 42 /
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| 444 | | -----------
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| 445 | |
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| 446 | V
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| 447 | +--------------------------------------------------+
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| 448 | | `/sys/bus/counter/devices/counterX/countY/count` |
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| 449 | +--------------------------------------------------+
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| 450 | \ Count: "42" /
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| 451 | --------------------------------------------------
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| 452 |
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| 453 | There are four primary components involved:
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| 454 |
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| 455 | Counter device driver
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| 456 | ---------------------
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| 457 | Communicates with the hardware device to read/write data; e.g. counter
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| 458 | drivers for quadrature encoders, timers, etc.
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| 459 |
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| 460 | Counter core
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| 461 | ------------
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| 462 | Registers the counter device driver to the system so that the respective
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| 463 | callbacks are called during userspace interaction.
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| 464 |
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| 465 | Counter sysfs
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| 466 | -------------
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| 467 | Translates counter data to the standard Counter sysfs interface format
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| 468 | and vice versa.
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| 469 |
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| 470 | Please refer to the Documentation/ABI/testing/sysfs-bus-counter file
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| 471 | for a detailed breakdown of the available Generic Counter interface
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| 472 | sysfs attributes.
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| 473 |
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| 474 | Counter chrdev
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| 475 | --------------
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| 476 | Translates Counter events to the standard Counter character device; data
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| 477 | is transferred via standard character device read calls, while Counter
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| 478 | events are configured via ioctl calls.
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| 479 |
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| 480 | Sysfs Interface
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| 481 | ===============
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| 482 |
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| 483 | Several sysfs attributes are generated by the Generic Counter interface,
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| 484 | and reside under the ``/sys/bus/counter/devices/counterX`` directory,
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| 485 | where ``X`` is to the respective counter device id. Please see
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| 486 | Documentation/ABI/testing/sysfs-bus-counter for detailed information
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| 487 | on each Generic Counter interface sysfs attribute.
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| 488 |
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| 489 | Through these sysfs attributes, programs and scripts may interact with
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| 490 | the Generic Counter paradigm Counts, Signals, and Synapses of respective
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| 491 | counter devices.
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| 492 |
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| 493 | Counter Character Device
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| 494 | ========================
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| 495 |
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| 496 | Counter character device nodes are created under the ``/dev`` directory
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| 497 | as ``counterX``, where ``X`` is the respective counter device id.
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| 498 | Defines for the standard Counter data types are exposed via the
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| 499 | userspace ``include/uapi/linux/counter.h`` file.
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| 500 |
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| 501 | Counter events
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| 502 | --------------
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| 503 | Counter device drivers can support Counter events by utilizing the
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| 504 | ``counter_push_event`` function::
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| 505 |
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| 506 | void counter_push_event(struct counter_device *const counter, const u8 event,
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| 507 | const u8 channel);
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| 508 |
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| 509 | The event id is specified by the ``event`` parameter; the event channel
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| 510 | id is specified by the ``channel`` parameter. When this function is
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| 511 | called, the Counter data associated with the respective event is
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| 512 | gathered, and a ``struct counter_event`` is generated for each datum and
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| 513 | pushed to userspace.
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| 514 |
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| 515 | Counter events can be configured by users to report various Counter
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| 516 | data of interest. This can be conceptualized as a list of Counter
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| 517 | component read calls to perform. For example:
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| 518 |
|
| 519 | +------------------------+------------------------+
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| 520 | | COUNTER_EVENT_OVERFLOW | COUNTER_EVENT_INDEX |
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| 521 | +========================+========================+
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| 522 | | Channel 0 | Channel 0 |
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| 523 | +------------------------+------------------------+
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| 524 | | * Count 0 | * Signal 0 |
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| 525 | | * Count 1 | * Signal 0 Extension 0 |
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| 526 | | * Signal 3 | * Extension 4 |
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| 527 | | * Count 4 Extension 2 +------------------------+
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| 528 | | * Signal 5 Extension 0 | Channel 1 |
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| 529 | | +------------------------+
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| 530 | | | * Signal 4 |
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| 531 | | | * Signal 4 Extension 0 |
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| 532 | | | * Count 7 |
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| 533 | +------------------------+------------------------+
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| 534 |
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| 535 | When ``counter_push_event(counter, COUNTER_EVENT_INDEX, 1)`` is called
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| 536 | for example, it will go down the list for the ``COUNTER_EVENT_INDEX``
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| 537 | event channel 1 and execute the read callbacks for Signal 4, Signal 4
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| 538 | Extension 0, and Count 7 -- the data returned for each is pushed to a
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| 539 | kfifo as a ``struct counter_event``, which userspace can retrieve via a
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| 540 | standard read operation on the respective character device node.
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| 541 |
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| 542 | Userspace
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| 543 | ---------
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| 544 | Userspace applications can configure Counter events via ioctl operations
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| 545 | on the Counter character device node. There following ioctl codes are
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| 546 | supported and provided by the ``linux/counter.h`` userspace header file:
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| 547 |
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| 548 | * :c:macro:`COUNTER_ADD_WATCH_IOCTL`
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| 549 |
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| 550 | * :c:macro:`COUNTER_ENABLE_EVENTS_IOCTL`
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| 551 |
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| 552 | * :c:macro:`COUNTER_DISABLE_EVENTS_IOCTL`
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| 553 |
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| 554 | To configure events to gather Counter data, users first populate a
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| 555 | ``struct counter_watch`` with the relevant event id, event channel id,
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| 556 | and the information for the desired Counter component from which to
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| 557 | read, and then pass it via the ``COUNTER_ADD_WATCH_IOCTL`` ioctl
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| 558 | command.
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| 559 |
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| 560 | Note that an event can be watched without gathering Counter data by
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| 561 | setting the ``component.type`` member equal to
|
| 562 | ``COUNTER_COMPONENT_NONE``. With this configuration the Counter
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| 563 | character device will simply populate the event timestamps for those
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| 564 | respective ``struct counter_event`` elements and ignore the component
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| 565 | value.
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| 566 |
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| 567 | The ``COUNTER_ADD_WATCH_IOCTL`` command will buffer these Counter
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| 568 | watches. When ready, the ``COUNTER_ENABLE_EVENTS_IOCTL`` ioctl command
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| 569 | may be used to activate these Counter watches.
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| 570 |
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| 571 | Userspace applications can then execute a ``read`` operation (optionally
|
| 572 | calling ``poll`` first) on the Counter character device node to retrieve
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| 573 | ``struct counter_event`` elements with the desired data.
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