| 1 | |
| 2 |
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| 3 | ====================
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| 4 | Reset controller API
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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 | Reset controllers are central units that control the reset signals to multiple
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| 11 | peripherals.
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| 12 | The reset controller API is split into two parts:
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| 13 | the `consumer driver interface <#consumer-driver-interface>`__ (`API reference
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| 14 | <#reset-consumer-api>`__), which allows peripheral drivers to request control
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| 15 | over their reset input signals, and the `reset controller driver interface
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| 16 | <#reset-controller-driver-interface>`__ (`API reference
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| 17 | <#reset-controller-driver-api>`__), which is used by drivers for reset
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| 18 | controller devices to register their reset controls to provide them to the
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| 19 | consumers.
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| 20 |
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| 21 | While some reset controller hardware units also implement system restart
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| 22 | functionality, restart handlers are out of scope for the reset controller API.
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| 23 |
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| 24 | Glossary
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| 25 | --------
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| 26 |
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| 27 | The reset controller API uses these terms with a specific meaning:
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| 28 |
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| 29 | Reset line
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| 30 |
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| 31 | Physical reset line carrying a reset signal from a reset controller
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| 32 | hardware unit to a peripheral module.
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| 33 |
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| 34 | Reset control
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| 35 |
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| 36 | Control method that determines the state of one or multiple reset lines.
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| 37 | Most commonly this is a single bit in reset controller register space that
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| 38 | either allows direct control over the physical state of the reset line, or
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| 39 | is self-clearing and can be used to trigger a predetermined pulse on the
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| 40 | reset line.
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| 41 | In more complicated reset controls, a single trigger action can launch a
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| 42 | carefully timed sequence of pulses on multiple reset lines.
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| 43 |
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| 44 | Reset controller
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| 45 |
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| 46 | A hardware module that provides a number of reset controls to control a
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| 47 | number of reset lines.
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| 48 |
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| 49 | Reset consumer
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| 50 |
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| 51 | Peripheral module or external IC that is put into reset by the signal on a
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| 52 | reset line.
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| 53 |
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| 54 | Consumer driver interface
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| 55 | =========================
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| 56 |
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| 57 | This interface provides an API that is similar to the kernel clock framework.
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| 58 | Consumer drivers use get and put operations to acquire and release reset
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| 59 | controls.
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| 60 | Functions are provided to assert and deassert the controlled reset lines,
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| 61 | trigger reset pulses, or to query reset line status.
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| 62 |
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| 63 | When requesting reset controls, consumers can use symbolic names for their
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| 64 | reset inputs, which are mapped to an actual reset control on an existing reset
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| 65 | controller device by the core.
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| 66 |
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| 67 | A stub version of this API is provided when the reset controller framework is
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| 68 | not in use in order to minimize the need to use ifdefs.
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| 69 |
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| 70 | Shared and exclusive resets
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| 71 | ---------------------------
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| 72 |
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| 73 | The reset controller API provides either reference counted deassertion and
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| 74 | assertion or direct, exclusive control.
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| 75 | The distinction between shared and exclusive reset controls is made at the time
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| 76 | the reset control is requested, either via devm_reset_control_get_shared() or
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| 77 | via devm_reset_control_get_exclusive().
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| 78 | This choice determines the behavior of the API calls made with the reset
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| 79 | control.
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| 80 |
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| 81 | Shared resets behave similarly to clocks in the kernel clock framework.
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| 82 | They provide reference counted deassertion, where only the first deassert,
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| 83 | which increments the deassertion reference count to one, and the last assert
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| 84 | which decrements the deassertion reference count back to zero, have a physical
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| 85 | effect on the reset line.
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| 86 |
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| 87 | Exclusive resets on the other hand guarantee direct control.
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| 88 | That is, an assert causes the reset line to be asserted immediately, and a
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| 89 | deassert causes the reset line to be deasserted immediately.
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| 90 |
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| 91 | Assertion and deassertion
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| 92 | -------------------------
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| 93 |
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| 94 | Consumer drivers use the reset_control_assert() and reset_control_deassert()
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| 95 | functions to assert and deassert reset lines.
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| 96 | For shared reset controls, calls to the two functions must be balanced.
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| 97 |
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| 98 | Note that since multiple consumers may be using a shared reset control, there
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| 99 | is no guarantee that calling reset_control_assert() on a shared reset control
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| 100 | will actually cause the reset line to be asserted.
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| 101 | Consumer drivers using shared reset controls should assume that the reset line
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| 102 | may be kept deasserted at all times.
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| 103 | The API only guarantees that the reset line can not be asserted as long as any
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| 104 | consumer has requested it to be deasserted.
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| 105 |
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| 106 | Triggering
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| 107 | ----------
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| 108 |
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| 109 | Consumer drivers use reset_control_reset() to trigger a reset pulse on a
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| 110 | self-deasserting reset control.
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| 111 | In general, these resets can not be shared between multiple consumers, since
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| 112 | requesting a pulse from any consumer driver will reset all connected
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| 113 | peripherals.
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| 114 |
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| 115 | The reset controller API allows requesting self-deasserting reset controls as
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| 116 | shared, but for those only the first trigger request causes an actual pulse to
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| 117 | be issued on the reset line.
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| 118 | All further calls to this function have no effect until all consumers have
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| 119 | called reset_control_rearm().
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| 120 | For shared reset controls, calls to the two functions must be balanced.
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| 121 | This allows devices that only require an initial reset at any point before the
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| 122 | driver is probed or resumed to share a pulsed reset line.
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| 123 |
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| 124 | Querying
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| 125 | --------
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| 126 |
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| 127 | Only some reset controllers support querying the current status of a reset
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| 128 | line, via reset_control_status().
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| 129 | If supported, this function returns a positive non-zero value if the given
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| 130 | reset line is asserted.
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| 131 | The reset_control_status() function does not accept a
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| 132 | `reset control array <#reset-control-arrays>`__ handle as its input parameter.
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| 133 |
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| 134 | Optional resets
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| 135 | ---------------
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| 136 |
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| 137 | Often peripherals require a reset line on some platforms but not on others.
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| 138 | For this, reset controls can be requested as optional using
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| 139 | devm_reset_control_get_optional_exclusive() or
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| 140 | devm_reset_control_get_optional_shared().
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| 141 | These functions return a NULL pointer instead of an error when the requested
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| 142 | reset control is not specified in the device tree.
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| 143 | Passing a NULL pointer to the reset_control functions causes them to return
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| 144 | quietly without an error.
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| 145 |
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| 146 | Reset control arrays
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| 147 | --------------------
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| 148 |
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| 149 | Some drivers need to assert a bunch of reset lines in no particular order.
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| 150 | devm_reset_control_array_get() returns an opaque reset control handle that can
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| 151 | be used to assert, deassert, or trigger all specified reset controls at once.
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| 152 | The reset control API does not guarantee the order in which the individual
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| 153 | controls therein are handled.
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| 154 |
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| 155 | Reset controller driver interface
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| 156 | =================================
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| 157 |
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| 158 | Drivers for reset controller modules provide the functionality necessary to
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| 159 | assert or deassert reset signals, to trigger a reset pulse on a reset line, or
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| 160 | to query its current state.
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| 161 | All functions are optional.
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| 162 |
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| 163 | Initialization
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| 164 | --------------
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| 165 |
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| 166 | Drivers fill a struct :c:type:`reset_controller_dev` and register it with
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| 167 | reset_controller_register() in their probe function.
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| 168 | The actual functionality is implemented in callback functions via a struct
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| 169 | :c:type:`reset_control_ops`.
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| 170 |
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| 171 | API reference
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| 172 | =============
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| 173 |
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| 174 | The reset controller API is documented here in two parts:
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| 175 | the `reset consumer API <#reset-consumer-api>`__ and the `reset controller
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| 176 | driver API <#reset-controller-driver-api>`__.
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| 177 |
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| 178 | Reset consumer API
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| 179 | ------------------
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| 180 |
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| 181 | Reset consumers can control a reset line using an opaque reset control handle,
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| 182 | which can be obtained from devm_reset_control_get_exclusive() or
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| 183 | devm_reset_control_get_shared().
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| 184 | Given the reset control, consumers can call reset_control_assert() and
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| 185 | reset_control_deassert(), trigger a reset pulse using reset_control_reset(), or
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| 186 | query the reset line status using reset_control_status().
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| 187 |
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| 188 | .. kernel-doc:: include/linux/reset.h
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| 189 | :internal:
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| 190 |
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| 191 | .. kernel-doc:: drivers/reset/core.c
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| 192 | :functions: reset_control_reset
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| 193 | reset_control_assert
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| 194 | reset_control_deassert
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| 195 | reset_control_status
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| 196 | reset_control_acquire
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| 197 | reset_control_release
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| 198 | reset_control_rearm
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| 199 | reset_control_put
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| 200 | of_reset_control_get_count
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| 201 | devm_reset_control_array_get
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| 202 | reset_control_get_count
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| 203 |
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| 204 | Reset controller driver API
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| 205 | ---------------------------
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| 206 |
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| 207 | Reset controller drivers are supposed to implement the necessary functions in
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| 208 | a static constant structure :c:type:`reset_control_ops`, allocate and fill out
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| 209 | a struct :c:type:`reset_controller_dev`, and register it using
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| 210 | devm_reset_controller_register().
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| 211 |
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| 212 | .. kernel-doc:: include/linux/reset-controller.h
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| 213 | :internal:
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| 214 |
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| 215 | .. kernel-doc:: drivers/reset/core.c
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| 216 | :functions: fwnode_reset_simple_xlate
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| 217 | reset_controller_register
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| 218 | reset_controller_unregister
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| 219 | devm_reset_controller_register
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