222 строки
8.3 KiB
ReStructuredText
222 строки
8.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0-only
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====================
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Reset controller API
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====================
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Introduction
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============
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Reset controllers are central units that control the reset signals to multiple
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peripherals.
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The reset controller API is split into two parts:
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the `consumer driver interface <#consumer-driver-interface>`__ (`API reference
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<#reset-consumer-api>`__), which allows peripheral drivers to request control
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over their reset input signals, and the `reset controller driver interface
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<#reset-controller-driver-interface>`__ (`API reference
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<#reset-controller-driver-api>`__), which is used by drivers for reset
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controller devices to register their reset controls to provide them to the
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consumers.
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While some reset controller hardware units also implement system restart
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functionality, restart handlers are out of scope for the reset controller API.
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Glossary
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--------
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The reset controller API uses these terms with a specific meaning:
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Reset line
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Physical reset line carrying a reset signal from a reset controller
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hardware unit to a peripheral module.
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Reset control
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Control method that determines the state of one or multiple reset lines.
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Most commonly this is a single bit in reset controller register space that
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either allows direct control over the physical state of the reset line, or
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is self-clearing and can be used to trigger a predetermined pulse on the
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reset line.
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In more complicated reset controls, a single trigger action can launch a
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carefully timed sequence of pulses on multiple reset lines.
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Reset controller
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A hardware module that provides a number of reset controls to control a
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number of reset lines.
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Reset consumer
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Peripheral module or external IC that is put into reset by the signal on a
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reset line.
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Consumer driver interface
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=========================
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This interface provides an API that is similar to the kernel clock framework.
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Consumer drivers use get and put operations to acquire and release reset
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controls.
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Functions are provided to assert and deassert the controlled reset lines,
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trigger reset pulses, or to query reset line status.
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When requesting reset controls, consumers can use symbolic names for their
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reset inputs, which are mapped to an actual reset control on an existing reset
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controller device by the core.
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A stub version of this API is provided when the reset controller framework is
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not in use in order to minimize the need to use ifdefs.
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Shared and exclusive resets
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---------------------------
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The reset controller API provides either reference counted deassertion and
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assertion or direct, exclusive control.
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The distinction between shared and exclusive reset controls is made at the time
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the reset control is requested, either via devm_reset_control_get_shared() or
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via devm_reset_control_get_exclusive().
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This choice determines the behavior of the API calls made with the reset
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control.
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Shared resets behave similarly to clocks in the kernel clock framework.
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They provide reference counted deassertion, where only the first deassert,
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which increments the deassertion reference count to one, and the last assert
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which decrements the deassertion reference count back to zero, have a physical
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effect on the reset line.
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Exclusive resets on the other hand guarantee direct control.
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That is, an assert causes the reset line to be asserted immediately, and a
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deassert causes the reset line to be deasserted immediately.
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Assertion and deassertion
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-------------------------
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Consumer drivers use the reset_control_assert() and reset_control_deassert()
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functions to assert and deassert reset lines.
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For shared reset controls, calls to the two functions must be balanced.
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Note that since multiple consumers may be using a shared reset control, there
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is no guarantee that calling reset_control_assert() on a shared reset control
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will actually cause the reset line to be asserted.
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Consumer drivers using shared reset controls should assume that the reset line
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may be kept deasserted at all times.
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The API only guarantees that the reset line can not be asserted as long as any
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consumer has requested it to be deasserted.
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Triggering
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----------
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Consumer drivers use reset_control_reset() to trigger a reset pulse on a
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self-deasserting reset control.
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In general, these resets can not be shared between multiple consumers, since
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requesting a pulse from any consumer driver will reset all connected
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peripherals.
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The reset controller API allows requesting self-deasserting reset controls as
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shared, but for those only the first trigger request causes an actual pulse to
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be issued on the reset line.
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All further calls to this function have no effect until all consumers have
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called reset_control_rearm().
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For shared reset controls, calls to the two functions must be balanced.
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This allows devices that only require an initial reset at any point before the
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driver is probed or resumed to share a pulsed reset line.
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Querying
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--------
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Only some reset controllers support querying the current status of a reset
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line, via reset_control_status().
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If supported, this function returns a positive non-zero value if the given
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reset line is asserted.
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The reset_control_status() function does not accept a
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`reset control array <#reset-control-arrays>`__ handle as its input parameter.
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Optional resets
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---------------
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Often peripherals require a reset line on some platforms but not on others.
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For this, reset controls can be requested as optional using
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devm_reset_control_get_optional_exclusive() or
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devm_reset_control_get_optional_shared().
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These functions return a NULL pointer instead of an error when the requested
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reset control is not specified in the device tree.
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Passing a NULL pointer to the reset_control functions causes them to return
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quietly without an error.
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Reset control arrays
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--------------------
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Some drivers need to assert a bunch of reset lines in no particular order.
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devm_reset_control_array_get() returns an opaque reset control handle that can
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be used to assert, deassert, or trigger all specified reset controls at once.
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The reset control API does not guarantee the order in which the individual
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controls therein are handled.
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Reset controller driver interface
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=================================
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Drivers for reset controller modules provide the functionality necessary to
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assert or deassert reset signals, to trigger a reset pulse on a reset line, or
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to query its current state.
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All functions are optional.
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Initialization
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--------------
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Drivers fill a struct :c:type:`reset_controller_dev` and register it with
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reset_controller_register() in their probe function.
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The actual functionality is implemented in callback functions via a struct
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:c:type:`reset_control_ops`.
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API reference
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=============
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The reset controller API is documented here in two parts:
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the `reset consumer API <#reset-consumer-api>`__ and the `reset controller
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driver API <#reset-controller-driver-api>`__.
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Reset consumer API
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------------------
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Reset consumers can control a reset line using an opaque reset control handle,
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which can be obtained from devm_reset_control_get_exclusive() or
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devm_reset_control_get_shared().
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Given the reset control, consumers can call reset_control_assert() and
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reset_control_deassert(), trigger a reset pulse using reset_control_reset(), or
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query the reset line status using reset_control_status().
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.. kernel-doc:: include/linux/reset.h
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:internal:
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.. kernel-doc:: drivers/reset/core.c
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:functions: reset_control_reset
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reset_control_assert
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reset_control_deassert
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reset_control_status
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reset_control_acquire
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reset_control_release
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reset_control_rearm
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reset_control_put
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of_reset_control_get_count
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of_reset_control_array_get
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devm_reset_control_array_get
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reset_control_get_count
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Reset controller driver API
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---------------------------
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Reset controller drivers are supposed to implement the necessary functions in
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a static constant structure :c:type:`reset_control_ops`, allocate and fill out
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a struct :c:type:`reset_controller_dev`, and register it using
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devm_reset_controller_register().
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.. kernel-doc:: include/linux/reset-controller.h
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:internal:
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.. kernel-doc:: drivers/reset/core.c
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:functions: of_reset_simple_xlate
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reset_controller_register
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reset_controller_unregister
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devm_reset_controller_register
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reset_controller_add_lookup
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