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diff --git a/Documentation/driver-api/reset.rst b/Documentation/driver-api/reset.rst new file mode 100644 index 000000000..84e03d703 --- /dev/null +++ b/Documentation/driver-api/reset.rst @@ -0,0 +1,221 @@ +.. SPDX-License-Identifier: GPL-2.0-only + +==================== +Reset controller API +==================== + +Introduction +============ + +Reset controllers are central units that control the reset signals to multiple +peripherals. +The reset controller API is split into two parts: +the `consumer driver interface <#consumer-driver-interface>`__ (`API reference +<#reset-consumer-api>`__), which allows peripheral drivers to request control +over their reset input signals, and the `reset controller driver interface +<#reset-controller-driver-interface>`__ (`API reference +<#reset-controller-driver-api>`__), which is used by drivers for reset +controller devices to register their reset controls to provide them to the +consumers. + +While some reset controller hardware units also implement system restart +functionality, restart handlers are out of scope for the reset controller API. + +Glossary +-------- + +The reset controller API uses these terms with a specific meaning: + +Reset line + + Physical reset line carrying a reset signal from a reset controller + hardware unit to a peripheral module. + +Reset control + + Control method that determines the state of one or multiple reset lines. + Most commonly this is a single bit in reset controller register space that + either allows direct control over the physical state of the reset line, or + is self-clearing and can be used to trigger a predetermined pulse on the + reset line. + In more complicated reset controls, a single trigger action can launch a + carefully timed sequence of pulses on multiple reset lines. + +Reset controller + + A hardware module that provides a number of reset controls to control a + number of reset lines. + +Reset consumer + + Peripheral module or external IC that is put into reset by the signal on a + reset line. + +Consumer driver interface +========================= + +This interface provides an API that is similar to the kernel clock framework. +Consumer drivers use get and put operations to acquire and release reset +controls. +Functions are provided to assert and deassert the controlled reset lines, +trigger reset pulses, or to query reset line status. + +When requesting reset controls, consumers can use symbolic names for their +reset inputs, which are mapped to an actual reset control on an existing reset +controller device by the core. + +A stub version of this API is provided when the reset controller framework is +not in use in order to minimize the need to use ifdefs. + +Shared and exclusive resets +--------------------------- + +The reset controller API provides either reference counted deassertion and +assertion or direct, exclusive control. +The distinction between shared and exclusive reset controls is made at the time +the reset control is requested, either via devm_reset_control_get_shared() or +via devm_reset_control_get_exclusive(). +This choice determines the behavior of the API calls made with the reset +control. + +Shared resets behave similarly to clocks in the kernel clock framework. +They provide reference counted deassertion, where only the first deassert, +which increments the deassertion reference count to one, and the last assert +which decrements the deassertion reference count back to zero, have a physical +effect on the reset line. + +Exclusive resets on the other hand guarantee direct control. +That is, an assert causes the reset line to be asserted immediately, and a +deassert causes the reset line to be deasserted immediately. + +Assertion and deassertion +------------------------- + +Consumer drivers use the reset_control_assert() and reset_control_deassert() +functions to assert and deassert reset lines. +For shared reset controls, calls to the two functions must be balanced. + +Note that since multiple consumers may be using a shared reset control, there +is no guarantee that calling reset_control_assert() on a shared reset control +will actually cause the reset line to be asserted. +Consumer drivers using shared reset controls should assume that the reset line +may be kept deasserted at all times. +The API only guarantees that the reset line can not be asserted as long as any +consumer has requested it to be deasserted. + +Triggering +---------- + +Consumer drivers use reset_control_reset() to trigger a reset pulse on a +self-deasserting reset control. +In general, these resets can not be shared between multiple consumers, since +requesting a pulse from any consumer driver will reset all connected +peripherals. + +The reset controller API allows requesting self-deasserting reset controls as +shared, but for those only the first trigger request causes an actual pulse to +be issued on the reset line. +All further calls to this function have no effect until all consumers have +called reset_control_rearm(). +For shared reset controls, calls to the two functions must be balanced. +This allows devices that only require an initial reset at any point before the +driver is probed or resumed to share a pulsed reset line. + +Querying +-------- + +Only some reset controllers support querying the current status of a reset +line, via reset_control_status(). +If supported, this function returns a positive non-zero value if the given +reset line is asserted. +The reset_control_status() function does not accept a +`reset control array <#reset-control-arrays>`__ handle as its input parameter. + +Optional resets +--------------- + +Often peripherals require a reset line on some platforms but not on others. +For this, reset controls can be requested as optional using +devm_reset_control_get_optional_exclusive() or +devm_reset_control_get_optional_shared(). +These functions return a NULL pointer instead of an error when the requested +reset control is not specified in the device tree. +Passing a NULL pointer to the reset_control functions causes them to return +quietly without an error. + +Reset control arrays +-------------------- + +Some drivers need to assert a bunch of reset lines in no particular order. +devm_reset_control_array_get() returns an opaque reset control handle that can +be used to assert, deassert, or trigger all specified reset controls at once. +The reset control API does not guarantee the order in which the individual +controls therein are handled. + +Reset controller driver interface +================================= + +Drivers for reset controller modules provide the functionality necessary to +assert or deassert reset signals, to trigger a reset pulse on a reset line, or +to query its current state. +All functions are optional. + +Initialization +-------------- + +Drivers fill a struct :c:type:`reset_controller_dev` and register it with +reset_controller_register() in their probe function. +The actual functionality is implemented in callback functions via a struct +:c:type:`reset_control_ops`. + +API reference +============= + +The reset controller API is documented here in two parts: +the `reset consumer API <#reset-consumer-api>`__ and the `reset controller +driver API <#reset-controller-driver-api>`__. + +Reset consumer API +------------------ + +Reset consumers can control a reset line using an opaque reset control handle, +which can be obtained from devm_reset_control_get_exclusive() or +devm_reset_control_get_shared(). +Given the reset control, consumers can call reset_control_assert() and +reset_control_deassert(), trigger a reset pulse using reset_control_reset(), or +query the reset line status using reset_control_status(). + +.. kernel-doc:: include/linux/reset.h + :internal: + +.. kernel-doc:: drivers/reset/core.c + :functions: reset_control_reset + reset_control_assert + reset_control_deassert + reset_control_status + reset_control_acquire + reset_control_release + reset_control_rearm + reset_control_put + of_reset_control_get_count + of_reset_control_array_get + devm_reset_control_array_get + reset_control_get_count + +Reset controller driver API +--------------------------- + +Reset controller drivers are supposed to implement the necessary functions in +a static constant structure :c:type:`reset_control_ops`, allocate and fill out +a struct :c:type:`reset_controller_dev`, and register it using +devm_reset_controller_register(). + +.. kernel-doc:: include/linux/reset-controller.h + :internal: + +.. kernel-doc:: drivers/reset/core.c + :functions: of_reset_simple_xlate + reset_controller_register + reset_controller_unregister + devm_reset_controller_register + reset_controller_add_lookup |