1.. SPDX-License-Identifier: GPL-2.0-only
2.. Copyright 2024 Linaro Ltd.
3
4====================
5Power Sequencing API
6====================
7
8:Author: Bartosz Golaszewski
9
10Introduction
11============
12
13This framework is designed to abstract complex power-up sequences that are
14shared between multiple logical devices in the linux kernel.
15
16The intention is to allow consumers to obtain a power sequencing handle
17exposed by the power sequence provider and delegate the actual requesting and
18control of the underlying resources as well as to allow the provider to
19mitigate any potential conflicts between multiple users behind the scenes.
20
21Glossary
22--------
23
24The power sequencing API uses a number of terms specific to the subsystem:
25
26Unit
27
28    A unit is a discreet chunk of a power sequence. For instance one unit may
29    enable a set of regulators, another may enable a specific GPIO. Units can
30    define dependencies in the form of other units that must be enabled before
31    it itself can be.
32
33Target
34
35    A target is a set of units (composed of the "final" unit and its
36    dependencies) that a consumer selects by its name when requesting a handle
37    to the power sequencer. Via the dependency system, multiple targets may
38    share the same parts of a power sequence but ignore parts that are
39    irrelevant.
40
41Descriptor
42
43    A handle passed by the pwrseq core to every consumer that serves as the
44    entry point to the provider layer. It ensures coherence between different
45    users and keeps reference counting consistent.
46
47Consumer interface
48==================
49
50The consumer API is aimed to be as simple as possible. The driver interested in
51getting a descriptor from the power sequencer should call pwrseq_get() and
52specify the name of the target it wants to reach in the sequence after calling
53pwrseq_power_up(). The descriptor can be released by calling pwrseq_put() and
54the consumer can request the powering down of its target with
55pwrseq_power_off(). Note that there is no guarantee that pwrseq_power_off()
56will have any effect as there may be multiple users of the underlying resources
57who may keep them active.
58
59Provider interface
60==================
61
62The provider API is admittedly not nearly as straightforward as the one for
63consumers but it makes up for it in flexibility.
64
65Each provider can logically split the power-up sequence into descrete chunks
66(units) and define their dependencies. They can then expose named targets that
67consumers may use as the final point in the sequence that they wish to reach.
68
69To that end the providers fill out a set of configuration structures and
70register with the pwrseq subsystem by calling pwrseq_device_register().
71
72Dynamic consumer matching
73-------------------------
74
75The main difference between pwrseq and other linux kernel providers is the
76mechanism for dynamic matching of consumers and providers. Every power sequence
77provider driver must implement the `match()` callback and pass it to the pwrseq
78core when registering with the subsystems.
79
80When a client requests a sequencer handle, the core will call this callback for
81every registered provider and let it flexibly figure out whether the proposed
82client device is indeed its consumer. For example: if the provider binds to the
83device-tree node representing a power management unit of a chipset and the
84consumer driver controls one of its modules, the provider driver may parse the
85relevant regulator supply properties in device tree and see if they lead from
86the PMU to the consumer.
87
88API reference
89=============
90
91.. kernel-doc:: include/linux/pwrseq/provider.h
92   :internal:
93
94.. kernel-doc:: drivers/power/sequencing/core.c
95   :export:
96