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