1#
2# (C) Copyright 2014 Samsung Electronics
3# Lukasz Majewski <l.majewski@samsung.com>
4#
5# SPDX-License-Identifier:      GPL-2.0+
6#
7
8Introduction
9------------
10
11This document describes the second version of the u-boot's PMIC (Power
12Management IC) framework. As a reference boards please consider Samsungs' Trats
13and Trats2.
14
15Background
16----------
17
18Boards supported by u-boot are getting increasingly complex. Developers and
19designers strive to cut down power consumption. Hence several different types of
20devices are now available on the board - namely power managers (PMIC), fuel
21gauges (FG), micro USB interface controllers (MUIC), batteries, multi-function
22devices (MFD).
23
24Explanation of key design decisions
25-----------------------------------
26
27One package can integrate PMIC and MUIC with different addresses on the I2C bus.
28The same device - e.g. MAX8997 uses two different I2C busses and addresses.
29
30Power devices use not only I2C for communication, but SPI as well. Additionally
31different ICs use different endianess. For this reason struct pmic holds
32information about I2C/SPI transmission, which is used with generic
33pmic_req_write() function.
34
35The "flat" hierarchy for power devices works well when each device performs only
36one operation - e.g. PMIC enables LDO.
37
38The problem emerges when we have a device (battery) which conceptually shall be
39the master and uses methods exported by other devices. We need to control MUIC
40to start charging the battery, use PMIC to reduce board's overall power
41consumption (by disabling not needed LDOs, BUCKs) and get current state of
42energy on the battery from FG.
43Up till now u-boot doesn't support device model, so a simple one had to be
44added.
45
46The directory hierarchy has following structure:
47./include/power/<device_name>_<device_function>.h
48e.g. ./include/power/max8997_pmic.h
49
50./drivers/power/pmic/power_{core files}.c
51e.g. ./drivers/power/pmic/power_core.c
52
53./drivers/power/pmic/<device_function>/<device_function>_<device_name>.c
54e.g. ./drivers/power/pmic/pmic_max8997.c
55e.g. ./drivers/power/battery/trats/bat_trats.c
56e.g. ./drivers/power/fuel_gauge/fg_max17042.c
57
58The framework classifies devices by their function - separate directories should
59be maintained for different classes of devices.
60
61Current design
62--------------
63
64Everything is a power device described by struct pmic. Even battery is
65considered as a valid power device. This helps for better management of those
66devices.
67
68- Block diagram of the hierarchy:
69			-----------------
70		--------| BAT           |------------
71		|       |               |           |
72		|       -----------------           |
73		|               |                   |
74	       \|/             \|/                 \|/
75	-----------     -----------------       ---------
76	|FG       |     |MUIC           |       |CHRG   |
77	|         |     |               |       |       |
78	-----------     -----------------       ---------
79
80
811. When hierarchy is not needed (no complex battery charge):
82
83Definition of the struct pmic is only required with proper name and parameters
84for communication. This is enough to use the "pmic" command in the u-boot
85prompt to change values of device's register (enable/disable LDO, BUCK).
86
87The PG, MUIC and CHRG above are regarded to be in the same level in the
88hierarchy.
89
902. Complex battery charging.
91
92To charge a battery, information from several "abstract" power devices is
93needed (defined at ./include/power/pmic.h):
94- FG device (struct power_fg):
95	     -- *fg_battery_check - check if battery is not above its limits
96	     -- *fg_battery_update - update the pmic framework with current
97				     battery state(voltage and current capacity)
98
99- Charger device (struct power_chrq):
100	     -- *chrg_type - type/capacity of the charger (including information
101			     about USB cable disconnection)
102	     -- *chrg_bat_present - detection if battery to be charged is
103				    present
104	     -- *chrg_state - status of the charger - if it is enabled or
105			      disabled
106
107- Battery device (struct power_battery):
108	     -- *battery_init - assign proper callbacks to be used by top
109				hierarchy battery device
110	     -- *battery_charge - called from "pmic" command, responsible
111				  for performing the charging
112
113Now two batteries are supported; trats and trats2 [*]. Those differ in the way
114how they handle the exact charging. Trats uses polling (MAX8997) and trats2
115relies on the PMIC/MUIC HW completely (MAX77693).
116
117__Example for trats (this can be very different for other board):__
118	     -- *fg_battery_check       -> FG device (fg_max17042.c)
119	     -- *fg_battery_update      -> FG device (fg_max17042.c)
120	     -- *chrg_type              -> MUIC device (muic_max8997.c)
121	     -- *chrg_bat_present       -> PMIC device (pmic_max8997.c)
122	     -- *chrg_state             -> PMIC device (pmic_max8997.c)
123	     -- *battery_init           -> BAT device (bat_trats.c)
124	     -- *battery_charge         -> BAT device (bat_trats.c)
125
126Also the struct pmic holds method (*low_power_mode) for reducing board's
127power consumption when one calls "pmic BAT_TRATS bat charge" command.
128
129How to add a new power device
130-----------------------------
131
1321. Simple device should be added with creation of file
133<pmic_function>_<pmic_name>.c, <pmic_name>_<pmic_function>.h  according to the
134proposed and described above scheme.
135
136Then "pmic" command supports reading/writing/dump of device's internal
137registers.
138
1392. Charging battery with hierarchy
140Define devices as listed at 1.
141
142Define battery file (bat_<board>.c). Please also note that one might need a
143corresponding battery model description for FG.
144
145For points 1 and 2 use a generic function power_init_board() to initialise the
146power framework on your board.
147
148For reference, please look into the trats/trats2 boards.
149
150TO DO list (for PMICv3) - up till v2014.04
151------------------------------------------
152
1531. Description of the devices related to power via device tree is not available.
154This is the main problem when a developer tries to build a multi-boot u-boot
155binary. The best would be to parse the DTS from Linux kernel.
156
1572. To support many instances of the same IC, like two MAX8997, one needs to
158copy the corresponding pmic_max8997.c file with changed name to "MAX8997_PMICX",
159where X is the device number. This problem will be addressed when extended
160pmic_core.c will support storing available devices in a list.
161
1623. Definition of batteries [*] (for trats/trats2) should be excluded from the
163code responsible for charging them and since it in fact describes the charging
164profile it should be put to a separate file.
165
1664. Adjust the framework to work with the device model.
167