1# Device Tree GPIO Naming in OpenBMC
2
3Author: Andrew Geissler (geissonator)
4
5Primary assignee: Andrew Geissler (geissonator)
6
7Other contributors:
8  < None >
9
10Created: April 3, 2020
11
12## Problem Description
13The Linux kernel has deprecated the use of sysfs to interact with the GPIO
14subsystem. The replacement is a "descriptor-based" character device interface.
15
16[libgpiod][1] is a suite of tools and library implemented in C and C++ which
17provides an abstraction to this new character device gpio interface.
18
19libgpiod provides a feature where you can access gpios by a name given to
20them in the kernel device tree files. The problem is there are no naming
21conventions for these GPIO names and if you want userspace code to be able
22to be consistent across different machines, these names would need to be
23consistent.
24
25## Background and References
26The kernel [documentation][2] has a good summary of the GPIO subsystem. The
27specific field used to name the GPIO's in the DTS is `gpio-line-names`.
28This [patch][3] shows an example of naming the GPIO's for a system.
29
30GPIOs are used for arbitrary things. It's pretty hard to have a coherent naming
31scheme in the face of a universe of potential use-cases.
32
33Scoping the problem down to just the vastness of OpenBMC narrows the
34possibilities quite a bit and allows the possibility of a naming scheme to
35emerge.
36
37## Requirements
38- Ensure common function GPIO's within OpenBMC use the same naming convention
39
40## Proposed Design
41Below are the standard categories. The "Pattern" in each section describes the
42naming convention and then the "Defined" portion lists the common GPIO names to
43be used (when available on an OpenBMC system). This naming convention must be
44followed for all common GPIO's.
45
46This list below includes all common GPIO's within OpenBMC. Any OpenBMC
47system which provides one of the below GPIO's must name it as listed in
48this document. This document must be updated as new common GPIO's are added.
49
50### LEDs
51Pattern: `led-*`
52
53Defined:
54- led-fault
55- led-identify
56- led-power
57- led-sys-boot-status
58- led-attention
59- led-hdd-fault
60- led-rear-fault
61- led-rear-power
62- led-rear-id
63
64### Power
65Pattern: `power-*`
66
67Defined:
68#### power-button
69
70#### power-chassis-control
71Set to initiate power-on or power-off of the chassis.
72
73#### power-chassis-good
74Indicates the power good state of the chassis.
75
76### Buttons
77Pattern: `*-button`
78
79Defined:
80- power-button
81
82### Presence
83Pattern: `presence-*`
84
85Defined:
86- presence-ps0
87- presence-ps1
88- ...
89- presence-ps`<N>`
90
91### Special
92These are special case and/or grandfathered in pin names.
93
94Defined:
95#### air-water
96Indicates whether system is air or water cooled
97
98#### factory-reset-toggle
99The software records the state of this GPIO and checks upon reboot if the state
100has changed since the last reboot. If it has, it indicates that a factory reset
101should be performed.
102
103### POWER Specific GPIO's
104Below are GPIO names specific to the POWER processor based servers.
105
106#### FSI
107Pattern: `fsi-*`
108
109Defined:
110- fsi-mux
111- fsi-enable
112- fsi-trans
113- fsi-clock
114- fsi-data
115- fsi-routing
116
117#### Special
118These are special case and/or grandfathered in pin names.
119
120Defined:
121- cfam-reset
122- checkstop
123
124## Alternatives Considered
125- Continue to hard code a config file per system type that has the
126gpio bank and pin number. This removes a dependency on the device tree to
127have consistent names but adds overhead in supporting each new system.
128
129- Have the device tree GPIO names match the hardware schematics and then
130have another userspace config file that maps between the schematic names
131and logical pin names. This makes the GPIO to schematic mapping easy but
132adds an additional layer of work with the userspace config.
133
134## Impacts
135Need to ensure OpenBMC device trees conform to the above naming conventions.
136
137## Testing
138Userspace utilization of the GPIO names will provide some testing coverage
139during CI.
140
141[1]: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/about/
142[2]: https://www.kernel.org/doc/html/latest/driver-api/gpio/index.html
143[3]: https://lore.kernel.org/linux-arm-kernel/20200306170218.79698-1-geissonator@yahoo.com/
144