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### Buttons
71Pattern: `*-button`
72
73Defined:
74- power-button
75
76### Presence
77Pattern: `presence-*`
78
79Defined:
80- presence-ps0
81- presence-ps1
82- ...
83- presence-ps`<N>`
84
85### Special
86These are special case and/or grandfathered in pin names.
87
88Defined:
89- air-water (indicates whether system is air or water cooled)
90
91### POWER Specific GPIO's
92Below are GPIO names specific to the POWER processor based servers.
93
94#### FSI
95Pattern: `fsi-*`
96
97Defined:
98- fsi-mux
99- fsi-enable
100- fsi-trans
101- fsi-clock
102- fsi-data
103- fsi-routing
104
105#### Special
106These are special case and/or grandfathered in pin names.
107
108Defined:
109- cfam-reset
110- checkstop
111
112## Alternatives Considered
113- Continue to hard code a config file per system type that has the
114gpio bank and pin number. This removes a dependency on the device tree to
115have consistent names but adds overhead in supporting each new system.
116
117- Have the device tree GPIO names match the hardware schematics and then
118have another userspace config file that maps between the schematic names
119and logical pin names. This makes the GPIO to schematic mapping easy but
120adds an additional layer of work with the userspace config.
121
122## Impacts
123Need to ensure OpenBMC device trees conform to the above naming conventions.
124
125## Testing
126Userspace utilization of the GPIO names will provide some testing coverage
127during CI.
128
129[1]: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/about/
130[2]: https://www.kernel.org/doc/html/latest/driver-api/gpio/index.html
131[3]: https://lore.kernel.org/linux-arm-kernel/20200306170218.79698-1-geissonator@yahoo.com/
132