xref: /openbmc/docs/designs/device-tree-gpio-naming.md (revision 785a64ff6f07c743361cc2fc208fd53a5bbc2e41)
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 GPIOs in the DTS is `gpio-line-names`.
28This [patch][3] shows an example of naming the GPIOs 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 GPIOs 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 sub bullets list the common GPIO names to
43be used (when available on an OpenBMC system). This naming convention must be
44followed for all common GPIOs.
45
46This list below includes all common GPIOs within OpenBMC. Any OpenBMC
47system which provides one of the below GPIOs must name it as listed in
48this document. This document must be updated as new common GPIOs are added.
49
50### Buttons
51Pattern: `*-button`
52
53Buttons should always be considered 'input' to the BMC.  There may be cases
54where BMC-less machines use a button to trigger system behavior and in a
55BMC-managed system this signal is emulated by the BMC (as output).  These
56should not be considered a button in this document but enumerated as another
57signal type.
58
59#### power-button
60
61### Host Ready
62Below are input GPIO names specific to Host ready. The name of Host ready GPIO
63depends on the index of Host and the active state is high or low.
64
65Pattern:
66- `host*-ready`: Host ready, active high
67- `host*-ready-n`: Host ready, active low
68
69Defined:
70- host0-ready
71- host1-ready-n
72- ...
73
74### LEDs
75Pattern: `led-*`
76
77#### led-fault
78#### led-identify
79#### led-power
80#### led-sys-boot-status
81#### led-attention
82#### led-hdd-fault
83#### led-rear-fault
84#### led-rear-power
85#### led-rear-id
86
87### Power and Regulators
88Pattern: `power-*`, `regulator-*`
89
90#### power-chassis-control
91Set to initiate power-on or power-off of the chassis.
92
93#### power-chassis-good
94Indicates the power good state of the chassis.
95
96#### power-config-full-load
97Output GPIO set by the power managing application that indicates to the hardware
98the threshold of power supplies that are expected to be present and working for
99this type of system for the case where a system has a model that supports two
100different number of power supplies (example 2 or 4). Default is the lowest
101number of power supplies expected of the two models. In the case when the number
102of power supplies that are present are less than the ones indicated by this
103GPIO, the hardware can then take actions such as reducing the system's
104performance so that it can maintain its powered on state.
105
106#### power-ffs-sync-history
107Output GPIO set by the power managing applications. The IBM Common Form Factor
108power supplies use this line as an input. When set low and left low the power
109supply fans run at full speed (Fans Full Speed). When toggled low, then high,
110it resets the power supply input history (average and maximum power values over
111a time range).
112
113#### regulator-standby-faulted
114This GPIO value represents the status of standby power regulator fault detection
115logic. This GPIO is an input only. The status will reflect a regulator
116non-faulted condition after AC power cycle when no standby power regulator fault
117condition is present. The status will reflect a standby regulator power faulted
118condition when an unexpected drop in standby power is detected.
119
120#### rtc-battery-voltage-read-enable
121Output pin that enables the ADC to read the board RTC battery voltage.
122
123### Presence
124Pattern: `presence-*`
125
126#### presence-ps0, presence-ps1, ..., presence-ps\<N>
127
128### Reset Cause
129These are GPIOs that provide more detail on the reason for a BMC reset. BMC
130hardware generally provides some information on a BMC reboot, like a EXTRST
131(i.e. a BMC reset was reset by some external source). At times though,
132firmware needs more details on the cause of a reset. Hardware can be configured
133to latch an event into a GPIO for firmware to then utilize for different
134software logic.
135
136Pattern: `reset-cause-*`
137
138#### reset-cause-pinhole
139The pinhole reset cause will be utilized by BMC firmware to know when it
140has been reset due to a user initiated pinhole reset. This is commonly done in
141error scenarios where the BMC is hanging or otherwise unresponsive. Note that
142this GPIO is not utilized to cause the actual reset, it is a GPIO that can be
143read after the BMC reset to know the reason for the reboot was a pinhole reset.
144
145### Secure Boot
146
147#### bmc-secure-boot
148Input pin that indicates that the BMC is configured to boot with security
149protections enforced.
150
151Pulled up by default (secure). Placing the jumper will pull the pin down
152(bypass security).
153
154### Special
155These are special case and/or grandfathered in pin names.
156
157#### air-water
158Indicates whether system is air or water cooled
159
160#### factory-reset-toggle
161The software records the state of this GPIO and checks upon reboot if the state
162has changed since the last reboot. If it has, it indicates that a factory reset
163should be performed.
164
165### POWER Specific GPIOs
166Below are GPIO names specific to the POWER processor based servers.
167
168#### Special
169These are special case and/or grandfathered in pin names.
170
171##### cfam-reset
172Utilized to issue a processor logic reset to a IBM POWER processor.
173
174##### checkstop
175Utilized to indicate a IBM POWER processor has entered an unrecoverable error
176state.
177
178## Alternatives Considered
179- Continue to hard code a config file per system type that has the
180gpio bank and pin number. This removes a dependency on the device tree to
181have consistent names but adds overhead in supporting each new system.
182
183- Have the device tree GPIO names match the hardware schematics and then
184have another userspace config file that maps between the schematic names
185and logical pin names. This makes the GPIO to schematic mapping easy but
186adds an additional layer of work with the userspace config.
187
188## Impacts
189Need to ensure OpenBMC device trees conform to the above naming conventions.
190
191## Testing
192Userspace utilization of the GPIO names will provide some testing coverage
193during CI.
194
195[1]: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/about/
196[2]: https://www.kernel.org/doc/html/latest/driver-api/gpio/index.html
197[3]: https://lore.kernel.org/linux-arm-kernel/20200306170218.79698-1-geissonator@yahoo.com/
198