# Device Tree GPIO Naming in OpenBMC

Author: Andrew Geissler (geissonator)

Primary assignee: Andrew Geissler (geissonator)

Other contributors:
  < None >

Created: April 3, 2020

## Problem Description
The Linux kernel has deprecated the use of sysfs to interact with the GPIO
subsystem. The replacement is a "descriptor-based" character device interface.

[libgpiod][1] is a suite of tools and library implemented in C and C++ which
provides an abstraction to this new character device gpio interface.

libgpiod provides a feature where you can access gpios by a name given to
them in the kernel device tree files. The problem is there are no naming
conventions for these GPIO names and if you want userspace code to be able
to be consistent across different machines, these names would need to be
consistent.

## Background and References
The kernel [documentation][2] has a good summary of the GPIO subsystem. The
specific field used to name the GPIO's in the DTS is `gpio-line-names`.
This [patch][3] shows an example of naming the GPIO's for a system.

GPIOs are used for arbitrary things. It's pretty hard to have a coherent naming
scheme in the face of a universe of potential use-cases.

Scoping the problem down to just the vastness of OpenBMC narrows the
possibilities quite a bit and allows the possibility of a naming scheme to
emerge.

## Requirements
- Ensure common function GPIO's within OpenBMC use the same naming convention

## Proposed Design
Below are the standard categories. The "Pattern" in each section describes the
naming convention and then the "Defined" portion lists the common GPIO names to
be used (when available on an OpenBMC system). This naming convention must be
followed for all common GPIO's.

This list below includes all common GPIO's within OpenBMC. Any OpenBMC
system which provides one of the below GPIO's must name it as listed in
this document. This document must be updated as new common GPIO's are added.

### LEDs
Pattern: `led-*`

Defined:
- led-fault
- led-identify
- led-power
- led-sys-boot-status
- led-attention
- led-hdd-fault
- led-rear-fault
- led-rear-power
- led-rear-id

### Power
Pattern: `power-*`

Defined:
- power-button

### Buttons
Pattern: `*-button`

Defined:
- power-button

### Presence
Pattern: `presence-*`

Defined:
- presence-ps0
- presence-ps1
- ...
- presence-ps`<N>`

### Special
These are special case and/or grandfathered in pin names.

Defined:
- air-water (indicates whether system is air or water cooled)

### POWER Specific GPIO's
Below are GPIO names specific to the POWER processor based servers.

#### FSI
Pattern: `fsi-*`

Defined:
- fsi-mux
- fsi-enable
- fsi-trans
- fsi-clock
- fsi-data
- fsi-routing

#### Special
These are special case and/or grandfathered in pin names.

Defined:
- cfam-reset
- checkstop

## Alternatives Considered
- Continue to hard code a config file per system type that has the
gpio bank and pin number. This removes a dependency on the device tree to
have consistent names but adds overhead in supporting each new system.

- Have the device tree GPIO names match the hardware schematics and then
have another userspace config file that maps between the schematic names
and logical pin names. This makes the GPIO to schematic mapping easy but
adds an additional layer of work with the userspace config.

## Impacts
Need to ensure OpenBMC device trees conform to the above naming conventions.

## Testing
Userspace utilization of the GPIO names will provide some testing coverage
during CI.

[1]: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/about/
[2]: https://www.kernel.org/doc/html/latest/driver-api/gpio/index.html
[3]: https://lore.kernel.org/linux-arm-kernel/20200306170218.79698-1-geissonator@yahoo.com/