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