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 and Regulators 65Pattern: `power-*`, `regulator-*` 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#### regulator-standby-faulted 77This GPIO value represents the status of standby power regulator fault detection 78logic. This GPIO is an input only. The status will reflect a regulator 79non-faulted condition after AC power cycle when no standby power regulator fault 80condition is present. The status will reflect a standby regulator power faulted 81condition when an unexpected drop in standby power is detected. 82 83### Buttons 84Pattern: `*-button` 85 86Defined: 87- power-button 88 89### Presence 90Pattern: `presence-*` 91 92Defined: 93- presence-ps0 94- presence-ps1 95- ... 96- presence-ps`<N>` 97 98### Secure Boot 99 100#### bmc-secure-boot 101 102Input pin that indicates that the BMC is configured to boot with security 103protections enforced. 104 105Pulled up by default (secure). Placing the jumper will pull the pin down 106(bypass security). 107 108### Special 109These are special case and/or grandfathered in pin names. 110 111Defined: 112#### air-water 113Indicates whether system is air or water cooled 114 115#### factory-reset-toggle 116The software records the state of this GPIO and checks upon reboot if the state 117has changed since the last reboot. If it has, it indicates that a factory reset 118should be performed. 119 120### POWER Specific GPIO's 121Below are GPIO names specific to the POWER processor based servers. 122 123#### Special 124These are special case and/or grandfathered in pin names. 125 126Defined: 127- cfam-reset 128- checkstop 129 130## Alternatives Considered 131- Continue to hard code a config file per system type that has the 132gpio bank and pin number. This removes a dependency on the device tree to 133have consistent names but adds overhead in supporting each new system. 134 135- Have the device tree GPIO names match the hardware schematics and then 136have another userspace config file that maps between the schematic names 137and logical pin names. This makes the GPIO to schematic mapping easy but 138adds an additional layer of work with the userspace config. 139 140## Impacts 141Need to ensure OpenBMC device trees conform to the above naming conventions. 142 143## Testing 144Userspace utilization of the GPIO names will provide some testing coverage 145during CI. 146 147[1]: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/about/ 148[2]: https://www.kernel.org/doc/html/latest/driver-api/gpio/index.html 149[3]: https://lore.kernel.org/linux-arm-kernel/20200306170218.79698-1-geissonator@yahoo.com/ 150