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