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#### rtc-battery-voltage-read-enable 108Output pin that enables the ADC to read the board RTC battery voltage. 109 110### Presence 111Pattern: `presence-*` 112 113#### presence-ps0, presence-ps1, ..., presence-ps\<N> 114 115### Reset Cause 116These are GPIOs that provide more detail on the reason for a BMC reset. BMC 117hardware generally provides some information on a BMC reboot, like a EXTRST 118(i.e. a BMC reset was reset by some external source). At times though, 119firmware needs more details on the cause of a reset. Hardware can be configured 120to latch an event into a GPIO for firmware to then utilize for different 121software logic. 122 123Pattern: `reset-cause-*` 124 125#### reset-cause-pinhole 126The pinhole reset cause will be utilized by BMC firmware to know when it 127has been reset due to a user initiated pinhole reset. This is commonly done in 128error scenarios where the BMC is hanging or otherwise unresponsive. Note that 129this GPIO is not utilized to cause the actual reset, it is a GPIO that can be 130read after the BMC reset to know the reason for the reboot was a pinhole reset. 131 132### Secure Boot 133 134#### bmc-secure-boot 135Input pin that indicates that the BMC is configured to boot with security 136protections enforced. 137 138Pulled up by default (secure). Placing the jumper will pull the pin down 139(bypass security). 140 141### Special 142These are special case and/or grandfathered in pin names. 143 144#### air-water 145Indicates whether system is air or water cooled 146 147#### factory-reset-toggle 148The software records the state of this GPIO and checks upon reboot if the state 149has changed since the last reboot. If it has, it indicates that a factory reset 150should be performed. 151 152### POWER Specific GPIOs 153Below are GPIO names specific to the POWER processor based servers. 154 155#### Special 156These are special case and/or grandfathered in pin names. 157 158##### cfam-reset 159Utilized to issue a processor logic reset to a IBM POWER processor. 160 161##### checkstop 162Utilized to indicate a IBM POWER processor has entered an unrecoverable error 163state. 164 165## Alternatives Considered 166- Continue to hard code a config file per system type that has the 167gpio bank and pin number. This removes a dependency on the device tree to 168have consistent names but adds overhead in supporting each new system. 169 170- Have the device tree GPIO names match the hardware schematics and then 171have another userspace config file that maps between the schematic names 172and logical pin names. This makes the GPIO to schematic mapping easy but 173adds an additional layer of work with the userspace config. 174 175## Impacts 176Need to ensure OpenBMC device trees conform to the above naming conventions. 177 178## Testing 179Userspace utilization of the GPIO names will provide some testing coverage 180during CI. 181 182[1]: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/about/ 183[2]: https://www.kernel.org/doc/html/latest/driver-api/gpio/index.html 184[3]: https://lore.kernel.org/linux-arm-kernel/20200306170218.79698-1-geissonator@yahoo.com/ 185