Author: Andrew Geissler (geissonator)
Primary assignee: Andrew Geissler (geissonator)
Other contributors: < None >
Created: April 3, 2020
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 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.
The kernel documentation has a good summary of the GPIO subsystem. The specific field used to name the GPIOs in the DTS is gpio-line-names. This patch shows an example of naming the GPIOs 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.
Below are the standard categories. The "Pattern" in each section describes the naming convention and then the sub bullets list the common GPIO names to be used (when available on an OpenBMC system). This naming convention must be followed for all common GPIOs.
This list below includes all common GPIOs within OpenBMC. Any OpenBMC system which provides one of the below GPIOs must name it as listed in this document. This document must be updated as new common GPIOs are added.
Pattern: *-button
Pattern: led-*
Pattern: power-*, regulator-*
Set to initiate power-on or power-off of the chassis.
Indicates the power good state of the chassis.
Output GPIO set by the power managing application that indicates to the hardware the threshold of power supplies that are expected to be present and working for this type of system for the case where a system has a model that supports two different number of power supplies (example 2 or 4). Default is the lowest number of power supplies expected of the two models. In the case when the number of power supplies that are present are less than the ones indicated by this GPIO, the hardware can then take actions such as reducing the system's performance so that it can maintain its powered on state.
This GPIO value represents the status of standby power regulator fault detection logic. This GPIO is an input only. The status will reflect a regulator non-faulted condition after AC power cycle when no standby power regulator fault condition is present. The status will reflect a standby regulator power faulted condition when an unexpected drop in standby power is detected.
Pattern: presence-*
These are GPIOs that provide more detail on the reason for a BMC reset. BMC hardware generally provides some information on a BMC reboot, like a EXTRST (i.e. a BMC reset was reset by some external source). At times though, firmware needs more details on the cause of a reset. Hardware can be configured to latch an event into a GPIO for firmware to then utilize for different software logic.
Pattern: reset-cause-*
The pinhole reset cause will be utilized by BMC firmware to know when it has been reset due to a user initiated pinhole reset. This is commonly done in error scenarios where the BMC is hanging or otherwise unresponsive. Note that this GPIO is not utilized to cause the actual reset, it is a GPIO that can be read after the BMC reset to know the reason for the reboot was a pinhole reset.
Input pin that indicates that the BMC is configured to boot with security protections enforced.
Pulled up by default (secure). Placing the jumper will pull the pin down (bypass security).
These are special case and/or grandfathered in pin names.
Indicates whether system is air or water cooled
The software records the state of this GPIO and checks upon reboot if the state has changed since the last reboot. If it has, it indicates that a factory reset should be performed.
Below are GPIO names specific to the POWER processor based servers.
These are special case and/or grandfathered in pin names.
Utilized to issue a processor logic reset to a IBM POWER processor.
Utilized to indicate a IBM POWER processor has entered an unrecoverable error state.
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.
Need to ensure OpenBMC device trees conform to the above naming conventions.
Userspace utilization of the GPIO names will provide some testing coverage during CI.