gpiod

A native Go library for Linux GPIO.

gpiod is a library for accessing GPIO pins/lines on Linux platforms using the GPIO character device.

The goal of this library is to provide the Go equivalent of the C libgpiod library. The intent is not to mirror the libgpiod API but to provide the equivalent functionality.

Roadmap

On the road to v1.0 the intent is to rename this library to gpiocdev to more clearly distinguish it from libgpiod, and to highlight that it operates on the GPIO character device. Additionally, the tools and testing will be more closely aligned to libgpiod v2, rather than libgpiod v1.

The plan is to work on the renaming in a gpiocdev branch and, once that is complete, to release a final 0.8.x version which deprecates the gpiod name, and to merge the gpiocdev branch into master and release v0.9.0 which will be the first release as gpiocdev. Coincident with this, the github repo will be renamed from gpiod to go-gpiocdev.

So, in summary, the planned changes are:

v0.8.x

• Deprecate github.com/warthog618/gpiod in favour of github.com/warthog618/go-gpiocdev

v0.9.0

• First release as gpiocdev (released simultaneously with the final 0.8.x)
• Switch tests from gpio-mockup to gpio-sim
• Rename gpiodctl to gpiocdev and upgrade tools to match libgpiod v2 tools
• Remove the libgpiod v1 equivalent tools (the gpiocdev tool will provide equivalents of the libgpiod v2 tools).

Features

Supports the following functionality per line and for collections of lines:

• direction (input/output)¹
• write (active/inactive)
• read (active/inactive)
• active high/low (defaults to high)
• output mode (push-pull/open-drain/open-source)
• pull up/down²
• watches and edge detection (rising/falling/both)
• chip and line labels
• debouncing input lines³
• different configurations for lines within a collection³

¹ Dynamically changing line direction without releasing the line requires Linux 5.5 or later.

² Requires Linux 5.5 or later.

³ Requires Linux 5.10 or later.

All library functions are safe to call from different goroutines.

Quick Start

A simple piece of wire example that reads the value of an input line (pin 2) and writes its value to an output line (pin 3):

import "github.com/warthog618/gpiod"

...

in, _ := gpiod.RequestLine("gpiochip0", 2, gpiod.AsInput)
val, _ := in.Value()
out, _ := gpiod.RequestLine("gpiochip0", 3, gpiod.AsOutput(val))

...

Error handling and releasing of resources omitted for brevity.

Usage

import "github.com/warthog618/gpiod"

Error handling is omitted from the following examples for brevity.

Line Requests

To read or alter the value of a line it must first be requested using gpiod.RequestLine:

l, _ := gpiod.RequestLine("gpiochip0", 4)   // in its existing state

or from the Chip object using Chip.RequestLine:

l, _ := c.RequestLine(4)                    // from a Chip object

The offset parameter identifies the line on the chip, and is specific to the GPIO chip. To improve readability, convenience mappings can be provided for specific devices, such as the Raspberry Pi:

l, _ := c.RequestLine(rpi.J8p7)             // using Raspberry Pi J8 mapping

The initial configuration of the line can be set by providing line configuration options, as shown in this AsOutput example:

l, _ := gpiod.RequestLine("gpiochip0", 4, gpiod.AsOutput(1))  // as an output line

Multiple lines from the same chip may be requested as a collection of lines using gpiod.RequestLines

ll, _ := gpiod.RequestLines("gpiochip0", []int{0, 1, 2, 3}, gpiod.AsOutput(0, 0, 1, 1))

or from a Chip object using Chip.RequestLines:

ll, _ := c.RequestLines([]int{0, 1, 2, 3}, gpiod.AsOutput(0, 0, 1, 1))

When no longer required, the line(s) should be closed to release resources:

l.Close()
ll.Close()

Line Values

Lines must be requsted using RequestLine before their values can be accessed.

Read Input

The current line value can be read with the Value method:

r, _ := l.Value()  // Read state from line (active / inactive)

For collections of lines, the level of all lines is read simultaneously using the Values method:

rr := []int{0, 0, 0, 0} // buffer to read into...
ll.Values(rr)           // Read the state of a collection of lines

Write Output

The current line value can be set with the SetValue method:

l.SetValue(1)     // Set line active
l.SetValue(0)     // Set line inactive

Also refer to the blinker example.

For collections of lines, all lines are set simultaneously using the SetValues method:

ll.SetValues([]int{0, 1, 0, 1}) // Set a collection of lines

Edge Watches

The value of an input line can be watched and trigger calls to handler functions.

The watch can be on rising or falling edges, or both.

The events are passed to a handler function provided using the WithEventHandler(eh) option. The handler function is passed a LineEvent, which contains details of the edge event including the offset of the triggering line, the time the edge was detected and the type of edge detected:

func handler(evt gpiod.LineEvent) {
  // handle edge event
}

l, _ = c.RequestLine(rpi.J8p7, gpiod.WithEventHandler(handler), gpiod.WithBothEdges)

To maintain event ordering, the event handler is called serially from a goroutine that reads the events from the kernel. The event handler is expected to be short lived, and so should hand off any potentially blocking operations to a separate goroutine.

An edge watch can be removed by closing the line:

l.Close()

or by reconfiguring the requested lines to disable edge detection:

l.Reconfigure(gpiod.WithoutEdges)

Note that the Close waits for the event handler to return and so must not be called from the event handler context - it should be called from a separate goroutine.

Also see the watcher example.

Line Configuration

Line configuration is set via options to Chip.RequestLine(s) and Line.Reconfigure. These override any default which may be set in NewChip.

Note that configuration options applied to a collection of lines apply to all lines in the collection, unless they are applied to a subset of the requested lines using the WithLines option.

Reconfiguration

Requested lines may be reconfigured using the Reconfigure method:

l.Reconfigure(gpiod.AsInput)            // set direction to Input
ll.Reconfigure(gpiod.AsOutput(1, 0))    // set direction to Output (and values to active and inactive)

The Line.Reconfigure method accepts differential changes to the configuration for the lines, so option categories not specified or overridden by the specified changes will remain unchanged.

The Line.Reconfigure method requires Linux 5.5 or later.

Complex Configurations

It is sometimes necessary for the configuration of lines within a request to have slightly different configurations. Line options may be applied to a subset of requested lines using the WithLines(offsets, options) option.

The following example requests a set of output lines and sets some of the lines in the request to active low:

ll, _ = c.RequestLines([]int{0, 1, 2, 3}, gpiod.AsOutput(0, 0, 1, 1),
    gpiod.WithLines([]int{0, 3}, gpiod.AsActiveLow),
    gpiod.AsOpenDrain)

The configuration of the subset of lines inherits the configuration of the request at the point the WithLines is invoked. Subsequent changes to the request configuration do not alter the configuration of the subset - in the example above, lines 0 and 3 will not be configured as open-drain.

Once a line's configuration has branched from the request configuration it can only be altered with WithLines options:

ll.Reconfigure(gpiod.WithLines([]int{0}, gpiod.AsActiveHigh))

or reset to the request configuration using the Defaulted option:

ll.Reconfigure(gpiod.WithLines([]int{3}, gpiod.Defaulted))

Complex configurations require Linux 5.10 or later.

Chip Initialization

The Chip object is used to discover details about avaialble lines and can be used to request lines from a GPIO chip.

A Chip object is constructed using the NewChip function.

c, _ := gpiod.NewChip("gpiochip0")

The parameter is the chip name, which corresponds to the name of the device in the /dev directory, so in this example /dev/gpiochip0.

The list of currently available GPIO chips is returned by the Chips function:

cc := gpiod.Chips()

Default attributes for Lines requested from the Chip can be set via configuration options to NewChip.

c, _ := gpiod.NewChip("gpiochip0", gpiod.WithConsumer("myapp"))

In this example the consumer label is defaulted to "myapp".

When no longer required, the chip should be closed to release resources:

c.Close()

Closing a chip does not close or otherwise alter the state of any lines requested from the chip.

Line Info

Info about a line can be read at any time from the chip using the LineInfo method:

inf, _ := c.LineInfo(4)
inf, _ := c.LineInfo(rpi.J8p7) // Using Raspberry Pi J8 mapping

Note that the line info does not include the value. The line must be requested from the chip to access the value.

Once requested, the line info can also be read from the line:

inf, _ := l.Info()
infs, _ := ll.Info()

Info Watches

Changes to the line info can be monitored by adding an info watch for the line:

func infoChangeHandler( evt gpiod.LineInfoChangeEvent) {
    // handle change in line info
}

inf, _ := c.WatchLineInfo(4, infoChangeHandler)

Note that the info watch does not monitor the line value (active or inactive) only its configuration. Refer to Edge Watches for monitoring line value.

An info watch can be cancelled by unwatching:

c.UnwatchLineInfo(4)

or by closing the chip.

Categories

Most line configuration options belong to one of the following categories:

• Active Level
• Direction
• Bias
• Drive
• Debounce
• Edge Detection
• Event Clock

Only one option from each category may be applied. If multiple options from a category are applied then all but the last are ignored.

Active Level

The values used throughout the API for line values are the logical value, which is 0 for inactive and 1 for active. The physical value considered active can be controlled using the AsActiveHigh and AsActiveLow options:

l, _ := c.RequestLine(4, gpiod.AsActiveLow) // during request
l.Reconfigure(gpiod.AsActiveHigh)           // once requested

Lines are typically active high by default.

Direction

The line direction can be controlled using the AsInput and AsOutput options:

l, _ := c.RequestLine(4, gpiod.AsInput) // during request
l.Reconfigure(gpiod.AsInput)            // set direction to Input
l.Reconfigure(gpiod.AsOutput(0))        // set direction to Output (and value to inactive)

Bias

The bias options control the pull up/down state of the line:

l, _ = c.RequestLine(4, gpiod.WithPullUp) // during request
l.Reconfigure(gpiod.WithBiasDisabled)     // once requested

The bias options require Linux 5.5 or later.

Drive

The drive options control how an output line is driven when active and inactive:

l,_ := c.RequestLine(4, gpiod.AsOpenDrain) // during request
l.Reconfigure(gpiod.AsOpenSource)          // once requested

The default drive for output lines is push-pull, which actively drives the line in both directions.

Debounce

Input lines may be debounced using the WithDebounce option. The debouncing will be performed by the underlying hardware, if supported, else by the Linux kernel.

period := 10 * time.Millisecond
l, _ = c.RequestLine(4, gpiod.WithDebounce(period))// during request
l.Reconfigure(gpiod.WithDebounce(period))         // once requested

The WithDebounce option requires Linux 5.10 or later.

Edge Detection

The edge options control which edges on input lines will generate edge events. Edge events are passed to the event handler specified in the WithEventHandler(eh) option.

By default edge detection is not enabled on requested lines.

Refer to Edge Watches for examples of the edge detection options.

Event Clock

The event clock options control the source clock used to timestamp edge events. This is only useful for Linux kernels 5.11 and later - prior to that the clock source is fixed.

The event clock source used by the kernel has changed over time as follows:

Determining which clock the edge event timestamps contain is currently left as an exercise for the user.

Configuration Options

The available configuration options are:

The options described as default are generally not required, except to override other options earlier in a chain of configuration options.

¹ Can be applied to either NewChip or Chip.RequestLine, but cannot be used with Line.Reconfigure.

² Can be applied to Chip.RequestLine, but cannot be used with NewChip or Line.Reconfigure.

³ Can be applied to either Chip.RequestLine or Line.Reconfigure, but cannot be used with NewChip.

⁴ Requires Linux 5.5 or later.

⁵ Requires Linux 5.10 or later.

⁶ Requires Linux 5.11 or later.

Installation

On Linux:

go get github.com/warthog618/gpiod

For other platforms, where you intend to cross-compile for Linux, don't attempt to compile the package when it is installed:

go get -d github.com/warthog618/gpiod

Tools

A command line utility, gpiodctl, can be found in the cmd directory and is provided to allow manual or scripted manipulation of GPIO lines. This utility combines the Go equivalent of all the libgpiod command line tools into a single tool.

gpiodctl is a utility to control GPIO lines on Linux GPIO character devices

Usage:
  gpiodctl [flags]
  gpiodctl [command]

Available Commands:
  detect      Detect available GPIO chips
  find        Find a GPIO line by name
  get         Get the state of a line or lines
  help        Help about any command
  info        Info about chip lines
  mon         Monitor the state of a line or lines
  set         Set the state of a line or lines
  version     Display the version
  watch       Watch lines for changes to the line info

Flags:
  -h, --help   help for gpiodctl

Use "gpiodctl [command] --help" for more information about a command.

The Go equivalent of each of the libgpiod command line tools can also be found in the cmd directory.

Those tools are:

Tests

The library is fully tested, other than some error cases and sanity checks that are difficult to trigger.

The tests require a kernel release 5.1.0 or later to run. For all the tests to pass a kernel 5.5.0 or later is required.

The test user must have access to the /dev/gpiochip0 character device.

Platforms

The tests can be run on either of two platforms:

• gpio-mockup (default)
• Raspberry Pi

gpio-mockup

The gpio-mockup platform is any Linux platform with a recent kernel that supports the gpio-mockup loadable module. gpio-mockup must be built as a module and the test user must have rights to load and unload the module.

The gpio-mockup is the default platform for tests and benchmarks as it does not interact with physical hardware and so is always safe to run.

Raspberry Pi

On Raspberry Pi, the tests are intended to be run on a board with J8 pins 11 and 12 floating and with pins 15 and 16 tied together, possibly using a jumper across the header.

⚠️ The tests set J8 pins 11, 12 and 16 to outputs so DO NOT run them on hardware where any of those pins is being externally driven.

The Raspberry Pi platform is selected by specifying the platform parameter on the test command line:

go test -platform=rpi

Tests have been run successfully on Raspberry Pi Zero W and Pi 4B. The library should also work on other Raspberry Pi variants, I just haven't gotten around to testing them yet.

The tests can be cross-compiled from other platforms using:

GOOS=linux GOARCH=arm GOARM=6 go test -c

Later Pis can also use ARM7 (GOARM=7).

Benchmarks

The tests include benchmarks on reads, writes, bulk reads and writes, and interrupt latency.

These are the results from a Raspberry Pi Zero W running Linux 5.10 and built with go1.15.6:

$ ./gpiod.test -platform=rpi -test.bench=.*
goos: linux
goarch: arm
pkg: github.com/warthog618/gpiod
BenchmarkChipNewClose              265       3949958 ns/op
BenchmarkLineInfo                28420         40192 ns/op
BenchmarkLineReconfigure         26079         46121 ns/op
BenchmarkLineValue              114961         10176 ns/op
BenchmarkLinesValues             66969         17367 ns/op
BenchmarkLineSetValue            92529         12531 ns/op
BenchmarkLinesSetValues          65965         17309 ns/op
BenchmarkInterruptLatency         1827        638202 ns/op
PASS

Prerequisites

The library targets Linux with support for the GPIO character device API. That generally means that /dev/gpiochip0 exists.

The caller must have access to the character device - typically /dev/gpiochip0. That is generally root unless you have changed the permissions of that device.

The Bias line options and the Line.Reconfigure method both require Linux 5.5 or later.

Debounce and other uAPI v2 features require Linux 5.10 or later.

The requirements for each configuration option are noted in that section.

Release Notes

v0.8.0

Add top level RequestLine and RequestLines functions to simplify common use cases.

blinker and watcher examples interwork with each other on a Raspberry Pi with a jumper across J8-15 and J8-16.

Fix deadlock in gpiodctl set no-wait.

v0.7.0

LineEvent exposes sequence numbers for uAPI v2 events.

Info tools (gpiodctl info and gpioinfo) report debounce-period.

gpiodctl mon and watcher example report event sequence numbers.

gpiodctl mon supports setting debounce period.

gpiodctl detect reports kernel uAPI version in use.

Watchers use Eventfd instead of pipes to reduce open file descriptors.

Start migrating to Go 1.17 go:build style build tags.

Make licensing REUSE compliant.

v0.6.0

gpiod now supports both the old GPIO uAPI (v1) and the newer (v2) introduced in Linux 5.10. The library automatically detects the available uAPI versions and makes use of the latest.

Applications written for uAPI v1 will continue to work with uAPI v2.

Applications that make use of v2 specific features will return errors when run on Linux kernels prior to 5.10.

Breaking API changes:

1. The event handler parameter has been moved from edge options into the WithEventHandler(eh) option to allow for reconfiguration of edge detection which is supported in Linux 5.10.

   Old edge options should be replaced with the WithEventHandler option and the now parameterless edge option, e.g.:

   s/gpiod\.WithBothEdges(/gpiod.WithBothEdges, gpiod.WithEventHandler(/g

2. WithBiasDisable is renamed WithBiasDisabled. This option is probably rarely used and the renaming is trivial, so no backward compatibility is provided.

3. FindLine has been dropped as line names are not guaranteed to be unique. Iterating over the available chips and lines to search for line by name can be easily done - the Chips function provides the list of available chips as a starting point.

   Refer to the find command in gpiodctl for example code.