Tony Blyler f6e9c70eb3 A quick and dirty implementation

2020-12-03 23:08:46 -05:00

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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.

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 (normal/open-drain/open-source)
pull up/down²
watches and edge detection (rising/falling/both)
chip and line labels

¹Dynamically changing line direction without releasing the line requires Linux v5.5 or later.

²Pull up/down support requires Linux v5.5 or later.

All library functions are safe to call from different goroutines.

Usage

import "github.com/warthog618/gpiod"

Error handling is omitted from the following examples for brevity.

Chip Initialization

The Chip object is 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.

Default attributes for Lines requested from the Chip can be set via 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 Requests

To alter the state of a line it must first be requested from the Chip, using Chip.RequestLine:

l, _ := c.RequestLine(4)                    // in its existing state

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

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

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

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

Note that line options applied to a collection of lines apply to all lines in the collection.

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

l.Close()
ll.Close()

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()

Direction

The line direction can be controlled using the AsInput and AsOutput line 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)

Read Input

The current line level 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 level 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

Watches

The state 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 handler function is passed a LineEvent, which contains the offset of the triggering line, the time the edge was detected and the type of edge detected:

func handler(evt gpiod.LineEvent) {
  // handle change in line state
}

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

A watch can be removed by closing the line:

l.Close()

Also see the watcher example.

Find

Lines can be found by the GPIO label name as returned in line info and set by device-tree using the FindLine function:

chipname, offset, _ := gpiod.FindLine("LED A")
c, _ := gpiod.NewChip(chipname, gpiod.WithConsumer("myapp"))
l, _ := c.RequestLine(offset)

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 level considered active can be controlled using the AsActiveHigh and AsActiveLow line options:

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

Lines are typically active high by default.

Bias

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

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

Note that bias options require Linux v5.5 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 drives the line in both directions.

Line Options

Line attributes are set via options to Chip.RequestLine(s) and Line.Reconfigure. These override any default which may be set in NewChip. Only one option from each category may be applied. If multiple options from a category are applied then all but the last are ignored.

The line options are:

Option	Category	Description
WithConsumer¹	Info	Set the consumer label for the lines
AsActiveLow	Level	Treat a low physical line level as active
AsActiveHigh	Level	Treat a high physical line level as active (default)
AsInput	Direction	Request lines as input
AsIs²	Direction	Request lines in their current input/output state (default)
AsOutput(<values>...)³	Direction	Request lines as output with initial values
AsPushPull	Drive³	Request output lines drive both high and low (default)
AsOpenDrain	Drive³	Request lines as open drain outputs
AsOpenSource	Drive³	Request lines as open source outputs
WithFallingEdge(eh)	Edge⁴	Request lines with falling edge detection, with events passed to the provided event handler
WithRisingEdge(eh)	Edge⁴	Request lines with rising edge detection, with events passed to the provided event handler
WithBothEdges(eh)	Edge⁴	Request lines with rising and falling edge detection, with events passed to the provided event handler
WithBiasDisable	Bias⁵	Request the lines have internal bias disabled
WithPullDown	Bias⁵	Request the lines have internal pull-down enabled
WithPullUp	Bias⁵	Request the lines have internal pull-up enabled

¹ WithConsumer can be provided to either NewChip or Chip.RequestLine(s), and cannot be used with Line.Reconfigure.

² The AsIs option can only be provided to Chip.RequestLine(s), and cannot be used with NewChip or Line.Reconfigure.

³ The AsOutput and Drive options can only be provided to either Chip.RequestLine(s) or Line.Reconfigure, and cannot be used with NewChip.

⁴ Edge options can only be provided to Chip.RequestLine(s), and cannot be used with NewChip or Line.Reconfigure.

⁵ Bias options require Linux v5.5 or later.

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
  help        Help about any command
  info        Info about chip lines
  mon         Monitor the state of a line
  set         Set the state of a line
  version     Display the version

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:

Tool	Description
gpiodetect	Report all the gpiochips available on the system.
gpioinfo	Report the details of all the lines available on gpiochips.
gpiofind	Find the gpiochip and offset of a line by name.
gpioget	Get the value of a line or a set of lines on one gpiochip.
gpioset	Set of value of a line or a set of lines on one gpiochip.
gpiomon	Report edges detected on a line or set of lines on one gpiochip.

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 built with Go 1.13:

$ ./gpiod.test -platform=rpi -test.bench=.*
goos: linux
goarch: arm
pkg: gpiod
BenchmarkLineValue             157851          7160 ns/op
BenchmarkLinesValues           152865          7599 ns/op
BenchmarkLineSetValue          171585          6782 ns/op
BenchmarkLinesSetValues        155041          7995 ns/op
BenchmarkInterruptLatency        2041        581938 ns/op
PASS

Prerequisites

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

The Bias line options and the Line.Reconfigure method both require Linux v5.5 or later.

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.

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