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https://github.com/rust-embedded/gpio-cdev

Rust interface to the Linux GPIO Character Device API (/dev/gpiochip...)
https://github.com/rust-embedded/gpio-cdev

embedded gpio gpio-cdev gpio-character-device linux rust

Last synced: 3 days ago
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Rust interface to the Linux GPIO Character Device API (/dev/gpiochip...)

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README 

        
# gpio-cdev



[![Build Status](https://github.com/rust-embedded/gpio-cdev/workflows/Build/badge.svg)](https://github.com/rust-embedded/gpio-cdev/actions)

[![Version](https://img.shields.io/crates/v/gpio-cdev.svg)](https://crates.io/crates/gpio-cdev)

[![License](https://img.shields.io/crates/l/gpio-cdev.svg)](https://github.com/rust-embedded/gpio-cdev/blob/master/README.md#license)



- [API Documentation](https://docs.rs/gpio-cdev)



rust-gpio-cdev is a Rust library/crate providing access to [GPIO character device

ABI](https://www.kernel.org/doc/Documentation/ABI/testing/gpio-cdev).  This API,

stabilized with Linux v4.4, deprecates the legacy sysfs interface to GPIOs that is

planned to be removed from the upstream kernel after

year 2020 (which is coming up quickly).



Use of this API is encouraged over the sysfs API used by this crate's

predecessor [sysfs_gpio](https://crates.io/crates/sysfs_gpio) if you don't need

to target older kernels.  For more information on differences see [Sysfs GPIO vs

GPIO Character Device](#sysfs-gpio-vs-gpio-character-device).



## Installation



Add the following to your `Cargo.toml`



```

[dependencies]

gpio-cdev = "0.4"

```



Note that the following features are available:



* `async-tokio`: Adds a Stream interface for consuming GPIO events in async code

  within a tokio runtime.



## Examples



There are several additional examples available in the [examples

directory](https://github.com/rust-embedded/rust-gpio-cdev/tree/master/examples).



### Read State



```rust

use gpio_cdev::{Chip, LineRequestFlags};



// Read the state of GPIO4 on a raspberry pi.  /dev/gpiochip0

// maps to the driver for the SoC (builtin) GPIO controller.

let mut chip = Chip::new("/dev/gpiochip0")?;

let handle = chip

    .get_line(4)?

    .request(LineRequestFlags::INPUT, 0, "read-input")?;

for _ in 1..4 {

    println!("Value: {:?}", handle.get_value()?);

}

```



### Mirror State (Read/Write)



```rust

use gpio_cdev::{Chip, LineRequestFlags, EventRequestFlags, EventType};



// Lines are offset within gpiochip0; see docs for more info on chips/lines

//

// This function will synchronously follow the state of one line

// on gpiochip0 and mirror its state on another line.  With this you

// could, for instance, control the state of an LED with a button

// if hooked up to the right pins on a raspberry pi.

fn mirror_gpio(inputline: u32, outputline: u32) -> Result<(), gpio_cdev::Error> {

    let mut chip = Chip::new("/dev/gpiochip0")?;

    let input = chip.get_line(inputline)?;

    let output = chip.get_line(outputline)?;

    let output_handle = output.request(LineRequestFlags::OUTPUT, 0, "mirror-gpio")?;

    for event in input.events(

        LineRequestFlags::INPUT,

        EventRequestFlags::BOTH_EDGES,

        "mirror-gpio",

    )? {

        let evt = event?;

        println!("{:?}", evt);

        match evt.event_type() {

            EventType::RisingEdge => {

                output_handle.set_value(1)?;

            }

            EventType::FallingEdge => {

                output_handle.set_value(0)?;

            }

        }

    }



    Ok(())

}

```



### Async Usage



Note that this requires the addition of the `async-tokio` feature.



```rust

use futures::stream::StreamExt;

use gpio_cdev::{Chip, AsyncLineEventHandle};



async fn gpiomon(chip: String, line: u32) -> gpio_cdev::Result<()> {

    let mut chip = Chip::new(args.chip)?;

    let line = chip.get_line(args.line)?;

    let mut events = AsyncLineEventHandle::new(line.events(

        LineRequestFlags::INPUT,

        EventRequestFlags::BOTH_EDGES,

        "gpioevents",

    )?)?;



    while let Some(event) = events.next().await {

        let event = event?;

        println!("GPIO Event: {:?}", event);

    }



    Ok(())

}

```



## Sysfs GPIO vs GPIO Character Device



Compared to the sysfs gpio interface (as made available by the sysfs_gpio crate)

the character device has several advantages and critical design differences

(some of which are driving the deprecation in the kernel).



Since many people are familiar with the sysfs interface (which is easily

accessible via basic commands in the shell) and few people are familiar with the

GPIO character device, an exploration of the two and key differences here may

prove useful.



### Getting Access to a GPIO



In the Linux kernel, individual GPIOs are exposed via drivers that on probe register

themselves as GPIO chips with the gpio subsystem.  Each of these chips provides

access to a set of GPIOs.  At present, when this chip is registered a global

base number is assigned to this driver.  The comments from the linux kernel

[`gpio_chip_add_data`](https://elixir.bootlin.com/linux/v4.9.85/source/drivers/gpio/gpiolib.c#L1087)

sum up the situation nicely when assigning the a base number to a GPIO chip

on registration.



    /*

     * TODO: this allocates a Linux GPIO number base in the global

     * GPIO numberspace for this chip. In the long run we want to

     * get *rid* of this numberspace and use only descriptors, but

     * it may be a pipe dream. It will not happen before we get rid

     * of the sysfs interface anyways.

     */



The entire sysfs interface to GPIO is based around offsets from the base number

assigned to a GPIO chip.  The base number is completely dependent on the order

in which the chip was registered with the subsystem and the number of GPIOs that

each of the previous chips registered.  The only reason this is usable at all is

that most GPIOs are accessed via SoC hardware that is registered consistently

during boot.  It's not great; in fact, it's not even good.



The GPIO character device ABI provides access to GPIOs owned by a GPIO chip via

a bus device, `/sys/bus/gpiochipN` (or `/dev/gpiochipN`).  Within a chip, the

programmer will still need to know some details about how to access the GPIO but

things are generally sane.  Figuring out which bus device is the desired GPIO

chip can be done by iterating over all that are present and/or setting up

appropriate udev rules.  One good example of this is the [`lsgpio` utility in

the kernel source](https://github.com/torvalds/linux/blob/master/tools/gpio/lsgpio.c).



In sysfs each GPIO within a chip would be exported and used individually. The

GPIO character device allows for one or more GPIOs (referenced via offsets) to

be read, written, configured, and monitored via a "linehandle" fd that is

created dynamically on request.



### "Exporting" a GPIO



Using the sysfs API, one would write the global GPIO number to the "export" file

to perform further operations using new files on the filesystem.  Using the

gpiochip character device, a handle for performing operations on one or more

GPIO offsets within a chip are available via a "linehandle" fd created using the

`GPIO_GET_LINEHANDLE_IOCTL`. A consequence of this is that a line will remember

its state only for as long as the fd is open; the line's state will be reset

once the fd is closed.



When a linehandle is requested, additional information is also included about

how the individual GPIOs will be used (input, output, as-is, active-low, open

drain, open source, etc).  Multiple lines can be grouped together in a single

request but they must all be configured the same way if being used in that way.

See `struct gpioevent_request`.



### Reading/Writing GPIOs



Via sysfs, GPIOs could be read/written using the value file.  For GPIO character

devices, the `GPIOHANDLE_GET_LINE_VALUES_IOCTL` and

`GPIOHANDLE_SET_LINE_VALUES_IOCTL` may be used to get/set the state of one or

more offsets within the chip.



### Input Events



Via sysfs, one could setup things up using the trigger file to notify userspace

(by polling on the value file) of a single event based on how things were setup.

With GPIO character devices, one can setup a `gpio_eventrequest` that will create

a new anonymous file (fd provided) for event notifications on a lines within a

gpiochip.  Contrary to sysfs gpio events, the event file will queue multiple events

and include with the event (best effort) nanosecond-precision timing and an

identifier with event type.



With this information one could more reasonably consider interpreting a basic

digital signal from userspace (with rising and falling edges) from userspace

using the queueing with timing information captured in the kernel.  Previously, one

would need to quickly handle the event notification, make another system call

to the value file to see the state, etc. which had far too many variables involved

to be considered reliable.



## Minimum Supported Rust Version (MSRV)



This crate is guaranteed to compile on stable Rust 1.65.0 and up. It *might*

compile with older versions but that may change in any new patch release.



## License



Licensed under either of



- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or

  http://www.apache.org/licenses/LICENSE-2.0)

- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)



at your option.



### Contribution



Unless you explicitly state otherwise, any contribution intentionally submitted

for inclusion in the work by you, as defined in the Apache-2.0 license, shall be

dual licensed as above, without any additional terms or conditions.



## Code of Conduct



Contribution to this crate is organized under the terms of the [Rust Code of

Conduct][CoC], the maintainer of this crate, the [Embedded Linux Team][team], promises

to intervene to uphold that code of conduct.



[CoC]: CODE_OF_CONDUCT.md

[team]: https://github.com/rust-embedded/wg#the-embedded-linux-team