Implementing a Component

Each Tock board defines the peripherals, capsules, kernel settings, and syscall drivers to customize Tock for that board. Often, instantiating different resources (particularly capsules and drivers) requires subtle setup steps that are easy to get wrong. The setup steps are often shared from board-to-board. Together, this makes configuring a board redundant and easy to make a mistake.

Components are the Tock mechanism to help address this. Each component includes the static memory allocations and setup steps required to implement a particular piece of kernel functionality (i.e. a capsule). You can read more technical documentation here.

In this guide we will create a component for a hypothetical system call driver called Notifier. Our system call driver is going to use an alarm as a resource and requires just one other parameter: a delay value in milliseconds. The steps should be the same for any capsule you want to create a component for.

Setup

This guide assumes you already have the capsule created, and ideally that you have set it up with a board to test. Making a component then just makes it easier to include on a new board and share among boards.

Overview

The high-level steps required are:

Define the static memory required for all objects used.
Create a struct that holds all of the resources and configuration necessary for the capsules.
Implement finalize() to initialize memory and perform setup.

Step-by-Step Guide

The steps from the overview are elaborated on here.

Define the static memory required for all objects used.

All objects in the kernel are statically allocated, so we need to statically allocate memory for the objects to live in. Due to constraints on the macros Tock provides for statically allocating memory, we must contain all calls to allocate this memory within another macro.

Create a file in boards/components/src to hold the component.

We need to define a macro to setup our state. We will use the static_buf!() macro to help with this. In the file, create a macro with the name <your capsule>_component_static. This naming convention must be followed.

In our hypothetical case, we need to allocate room for the notifier capsule and a buffer. Each capsule might need slightly different resources.

#![allow(unused)]
fn main() {
#[macro_export]
macro_rules! notifier_driver_component_static {
    ($A:ty $(,)?) => {{
        let notifier_buffer = kernel::static_buf!([u8; 16]);
        let notifier_driver = kernel::static_buf!(
            capsules_extra::notifier::NotifierDriver<'static, $A>
        );

        (notifier_buffer, notifier_driver)
    };};
}
}

Notice how the macro uses the type $A which is the type of the underlying alarm. We also use full paths to avoid errors when the macro is used. The macro then "returns" the two statically allocated resources.

Create a struct that holds all of the resources and configuration necessary for the capsules.

Now we create the actual component object which collects all of the resources and any configuration needed to successfully setup this capsule.

#![allow(unused)]
fn main() {
pub struct NotifierDriverComponent<A: 'static + time::Alarm<'static>> {
    board_kernel: &'static kernel::Kernel,
    driver_num: usize,
    alarm: &'static A,
    delay_ms: usize,
}
}

The component needs a reference to the board as well as the driver number to be used for this driver. This is to setup the grant, as we will see. If you are not setting up a syscall driver you will not need this. Finally we also need to keep track of the delay the kernel wants to use with this capsule.

Next we can create a constructor for this component object:

#![allow(unused)]
fn main() {
impl<A: 'static + time::Alarm<'static>> NotifierDriverComponent<A> {
    pub fn new(
        board_kernel: &'static kernel::Kernel,
        driver_num: usize,
        alarm: &'static A,
        delay_ms: usize,
    ) -> AlarmDriverComponent<A> {
        AlarmDriverComponent {
            board_kernel,
            driver_num,
            alarm,
            delay_ms,
        }
    }
}
}

Note, all configuration that is required must be passed in to this new() constructor.

Implement finalize() to initialize memory and perform setup.

The last step is to implement the Component trait and the finalize() method to actually setup the capsule.

The general format looks like:

#![allow(unused)]
fn main() {
impl<A: 'static + time::Alarm<'static>> Component for NotifierDriverComponent<A> {
    type StaticInput = (...);
    type Output = ...;

    fn finalize(self, static_buffer: Self::StaticInput) -> Self::Output {}
}
}

We need to define what statically allocated types we need, and what this method will produce:

#![allow(unused)]
fn main() {
impl<A: 'static + time::Alarm<'static>> Component for AlarmDriverComponent<A> {
    type StaticInput = (
        &'static mut MaybeUninit<[u8; 16]>,
        &'static mut MaybeUninit<NotifierDriver<'static, $A>>,
    );
    type Output = &'static NotifierDriver<'static, A>;

    fn finalize(self, static_buffer: Self::StaticInput) -> Self::Output {}
}
}

Notice that the static input types must match the output of the macro. The output type is what we are actually creating.

Inside the finalize() method we need to initialize the static memory and configure/setup the capsules:

#![allow(unused)]
fn main() {
impl<A: 'static + time::Alarm<'static>> Component for AlarmDriverComponent<A> {
    type StaticInput = (
        &'static mut MaybeUninit<[u8; 16]>,
        &'static mut MaybeUninit<NotifierDriver<'static, $A>>,
    );
    type Output = &'static NotifierDriver<'static, A>;

    fn finalize(self, static_buffer: Self::StaticInput) -> Self::Output {
    	 let grant_cap = create_capability!(capabilities::MemoryAllocationCapability);

    	 let buf = static_buffer.0.write([0; 16]);

    	 let notifier = static_buffer.1.write(NotifierDriver::new(
    	     self.alarm,
    	     self.board_kernel.create_grant(self.driver_num, &grant_cap),
    	     buf,
    	     self.delay_ms,
    	 ));

      // Very important we set the callback client correctly.
    	 self.alarm.set_client(notifier);

    	 notifier
    }
}
}

We initialize the memory for the static buffer, create the grant for the syscall driver to use, provide the driver with the alarm resource, and pass in the delay value to use. Lastly, we return a reference to the actual notifier driver object.

Summary

Our full component looks like:

#![allow(unused)]
fn main() {
use core::mem::MaybeUninit;

use capsules_extra::notifier::NotifierDriver;
use kernel::capabilities;
use kernel::component::Component;
use kernel::create_capability;
use kernel::hil::time::{self, Alarm};

#[macro_export]
macro_rules! notifier_driver_component_static {
    ($A:ty $(,)?) => {{
        let notifier_buffer = kernel::static_buf!([u8; 16]);
        let notifier_driver = kernel::static_buf!(
            capsules_extra::notifier::NotifierDriver<'static, $A>
        );

        (notifier_buffer, notifier_driver)
    };};
}

pub struct NotifierDriverComponent<A: 'static + time::Alarm<'static>> {
    board_kernel: &'static kernel::Kernel,
    driver_num: usize,
    alarm: &'static A,
    delay_ms: usize,
}

impl<A: 'static + time::Alarm<'static>> NotifierDriverComponent<A> {
    pub fn new(
        board_kernel: &'static kernel::Kernel,
        driver_num: usize,
        alarm: &'static A,
        delay_ms: usize,
    ) -> AlarmDriverComponent<A> {
        AlarmDriverComponent {
            board_kernel,
            driver_num,
            alarm,
            delay_ms,
        }
    }
}

impl<A: 'static + time::Alarm<'static>> Component for AlarmDriverComponent<A> {
    type StaticInput = (
        &'static mut MaybeUninit<[u8; 16]>,
        &'static mut MaybeUninit<NotifierDriver<'static, $A>>,
    );
    type Output = &'static NotifierDriver<'static, A>;

    fn finalize(self, static_buffer: Self::StaticInput) -> Self::Output {
		let grant_cap = create_capability!(capabilities::MemoryAllocationCapability);

		let buf = static_buffer.0.write([0; 16]);

		let notifier = static_buffer.1.write(NotifierDriver::new(
			self.alarm,
			self.board_kernel.create_grant(self.driver_num, &grant_cap),
			buf,
			self.delay_ms,
		));

		// Very important we set the callback client correctly.
		self.alarm.set_client(notifier);

		notifier
    }
}
}

Usage

In a board's main.rs file to use the component:

#![allow(unused)]
fn main() {
let notifier = components::notifier::NotifierDriverComponent::new(
    board_kernel,
    capsules_core::notifier::DRIVER_NUM,
    alarm,
    100,
)
.finalize(components::notifier_driver_component_static!(nrf52840::rtc::Rtc));
}

Wrap-Up

Congratulations! You have created a component to easily create a resource in the Tock kernel! We encourage you to submit a pull request to upstream this to the Tock repository.

The Tock Book