Kernel Boot and Setup

The goal of this module is to make you comfortable with the Tock kernel, how it is structured, how the kernel is setup at boot, and how capsules provide additional kernel functionality.

During this you will:

Learn how Tock uses Rust's memory safety to provide isolation for free
Read the Tock boot sequence, seeing how Tock uses static allocation
Learn about Tock's event-driven programming

The Tock Boot Sequence

The very first thing that runs on a Tock board is an assembly function called initialize_ram_jump_to_main(). Rust requires that memory is configured before any Rust code executes, so this must run first. As the function name implies, control is then transferred to the main() function in the board's main.rs file. Tock intentionally tries to give the board as much control over the operation of the system as possible, hence why there is very little between reset and the board's main function being called.

Open the main.rs file for your board in your favorite editor. This file defines the board's platform: how it boots, what capsules it uses, and what system calls it supports for userland applications.

How is everything organized?

Find the declaration of "platform" struct. This is typically called struct Platform or named based on the name of the board (it's pretty early in the file). This declares the structure representing the platform. It has many fields, many of which are capsules that make up the board's platform. These fields are resources that the board needs to maintain a reference to for future use, for example for handling system calls or implementing kernel policies.

Recall that everything in the kernel is statically allocated. We can see that here. Every field in the platform struct is a reference to an object with a static lifetime.

Many capsules themselves take a lifetime as a parameter, which is currently always 'static.

The boot process is primarily the construction of this platform structure. Once everything is set up, the board will pass the constructed platform object to kernel::kernel_loop and we're off to the races.

How do things get started?

After RAM initialization, the reset handler invokes the main() function in the board main.rs file. main() is typically rather long as it must setup and configure all of the drivers and capsules the board needs. Many capsules depend on other, lower layer abstractions that need to be created and initialized as well.

Take a look at the first few lines of main(). The boot sequence generally sets up any low-level microcontroller configuration, initializes the MCU peripherals, and sets up debugging capabilities.

How do capsules get created?

The bulk of main() create and initializes capsules which provide the main functionality of the Tock system. For example, to provide userspace applications with ability to display serial data, boards typically create a console capsule. An example of this looks like:


pub unsafe fn main() {
    ...

    // Create a virtualizer on top of an underlying UART device. Use 115200 as
    // the baud rate.
    let uart_mux = components::console::UartMuxComponent::new(channel, 115200)
        .finalize(components::uart_mux_component_static!());

    // Instantiate the console capsule. This uses the virtualized UART provided
    // by the uart_mux.
    let console = components::console::ConsoleComponent::new(
        board_kernel,
        capsules_core::console::DRIVER_NUM,
        uart_mux,
    )
    .finalize(components::console_component_static!());

    ...
}

Eventually, once all of the capsules have been created, we will populate the platform structure with them:

pub unsafe fn main() {
    ...

    let platform = Platform {
        console: console,
        gpio: gpio,
        ...
    }

}

What Are Components?

When setting up the capsules (such as console), we used objects in the components crate to help. In Tock, components are helper objects that make it easier to correctly create and initialize capsules.

For example, if we look under the hood of the console component, the main initialization of console looks like:

#![allow(unused)]
fn main() {
impl Component for ConsoleComponent {
    fn finalize(self, s: Self::StaticInput) -> Console {
        let grant_cap = create_capability!(capabilities::MemoryAllocationCapability);

        let write_buffer = static_init!([u8; DEFAULT_BUF_SIZE], [0; DEFAULT_BUF_SIZE]);
        let read_buffer = static_init!([u8; DEFAULT_BUF_SIZE], [0; DEFAULT_BUF_SIZE]);

        let console_uart = static_init!(
            UartDevice,
            UartDevice::new(self.uart_mux, true)
        );
        // Don't forget to call setup() to register our new UartDevice with the
        // mux!
        console_uart.setup();

        let console = static_init!(
            Console<'static>,
            console::Console::new(
                console_uart,
                write_buffer,
                read_buffer,
                self.board_kernel.create_grant(self.driver_num, &grant_cap),
            )
        );
        // Very easy to figure to set the client reference for callbacks!
        hil::uart::Transmit::set_transmit_client(console_uart, console);
        hil::uart::Receive::set_receive_client(console_uart, console);

        console
    }
}
}

Much of the code within components is boilerplate that is copied for each board and easy to subtlety miss an important setup step. Components encapsulate the setup complexity and can be reused on each board Tock supports.

The static_init! macro is simply an easy way to allocate a static variable with a call to new. The first parameter is the type, the second is the expression to produce an instance of the type.

Components end up looking somewhat complex because they can be re-used across multiple boards and different microcontrollers. More detail here.

A brief aside on buffers:

Notice that the console needs both a read and write buffer for it to use. These buffers have to have a 'static lifetime. This is because low-level hardware drivers, especially those that use DMA, require 'static buffers. Since we don't know exactly when the underlying operation will complete, and we must promise that the buffer outlives the operation, we use the one lifetime that is assured to be alive at the end of an operation: 'static. Other code with buffers without a 'static lifetime, such as userspace processes, use capsules like Console by copying data into internal 'static buffers before passing them to the console. The buffer passing architecture looks like this:

Let's Make a Tock Board!

The code continues on, creating all of the other capsules that are needed by the platform. Towards the end of main(), we've created all of the capsules we need, and it's time to create the actual platform structure (let platform = Platform {...}).

Boards must implement two traits to successfully run the Tock kernel: SyscallDriverLookup and KernelResources.

`SyscallDriverLookup`

The first, SyscallDriverLookup, is how the kernel maps system calls from userspace to the correct capsule within the kernel. The trait requires one function:

#![allow(unused)]
fn main() {
trait SyscallDriverLookup {
    /// Mapping of syscall numbers to capsules.
    fn with_driver<F, R>(&self, driver_num: usize, f: F) -> R
    where
        F: FnOnce(Option<&dyn SyscallDriver>) -> R;
}
}

The with_driver() function executes the provided function f() by passing it the correct capsule based on the provided driver_num. A brief example of an implementation of SyscallDriverLookup looks like:

#![allow(unused)]
fn main() {
impl SyscallDriverLookup for Platform {
    fn with_driver<F, R>(&self, driver_num: usize, f: F) -> R
    where
        F: FnOnce(Option<&dyn kernel::syscall::SyscallDriver>) -> R,
    {
        match driver_num {
            capsules_core::console::DRIVER_NUM => f(Some(self.console)),
            capsules_core::gpio::DRIVER_NUM => f(Some(self.gpio)),
            ...
            _ => f(None),
        }
    }
}
}

Why require each board to provide this mapping? Why not implement this mapping centrally in the kernel? Tock requires boards to implement this mapping as we consider the assignment of driver numbers to specific capsules as a platform-specific decisions. While Tock does have a default mapping of driver numbers, boards are not obligated to use them. This flexibility allows boards to expose multiple copies of the same capsule to userspace, for example.

`KernelResources`

The KernelResources trait is the main method for configuring the operation of the core Tock kernel. Policies such as the syscall mapping described above, syscall filtering, and watchdog timers are configured through this trait. More information is contained in a separate course module.

Loading processes

Once the platform is all set up, the board is responsible for loading processes into memory:

pub unsafe fn main() {
    ...

    kernel::process::load_processes(
        board_kernel,
        chip,
        core::slice::from_raw_parts(
            &_sapps as *const u8,
            &_eapps as *const u8 as usize - &_sapps as *const u8 as usize,
        ),
        core::slice::from_raw_parts_mut(
            &mut _sappmem as *mut u8,
            &_eappmem as *const u8 as usize - &_sappmem as *const u8 as usize,
        ),
        &mut PROCESSES,
        &FAULT_RESPONSE,
        &process_management_capability,
    )
    .unwrap_or_else(|err| {
        debug!("Error loading processes!");
        debug!("{:?}", err);
    });

    ...
}

A Tock process is represented by a kernel::Process struct. In principle, a platform could load processes by any means. In practice, all existing platforms write an array of Tock Binary Format (TBF) entries to flash. The kernel provides the load_processes helper function that takes in a flash address and begins iteratively parsing TBF entries and making Processes.

A brief aside on capabilities:

To call load_processes(), the board had to provide a reference to a &process_management_capability. The load_processes() function internally requires significant direct access to memory, and it should only be called in very specific places. To prevent its misuse (for example from within a capsule), calling it requires a capability to be passed in with the arguments. To create a capability, the calling code must be able to call unsafe, Code (i.e. capsules) which cannot use unsafe therefore has no way to create a capability and therefore cannot call the restricted function.

Starting the kernel

Finally, the board passes a reference to the current platform, the chip the platform is built on (used for interrupt and power handling), and optionally an IPC capsule to start the main kernel loop:

#![allow(unused)]
fn main() {
board_kernel.kernel_loop(&platform, chip, Some(&platform.ipc), &main_loop_capability);

}

From here, Tock is initialized, the kernel event loop takes over, and the system enters steady state operation.

The Tock Book