Crate kernel

Core Tock Kernel

The kernel crate implements the core features of Tock as well as shared code that many chips, capsules, and boards use. It also holds the Hardware Interface Layer (HIL) definitions.

Most unsafe code is in this kernel crate.

Core Kernel Visibility

As the root crate in the Tock operating system, this crate serves multiple purposes:

1. It includes the logic for the core kernel, including process management, grants, scheduling, etc.

2. It includes important interfaces for hardware and other device abstractions. These are generally in the HIL and platform folders.

3. It includes utility functions used elsewhere in the kernel, generally by multiple different crates such that it makes sense to have shared implementations in the core kernel crate.

Because of these different features of the core kernel, managing visibility of the various objects and functions is a bit tricky. In general, the kernel crate only exposes what it absolutely needs to. However, there are three cases where resources in this crate must be exposed.

1. The shared utility functions and structs must be exposed. These are marked pub and are used by many other kernel crates.

   Some utility objects and abstractions, however, expose memory unsafe behavior. These are marked as unsafe, and require an unsafe block to use them. One example of this is StaticRef which is used for accessing memory-mapped I/O registers. Since accessing the addresses through just a memory address is potentially very unsafe, instantiating a StaticRef requires an unsafe block.

2. The core kernel types generally have to be exposed as other layers of the OS need to use them. However, generally only a very small interface is exposed, and using that interface cannot compromise the overall system or the core kernel. These functions are also marked pub. For example, the AppSlice abstraction must be exposed to capsules to use shared memory between a process and the kernel. However, the constructor is not public, and the API exposed to capsules is very limited and confined by the Rust type system. The constructor and other sensitive interfaces are restricted to use only inside the kernel crate and are marked pub(crate).

   In some cases, more sensitive core kernel interfaces must be exposed. For example, the kernel exposes a function for starting the main scheduling loop in the kernel. Since board crates must be able to start this loop after all initialization is finished, the kernel loop function must be exposed and marked pub. However, this interface is not generally safe to use, since starting the loop a second time would compromise the stability of the overall system. It's also not necessarily memory unsafe to call the start loop function again, so we do not mark it as unsafe. Instead, we require that the caller hold a Capability to call the public but sensitive functions. More information is in capabilities.rs. This allows the kernel crate to still expose functions as public while restricting their use. Another example of this is the Grant constructor, which must be called outside of the core kernel, but should not be called except during the board setup.

3. Certain internal core kernel interfaces must also be exposed. These are needed for extensions of the core kernel that happen to be implemented in crates outside of the kernel crate. For example, additional implementations of ProcessType may live outside of the kernel crate. To successfully implement a new ProcessType requires access to certain in-core-kernel APIs, and these must be marked pub so that outside crates can access them.

   These interfaces are highly sensitive, so again we require the caller hold a Capability to call them. This helps restrict their use and makes it very clear that calling them requires special permissions. Additionally, to differentiate these interfaces, which are for external extensions of core kernel functionality, from the other public but sensitive interfaces (item 2 above), we append the name _external to the function name.

   One note is that there are currently very few extensions to the core kernel that live outside of the kernel crate. That means we have not necessarily created _extern functions for all the interfaces needed for this use case. It is likely we will have to create new interfaces as new use cases are discovered.

Modules

capabilities	Special restricted capabilities.
common	Common operations and types in Tock.
component	Components extend the functionality of the Tock kernel through a simple factory method interface.
debug	Support for in-kernel debugging.
hil	Public traits for interfaces between Tock components.
introspection	Mechanism for inspecting the status of the kernel.
ipc	Inter-process communication mechanism for Tock.
mpu	Interface for configuring the Memory Protection Unit.
procs	Publicly available process-related objects.
syscall	Tock syscall number definitions and arch-agnostic interface trait.
watchdog	Interface for configuring a watchdog

Macros

count_expressions	Count the number of passed expressions. Useful for constructing variable sized arrays in other macros. Taken from the Little Book of Rust Macros
create_capability	Create an object with the given capability.
debug	In-kernel println() debugging.
debug_enqueue	This macro prints a new line to an internal ring buffer, the contents of which are only flushed with debug_flush_queue! and in the panic handler.
debug_flush_queue	This macro flushes the contents of the debug queue into the regular debug output.
debug_gpio	In-kernel gpio debugging, accepts any GPIO HIL method
debug_verbose	In-kernel println() debugging with filename and line numbers.
static_buf	Allocates a statically-sized global array of memory for data structures but does not initialize the memory.
static_init	Allocates a statically-sized global array of memory and initializes the memory for a particular data structure.
static_init_half	This macro is deprecated. You should migrate to using static_buf! followed by a call to StaticUninitializedBuffer::initialize().
storage_volume	Allocates space in the kernel image for on-chip non-volatile storage.

Structs

AppId	Userspace app identifier.
AppSlice	Buffer of memory shared from an app to the kernel.
Callback	Type for calling a callback in a process.
CoopProcessNode	A node in the linked list the scheduler uses to track processes
CooperativeSched	Cooperative Scheduler
Grant	Region of process memory reserved for the kernel.
Kernel	Main object for the kernel. Each board will need to create one.
MLFQProcessNode	Nodes store per-process state
MLFQSched
NoClockControl	Helper struct for interfaces that expect clocks, but have no clock control.
PrioritySched	Priority scheduler based on the order of processes in the PROCESSES array.
Private	Type for specifying an AppSlice is hidden from the kernel.
RoundRobinProcessNode	A node in the linked list the scheduler uses to track processes Each node holds a pointer to a slot in the processes array
RoundRobinSched	Round Robin Scheduler
Shared	Type for specifying an AppSlice is shared with the kernel.
VirtualSchedulerTimer	Implementation of SchedulerTimer trait on top of a virtual alarm.

Enums

ReturnCode

Standard return errors in Tock.

Statics

NO_CLOCK_CONTROL

Instance of NoClockControl for things that need references to ClockInterface objects.

Traits

Chip	Interface for individual MCUs.
ClockInterface	Generic operations that clock-like things are expected to support.
Driver	Drivers implement the three driver-specific system calls: subscribe, command and allow.
Platform	Interface for individual boards.
Scheduler	Trait which any scheduler must implement.
SchedulerTimer	Interface for the system scheduler timer.