New in 2.0

Many core kernel APIs have been redesigned and rewritten.
- There are two new userspace system calls: AllowReadOnly and Exit. The old Allow system call has been renamed to AllowReadWrite. AllowReadOnly provides a mechanism for userspace to share a read-only buffer (e.g., constant data stored in flash) to the kernel. Exit allows a process to complete execution and request that the kernel either terminate or restart it.
- The system call ABI has been rewritten. System calls can now return up to 4 registers of values to userspace on return. The ABI defines the format and structure of the allowed return types. TRD104 documents the new ABI.
- The calling semantics and requirements for system calls is more clearly defined, especially with respect to calls to the Allow system calls and how buffers are managed. Furthermore, the lifetime of upcalls passed to the kernel with the subscribe system call has been defined. To enforce that the kernel doesn't maintain references to upcalls that it shouldn't (so userspace can reclaim any resources they require), upcalls are now managed by the core kernel. The changes to these calling semantics are documented in TRD104.
- Several types in the kernel have changed names, to better reflect their actual use and behavior.
  - AppSlice is now ReadOnlyProcessBuffer and ReadWriteProcessBuffer.
  - Callback is now Upcall (to distinguish upcalls from the kernel to userspace from general softare callbacks). Upcalls are now stored in a special block of memory in grant regions and are managed by the kernel rather than drivers. This allows the kernel to enforce their swapping semantics. #2639
  - Platform is now SyscallDriverLookup and Chip is now split into Chip for chip-specific operations and KernelResources for kernel operations.
  - Driver is now SyscallDriver.
The kernel namespace has been reorganized.
- tock#2659 reorganizes the kernel namespace. The actual abstractions and types exported were not changed, but their places in the namespace were.
- Almost everything is now exported as kernel::module::Type rather than kernel::Type.
- /common is split up into /utilities and /collections
There is increased chip and board support.
- RISC-V support has been extended to support progress and revisions to support microcontroller-type systems, including support for EPMP memory protection.
- There is support for ARM CortexM0+ and CortexM7.
- Board support adds:
  - Nano RP2040 Connect
  - Clue nRF52840
  - BBC Micro:bit v2
  - WeAct F401CCU6 Core Board
  - i.MX RT 1052 Evaluation Kit
  - Teensy 4.0
  - Pico Explorer Base
  - Rapsberry Pi Pico
  - LiteX on Digilent Arty A-7
  - Verilated LiteX simulation
  - ESP32-C3-DevKitM-1
Major HIL changes
- All HILs have changed significantly, to be in line with the new types within the kernel.
- ReturnCode has been removed from the kernel. HILs that used to return ReturnCode now return Result<(), ErrorCode>, so that Ok indicates a success result. #2508
- There is a draft of a TRD describing guidelines for HIL design, which enumerates 13 principles HIL traits should follow.
- The SPI, I2C, and CRC HILs have changed in how they handle buffers. SPI and I2C now correctly return buffers on error cases, and CRC now relies on LeasableBuffer to compute a CRC over a large block of memory.
- Digest has been extended to support multiple digest algorithms: in addition to HMAC SHA256 it now supports SHA224, SHA256, SHA384, SHA512, HMAC SHA384 and HMAC SHA512.
- The Time HIL has been updated to better support dyn references when needed, by adding a ConvertTicks trait. This change is documented in TRD 105 (which, when finalized, obsoletes 101).
- Blanket implementations for UART trait groups have been added. Now, if a structure implements both uart::Transmit and uart::Receive, it will automatically implement uart::UartData.
- New HILs added:
  - key/value store
  - 8080 bus (for LCDs)
  - text screen
  - screen
  - touch
In-kernel virtualizers for the following HILs have been added: AES, RNG, SHA
The kernel now checks whether loaded processes are compiled for the running kernel version. Because 2.0 changes the user/kernel ABI, processes compiled for Tock 1.x will not run correctly on a Tock 2.x kernel and vice versa. If the kernel detects that a process is compiled for the wrong kernel version it stops loading processes.
There have been changes to kernel internals and the build system to reduce code size. For example, kernel code that was highly replicated in monomorphized functions has been factored out (#2648).
All system call driver capsules that do not support use by multiple processes now use grant regions to store state and explicitly forbid access from multiple processes (e.g., #2518).
The process console has been improved and can now display memory maps for the kernel and processes.
Added tools/stack_analysis.sh and make stack-analysis for analyzing stack size.
Improvements to tools/print_tock_memory_usage.sh for displaying code size.
Transitioned uses of deprecated llvm_asm!() to asm!() macro for better compile-time checking (#2449, #2363).
Make it possible for boards to avoid using code space for peripherals they do not use (e.g., #2069).
Bug fixes.

New in 1.5

Major HIL Changes

None
Loading and Restarting Processes Improvements

Processes can now fault and be restarted by the kernel, and #1565 allows a board configuration file to specify the restart policy that the kernel should use.

Process discovery, parsing, and creation was also overhauled in #1480 to remove unsafe from the TBF header parsing code. This allows process::load_processes() to return errors if process loading fails. Boards now need to handle the Result return type.

New in 1.4

Major HIL Changes

Three HILs have been revised to better support embedded devices and clean up the interface for users of the HILs.
- #1211 revamps the UART interface to separate the transmit and receive paths.
- #1297 breaks the GPIO HIL into component subtraits so GPIO users can be specific about the features they need from GPIO pins.
- #1345 clearly defines the differences between counters, alarms, and timers.
Start on RISC-V Support

#1323, #1323, and #1345 add architecture support and boards to Tock for the RISC-V architecture.
Update Userland-Kernel Boundary Interface

#1318 updates the interface for switching to and returning from userspace to be less Cortex-M specific. The functions are more general and do not assume values are passed on the stack.

New in 1.2

Kernel debug module
- #1036, #1029, and #997 change debug::panic's signature. First, instead of taking a single LED, panic takes a slice of LEDs as its first argument. Second, the Rust now uses a PanicInfo struct to pass along information about where a panic occured, and debug::panic adopts the same structure. Third, architecture specific assembly code was removed from the kernel crate (including the debug module), requiring debug::panic to take in a particlar implementation of the nop instruction. Finally, debug::panic takes a reference to the process array (it is permissible to pass an empty array instead, but you won't get any information about process state on panic).
  
  Boards most likely call debug::panic from their panic_fmt function:
```
#[lang = "panic_fmt"]
pub unsafe extern "C" fn panic_fmt(args: Arguments, file: &'static str, line: u32) -> ! {
        let led = ...;
        let writer = ...;
        debug::panic(led, writer, args, file, line)
}
```
  should now be:
```
use core::panic::PanicInfo;
...
#[panic_implementation]
pub unsafe extern "C" fn panic_fmt(pi: &PanicInfo) -> ! {[lang = "panic_fmt"]
    let led = ...;
    let writer = ...;

    debug::panic(&mut [led], writer, pi, &cortexm4::support::nop, &PROCESSES)
```
- #1046 changes how the debug module in the kernel crate is structured. Instead of being a pseudo-process, debug is now treated more like a capsule, and needs a UART object to be passed to it. This means that main.rs needs to be updated to correctly set this up.
  
  First, if the debug UART bus is shared with console (or anything else), and this is likely the case, then a UART mux needs to be created. This is going to look slightly different depending on the underlying MCU, but for the SAM4L this looks like:
```
let uart_mux = static_init!(
    MuxUart<'static>,
    MuxUart::new(
        &sam4l::usart::USART0, // Choose the correct UART HW bus
        &mut capsules::virtual_uart::RX_BUF,
        115200
    )
);
hil::uart::UART::set_client(&sam4l::usart::USART0, uart_mux);
```
  With the mux created, a user of the UART bus must be defined. This will be passed to the debug module.
```
let debugger_uart = static_init!(UartDevice, UartDevice::new(uart_mux, false));
debugger_uart.setup();
```
  The following is the actual debug module, and must be created to use the debug!() macro. If debug is sharing a UART bus then the above mux and device is necessary, but if it is on a dedicated UART bus then that UART module can be passed in here instead.
```
let debugger = static_init!(
    kernel::debug::DebugWriter,
    kernel::debug::DebugWriter::new(
        debugger_uart, // Replace with just a HW UART if no sharing is needed.
        &mut kernel::debug::OUTPUT_BUF,
        &mut kernel::debug::INTERNAL_BUF,
    )
);
hil::uart::UART::set_client(debugger_uart, debugger);
```
  Finally, to get around Rust sharing rules, we need to create this wrapper:
```
let debug_wrapper = static_init!(
    kernel::debug::DebugWriterWrapper,
    kernel::debug::DebugWriterWrapper::new(debugger)
);
kernel::debug::set_debug_writer_wrapper(debug_wrapper);
```
Reorganization of the kernel crate: The kernel crate has been restructured to enable many improvements to Tock, and to move to a more consistent design between the kernel crate and other parts of Tock. This change has happened through several pull requests: #975, #1044, #1109, #1111, #1113, #1115, #1171, and #1191.

The primary motivation for this is making the kernel crate architecture agnostic, so that Tock can be ported non Cortex-M platforms (#985).

A part of this reorganization is the introduction of Capabilities, or a compile-time access control mechanism in Tock based on being able to forbid unsafe code. Capabilities restrict what code in Tock can call certain sensitive functions, like load_processes().
- The Chip in main.rs has to be instantiated with static_init! to ensure it has a long enough lifetime. Now:
```
let chip = static_init!(sam4l::chip::Sam4l, sam4l::chip::Sam4l::new());
```
- Capabilities need to be created. Creating a capability requires the ability to call unsafe, so capsules cannot create capabilities, and instead must be passed the capability if they need access to protected functions.
```
let process_management_capability =
    create_capability!(capabilities::ProcessManagementCapability);
let main_loop_capability = create_capability!(capabilities::MainLoopCapability);
```
- There is now a Kernel struct that needs to be instantiated by the board. Kernel has a method for the kernel's main loop, instead of a global function in the kernel's base module. Board configurations (i.e. each board's main.rs) as a result need to instantiate a statically allocate this new struct.
```
let board_kernel = static_init!(kernel::Kernel, kernel::Kernel::new(&PROCESSES));

board_kernel.kernel_loop(&hail, chip, Some(&hail.ipc), &main_loop_capability);
```
- load_processes takes the Kernel struct as an additional first argument, the chip as a new second argument, and the required capability as the last argument. Creating a Process (which load_processes() does) requires a reference to the chip because the process object needs to have access to architecture-specific context switching functions, as well as chip-specific MPU functions.
```
 kernel::procs::load_processes(
    board_kernel,
    chip,
    &_sapps as *const u8,
    &mut APP_MEMORY,
    &mut PROCESSES,
    FAULT_RESPONSE,
    &process_management_capability,
);
```
- Creating a grant requires a capability, as not just any code should be able to allocate memory in the grant regions.
```
let memory_allocation_cap = create_capability!(capabilities::MemoryAllocationCapability);
```
  To use:
```
board_kernel.create_grant(&memory_allocation_cap)
```
  Creating grants is now handled through the main Kernel struct so that it can check that no grants are created after processes are setup in memory, since grants require space allocated in process memory.

#1032 updates the ADC HIL to explicitly specify the resolution of the sample and to clarify that samples are always left-aligned in the u16 buffer. Previously, the only ADC implementation happened to be 12 bits and left-aligned, which callers only assumed. It also added a method to (if possible) report the reference voltage, which can be used to convert raw ADC samples to absolute voltages. Implementers of the ADC HIL must implement two new methods:

/// Function to ask the ADC how many bits of resolution are in the samples
/// it is returning.
fn get_resolution_bits(&self) -> usize;

/// Function to ask the ADC what reference voltage it used when taking the
/// samples. This allows the user of this interface to calculate an actual
/// voltage from the ADC reading.
///
/// The returned reference voltage is in millivolts, or `None` if unknown.
fn get_voltage_reference_mv(&self) -> Option<usize>;

UART HIL Refinements: This release saw several updates to the UART HIL, summarized in the UART HIL tracking issue.
- #1073 removes initialize from the UART HIL. Implementations will need to disentangle board-specific initialization code, such as enabling the peripheral or assigning pins, from UART configuration code, such as baud rate or parity. Initialization is no longer part of the UART HIL and should be performed by the top-level board before passing the UART object to any other code. UART configuration is now controlled by the new configure HIL method:
```
/// Configure UART
///
/// Returns SUCCESS, or
///
/// - EOFF: The underlying hardware is currently not available, perhaps
///         because it has not been initialized or in the case of a shared
///         hardware USART controller because it is set up for SPI.
/// - EINVAL: Impossible parameters (e.g. a `baud_rate` of 0)
/// - ENOSUPPORT: The underlying UART cannot satisfy this configuration.
fn configure(&self, params: UARTParameters) -> ReturnCode;
```
#1145 rewrites the HILs for random number generation. There are now two HILs, entropy and rng (random number generation). They differ in the guarantees they give about the bits they produce. The entropy traits guarantee high entropy bits: 1 bit of entropy per bit generated, such that every bit generated has an equal chance of being 0 or 1 and is independent of any other bit produced by the trait: that observing the stream of bits provides zero information on what the future bits will be. Entropy's guarantees make it suitable for use in security and cryptography. The rng traits provide bits that are assured to satisfy all standard NIST randomness tests, but do not promise that future bits cannot be guessed from past ones. E.g., the bits are random but not robust against an adversary.

It also adds library components for converting between different entropy sources as well as converting an entropy source into a random number generator (but not a random number generator into an entropy source!). Any software that needs entropy for security or cryptography should use an entropy trait and not an rng trait.

Updates to linker and toolchain: As of #993 and #1031, the Tock kernel no longer requires GCC for compilation, and entirely uses the LLVM toolchain.

Boards now need to explicitly define room for the kernel stack. Something like the following should be in the board's main.rs:

/// Dummy buffer that causes the linker to reserve enough space for the stack.
#[no_mangle]
#[link_section = ".stack_buffer"]
pub static mut STACK_MEMORY: [u8; 0x1000] = [0; 0x1000];

There are numerous changes to the shared board linker file. Individual boards need to be updated to not use variables, and instead define the entire MEMORY section:

/* Memory Spaces Definitions, 448K flash, 64K ram */
/* Bootloader is at address 0x00000000 */
MEMORY
{
  rom (rx)  : ORIGIN = 0x00010000, LENGTH = 0x00020000
  prog (rx) : ORIGIN = 0x00030000, LENGTH = 0x00040000
  ram (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00020000
}

MPU_MIN_ALIGN = 8K;

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