[−][src]Crate betrusted_rt

Minimal startup / runtime for RISC-V CPU's

Minimum Supported Rust Version (MSRV)

This crate is guaranteed to compile on stable Rust 1.31 and up. It might compile with older versions but that may change in any new patch release. Note that riscv64imac-unknown-none-elf and riscv64gc-unknown-none-elf targets are not supported on stable yet.

Features

This crate provides

  • Before main initialization of the .bss and .data sections.

  • #[entry] to declare the entry point of the program

  • #[pre_init] to run code before static variables are initialized

  • A linker script that encodes the memory layout of a generic RISC-V microcontroller. This linker script is missing some information that must be supplied through a memory.x file (see example below). This file must be supplied using rustflags and listed before link.x. Arbitrary filename can be use instead of memory.x.

  • A _sheap symbol at whose address you can locate a heap.

$ cargo new --bin app && cd $_

$ # add this crate as a dependency
$ edit Cargo.toml && cat $_
[dependencies]
riscv-rt = "0.6.1"
panic-halt = "0.2.0"

$ # memory layout of the device
$ edit memory.x && cat $_
MEMORY
{
  RAM : ORIGIN = 0x80000000, LENGTH = 16K
  FLASH : ORIGIN = 0x20000000, LENGTH = 16M
}

REGION_ALIAS("REGION_TEXT", FLASH);
REGION_ALIAS("REGION_RODATA", FLASH);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
REGION_ALIAS("REGION_HEAP", RAM);
REGION_ALIAS("REGION_STACK", RAM);

$ edit src/main.rs && cat $_
ⓘThis example is not tested
#![no_std]
#![no_main]

extern crate panic_halt;

use riscv_rt::entry;

// use `main` as the entry point of this application
// `main` is not allowed to return
#[entry]
fn main() -> ! {
    // do something here
    loop { }
}
$ mkdir .cargo && edit .cargo/config && cat $_
[target.riscv32imac-unknown-none-elf]
rustflags = [
  "-C", "link-arg=-Tmemory.x",
  "-C", "link-arg=-Tlink.x",
]

[build]
target = "riscv32imac-unknown-none-elf"
$ edit build.rs && cat $_
ⓘThis example is not tested
use std::env;
use std::fs::File;
use std::io::Write;
use std::path::Path;

/// Put the linker script somewhere the linker can find it.
fn main() {
    let out_dir = env::var("OUT_DIR").expect("No out dir");
    let dest_path = Path::new(&out_dir);
    let mut f = File::create(&dest_path.join("memory.x"))
        .expect("Could not create file");

    f.write_all(include_bytes!("memory.x"))
        .expect("Could not write file");

    println!("cargo:rustc-link-search={}", dest_path.display());

    println!("cargo:rerun-if-changed=memory.x");
    println!("cargo:rerun-if-changed=build.rs");
}
$ cargo build

$ riscv32-unknown-elf-objdump -Cd $(find target -name app) | head

Disassembly of section .text:

20000000 <_start>:
20000000:	800011b7          	lui	gp,0x80001
20000004:	80018193          	addi	gp,gp,-2048 # 80000800 <_stack_start+0xffffc800>
20000008:	80004137          	lui	sp,0x80004

Symbol interfaces

This crate makes heavy use of symbols, linker sections and linker scripts to provide most of its functionality. Below are described the main symbol interfaces.

memory.x

This file supplies the information about the device to the linker.

MEMORY

The main information that this file must provide is the memory layout of the device in the form of the MEMORY command. The command is documented here, but at a minimum you'll want to create at least one memory region.

To support different relocation models (RAM-only, FLASH+RAM) multiple regions are used:

  • REGION_TEXT - for .init, .trap and .text sections
  • REGION_RODATA - for .rodata section and storing initial values for .data section
  • REGION_DATA - for .data section
  • REGION_BSS - for .bss section
  • REGION_HEAP - for the heap area
  • REGION_STACK - for hart stacks

Specific aliases for these regions must be defined in memory.x file (see example below).

_stext

This symbol provides the loading address of .text section. This value can be changed to override the loading address of the firmware (for example, in case of bootloader present).

If omitted this symbol value will default to ORIGIN(REGION_TEXT).

_stack_start

This symbol provides the address at which the call stack will be allocated. The call stack grows downwards so this address is usually set to the highest valid RAM address plus one (this is an invalid address but the processor will decrement the stack pointer before using its value as an address).

In case of multiple harts present, this address defines the initial stack pointer for hart 0. Stack pointer for hart N is calculated as _stack_start - N * _hart_stack_size.

If omitted this symbol value will default to ORIGIN(REGION_STACK) + LENGTH(REGION_STACK).

Example

Allocating the call stack on a different RAM region.

MEMORY
{
  L2_LIM : ORIGIN = 0x08000000, LENGTH = 1M
  RAM : ORIGIN = 0x80000000, LENGTH = 16K
  FLASH : ORIGIN = 0x20000000, LENGTH = 16M
}

REGION_ALIAS("REGION_TEXT", FLASH);
REGION_ALIAS("REGION_RODATA", FLASH);
REGION_ALIAS("REGION_DATA", RAM);
REGION_ALIAS("REGION_BSS", RAM);
REGION_ALIAS("REGION_HEAP", RAM);
REGION_ALIAS("REGION_STACK", L2_LIM);

_stack_start = ORIGIN(L2_LIM) + LENGTH(L2_LIM);

_max_hart_id

This symbol defines the maximum hart id suppoted. All harts with id greater than _max_hart_id will be redirected to abort().

This symbol is supposed to be redefined in platform support crates for multi-core targets.

If omitted this symbol value will default to 0 (single core).

_hart_stack_size

This symbol defines stack area size for one hart.

If omitted this symbol value will default to 2K.

_heap_size

This symbol provides the size of a heap region. The default value is 0. You can set _heap_size to a non-zero value if you are planning to use heap allocations.

_sheap

This symbol is located in RAM right after the .bss and .data sections. You can use the address of this symbol as the start address of a heap region. This symbol is 4 byte aligned so that address will be a multiple of 4.

Example

extern crate some_allocator;

extern "C" {
    static _sheap: u8;
    static _heap_size: u8;
}

fn main() {
    unsafe {
        let heap_bottom = &_sheap as *const u8 as usize;
        let heap_size = &_heap_size as *const u8 as usize;
        some_allocator::initialize(heap_bottom, heap_size);
    }
}

_mp_hook

This function is called from all the harts and must return true only for one hart, which will perform memory initialization. For other harts it must return false and implement wake-up in platform-dependent way (e.g. after waiting for a user interrupt).

This function can be redefined in the following way:

#[export_name = "_mp_hook"]
pub extern "Rust" fn mp_hook() -> bool {
   // ...
}

Default implementation of this function wakes hart 0 and busy-loops all the other harts.

Functions

start_rust⚠

Rust entry point (_start_rust)

start_trap_rust

Trap entry point rust (_start_trap_rust)

Attribute Macros

entry

Attribute to declare the entry point of the program

pre_init

Attribute to mark which function will be called at the beginning of the reset handler.