//! Minimal startup / runtime for RISC-V Fomu CPU
//!
//! # 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.
//!
//! # 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.
//!
//! ``` text
//! $ cargo new --bin app && cd $_
//!
//! $ # add this crate as a dependency
//! $ edit Cargo.toml && cat $_
//! [dependencies]
//! fomu-rt = "0.0.1"
//! panic-halt = "0.2.0"
//!
//! $ edit src/main.rs && cat $_
//! ```
//!
//! ``` ignore,no_run
//! #![no_std]
//! #![no_main]
//!
//! extern crate panic_halt;
//!
//! use fomu_rt::entry;
//!
//! // use `main` as the entry point of this application
//! // `main` is not allowed to return
//! #[entry]
//! fn main() -> ! {
//!     // do something here
//!     loop { }
//! }
//! ```
//!
//! ``` text
//! $ mkdir .cargo && edit .cargo/config && cat $_
//! [target.riscv32i-unknown-none-elf]
//! rustflags = [
//!   "-C", "link-arg=-Tlink.x",
//! ]
//!
//! [build]
//! target = "riscv32i-unknown-none-elf"
//! $
//! ```
//!
//! ``` text
//! $ 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.
//!
//! #### Example
//!
//! ``` no_run
//! 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:
//!
//! ``` no_run
//! #[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.

// NOTE: Adapted from cortex-m/src/lib.rs
#![no_std]
#![deny(missing_docs)]
#![deny(warnings)]

extern crate vexriscv;
extern crate riscv_rt_macros as macros;
extern crate r0;

pub use macros::{entry, pre_init};

use vexriscv::register::mstatus;

#[export_name = "error: fomu-rt appears more than once in the dependency graph"]
#[doc(hidden)]
pub static __ONCE__: () = ();

extern "C" {
    // Boundaries of the .bss section
    static mut _ebss: u32;
    static mut _sbss: u32;

    // Boundaries of the .data section
    static mut _edata: u32;
    static mut _sdata: u32;

    // Initial values of the .data section (stored in Flash)
    static _sidata: u32;
}


/// Rust entry point (_start_rust)
///
/// Zeros bss section, initializes data section and calls main. This function
/// never returns.
#[link_section = ".init.rust"]
#[export_name = "_start_rust"]
pub unsafe extern "C" fn start_rust() -> ! {
    extern "Rust" {
        // This symbol will be provided by the user via `#[entry]`
        fn main() -> !;

        // This symbol will be provided by the user via `#[pre_init]`
        fn __pre_init();

        fn _mp_hook() -> bool;
    }

    if _mp_hook() {
        __pre_init();

        r0::zero_bss(&mut _sbss, &mut _ebss);
        r0::init_data(&mut _sdata, &mut _edata, &_sidata);
    }

    main();
}


/// Trap entry point rust (_start_trap_rust)
///
/// mcause is read to determine the cause of the trap. XLEN-1 bit indicates
/// if it's an interrupt or an exception. The result is converted to an element
/// of the Interrupt or Exception enum and passed to handle_interrupt or
/// handle_exception.
#[link_section = ".trap.rust"]
#[export_name = "_start_trap_rust"]
pub extern "C" fn start_trap_rust() {
    extern "C" {
        fn trap_handler();
    }

    unsafe {
        // dispatch trap to handler
        trap_handler();

        // mstatus, remain in M-mode after mret
        mstatus::set_mpp(mstatus::MPP::Machine);
    }
}


#[doc(hidden)]
#[no_mangle]
pub fn default_trap_handler() {}

#[doc(hidden)]
#[no_mangle]
pub unsafe extern "Rust" fn default_pre_init() {}

#[doc(hidden)]
#[no_mangle]
pub extern "Rust" fn default_mp_hook() -> bool {
    use vexriscv::register::mhartid;
    match mhartid::read() {
        0 => true,
        _ => loop {
            unsafe { vexriscv::asm::wfi() }
        },
    }
}