mips-mcu-alloc 0.6.2

//! A heap allocator for microcontollers with MIPS core such as PIC32 controllers
//!
//! The heap is placed at a location determined by the linker and automatically extended
//! to fullfil allocation requests. Automatic heap extension fails if the heap would collide
//! with the stack.
//!
//! # Example
//!
//! ```ignore
//! #![feature(global_allocator)]
//! #![feature(alloc_error_handler)]
//!
//! // Plug in the allocator crate
//! extern crate alloc;
//!
//! use alloc::Vec;
//! use mips_mcu_alloc::MipsMcuHeap;
//!
//! #[global_allocator]
//! static ALLOCATOR: MipsMcuHeap = MipsMcuHeap::empty();
//!
//! #[entry]
//! fn main() -> ! {
//!     ALLOCATOR.init();
//!
//!     let mut xs = Vec::new();
//!     xs.push(1);
//!
//!     loop { /* .. */ }
//! }
//!
//! #[alloc_error_handler]
//! fn alloc_error(layout: core::alloc::Layout) -> ! {
//!     panic!("Cannot allocate heap memory: {:?}", layout);
//! }
//!
//! ```

#![no_std]
#![feature(asm_experimental_arch)]

use core::alloc::{GlobalAlloc, Layout};
use core::arch::asm;
use core::cell::RefCell;
use core::ptr::{self, NonNull};

use critical_section::{self, Mutex};
use linked_list_allocator::Heap;
use mips_rt::heap_start;

#[cfg(feature = "log")]
use log::{debug, trace};

/// Heap extension is performed stepwise. This constant defines the size of one extension step.
const EXTEND_INCREMENT: usize = 1024;

pub struct MipsMcuHeap {
    heap: Mutex<RefCell<Heap>>,
}

#[derive(Debug)]
#[non_exhaustive]
pub enum Error {
    InsufficientHeadroom,
}

impl MipsMcuHeap {
    /// Create a new UNINITIALIZED heap allocator.
    ///
    /// You must initialize this heap using the
    /// [`init`](struct.CortexMHeap.html#method.init) method before using the allocator.
    pub const fn empty() -> MipsMcuHeap {
        MipsMcuHeap {
            heap: Mutex::new(RefCell::new(Heap::empty())),
        }
    }

    /// Initialize heap with heap start location from linker and a defined initial size.
    pub fn init(&self) {
        let bottom = heap_start() as *mut u8;
        #[cfg(feature = "log")]
        debug!("heap init, bottom = {bottom:?}, size = {EXTEND_INCREMENT}");
        critical_section::with(|cs| {
            unsafe {
                self.heap
                    .borrow(cs)
                    .borrow_mut()
                    .init(bottom, EXTEND_INCREMENT)
            };
        });
    }

    /// Returns an estimate of the amount of bytes in use.
    pub fn used(&self) -> usize {
        critical_section::with(|cs| self.heap.borrow(cs).borrow_mut().used())
    }

    /// Returns the amount of bytes currently available.
    ///
    /// This method does not consider possible heap extensions
    pub fn free(&self) -> usize {
        critical_section::with(|cs| self.heap.borrow(cs).borrow_mut().free())
    }

    /// Returns the start (bottom) of the heap.
    pub fn bottom(&self) -> *mut u8 {
        critical_section::with(|cs| self.heap.borrow(cs).borrow_mut().bottom())
    }

    /// Returns the end (top) of the heap.
    pub fn top(&self) -> *mut u8 {
        critical_section::with(|cs| self.heap.borrow(cs).borrow_mut().top())
    }

    /// Returns the current distance to the bottom of the stack.
    ///
    /// This is an estimate amount of memory that could be added to the heap by
    /// automatic extension. An estimate of the total available free heap memory
    /// is the sum of `free()` and `headroom()`.
    pub fn headroom(&self) -> usize {
        let sp = stack_pointer() as usize;
        let top = self.top() as usize;
        if sp >= top {
            sp - top
        } else {
            0
        }
    }

    /// Try to extend the stack so that it has at least an amount of
    /// `free_bytes` available.
    ///
    /// Fails if there is not enough headroom. Does nothing if there are already
    /// enough free bytes.
    ///
    /// # Safety
    ///
    /// This method is considered unsafe because it increases the likelihood of
    /// heap/stack collisions.
    pub unsafe fn reserve(&self, free_bytes: usize) -> Result<(), Error> {
        if free_bytes <= self.free() {
            return Ok(());
        }
        let additional_bytes = free_bytes - self.free();
        critical_section::with(|cs| {
            let mut heap = self.heap.borrow_ref_mut(cs);
            let new_top: *mut u8 = heap.top().add(additional_bytes);
            if new_top >= stack_pointer() {
                return Err(Error::InsufficientHeadroom);
            }
            heap.extend(additional_bytes);
            Ok(())
        })
    }
}

unsafe impl GlobalAlloc for MipsMcuHeap {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        // try to allocate and successively extend by EXTEND_INCREMENT until memory is exhausted
        loop {
            if let Ok(p) = critical_section::with(|cs| {
                self.heap.borrow(cs).borrow_mut().allocate_first_fit(layout)
            }) {
                #[cfg(feature = "log")]
                trace!("alloc at {:?}, {:?}, stack_pointer = {:?}", p.as_ptr(), layout, stack_pointer());
                break p.as_ptr();
            } else {
                // this must be a u8 pointer
                let new_top: *mut u8 =
                    critical_section::with(|cs| self.heap.borrow(cs).borrow_mut().top())
                        .add(EXTEND_INCREMENT);
                // avoid collision with stack
                if new_top < stack_pointer() {
                    critical_section::with(|cs| {
                        self.heap.borrow(cs).borrow_mut().extend(EXTEND_INCREMENT)
                    });
                    #[cfg(feature = "log")]
                    debug!("extended heap, top = {:?}, stack_pointer = {:?}", self.top(), stack_pointer());
                } else {
                    break ptr::null_mut();
                }
            }
        }
    }

    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        #[cfg(feature = "log")]
        trace!("dealloc at {ptr:?}, {layout:?}");
        critical_section::with(|cs| {
            self.heap
                .borrow(cs)
                .borrow_mut()
                .deallocate(NonNull::new_unchecked(ptr), layout)
        });
    }
}

fn stack_pointer() -> *mut u8 {
    let sp: *mut u8;
    unsafe {
        asm!(".set noat",
             "move {0}, $29", out(reg) sp);
    }
    sp
}