initial version of checked resources

src/checked.rs

@@ -0,0 +1,77 @@
+//! Safe, run-time checked resources
+
+use core::marker::PhantomData;
+use core::cell::UnsafeCell;
+
+use cortex_m::interrupt;
+use cortex_m::register::{basepri, basepri_max};
+
+use Ceiling;
+
+unsafe fn acquire(locked: &UnsafeCell<bool>, ceiling: u8) -> u8 {
+    interrupt::free(
+        |_| {
+            assert!(!*locked.get(), "resource already locked");
+            let old_basepri = basepri::read();
+            basepri_max::write(ceiling);
+            *locked.get() = true;
+            old_basepri
+        },
+    )
+}
+
+unsafe fn release(locked: &UnsafeCell<bool>, old_basepri: u8) {
+    interrupt::free(
+        |_| {
+            basepri::write(old_basepri);
+            *locked.get() = false;
+        },
+    );
+}
+
+/// A totally safe `Resource` that panics on misuse
+pub struct Resource<T, C> {
+    _marker: PhantomData<C>,
+    data: UnsafeCell<T>,
+    locked: UnsafeCell<bool>,
+}
+
+impl<T, C> Resource<T, C>
+where
+    C: Ceiling,
+{
+    /// Creates a new `Resource` with ceiling `C`
+    pub const fn new(data: T) -> Resource<T, C> {
+        Resource {
+            _marker: PhantomData,
+            data: UnsafeCell::new(data),
+            locked: UnsafeCell::new(false),
+        }
+    }
+
+    /// Locks the resource, blocking tasks with priority equal or smaller than
+    /// the ceiling `C`
+    pub fn lock<F>(&'static self, f: F)
+    where
+        F: FnOnce(&T),
+    {
+        unsafe {
+            let old_basepri = acquire(&self.locked, C::ceiling());
+            f(&*self.data.get());
+            release(&self.locked, old_basepri);
+        }
+    }
+
+    /// Mutably locks the resource, blocking tasks with priority equal or
+    /// smaller than the ceiling `C`
+    pub fn lock_mut<F>(&'static self, f: F)
+        where
+        F: FnOnce(&mut T),
+    {
+        unsafe {
+            let old_basepri = acquire(&self.locked, C::ceiling());
+            f(&mut *self.data.get());
+            release(&self.locked, old_basepri);
+        }
+    }
+}

src/lib.rs

@@ -10,6 +10,8 @@
 
 extern crate cortex_m;
 
+pub mod checked;
+
 use cortex_m::ctxt::Context;
 use cortex_m::interrupt::{CriticalSection, Nr};
 use cortex_m::peripheral::{Peripheral, NVIC};