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//! 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 vcell::VolatileCell;
use Ceiling;
unsafe fn acquire(locked: &VolatileCell<bool>, ceiling: u8) -> u8 {
interrupt::free(
|_| {
assert!(!locked.get(), "resource already locked");
let old_basepri = basepri::read();
basepri_max::write(ceiling);
locked.set(true);
old_basepri
},
)
}
unsafe fn release(locked: &VolatileCell<bool>, old_basepri: u8) {
// XXX Is it really memory safe to *not* use a global critical section here
// interrupt::free(
// |_| {
locked.set(false);
basepri::write(old_basepri);
// },
// );
}
/// A totally safe `Resource` that panics on misuse
pub struct Resource<T, C> {
_marker: PhantomData<C>,
data: UnsafeCell<T>,
locked: VolatileCell<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: VolatileCell::new(false),
}
}
/// Locks the resource, blocking tasks with priority equal or smaller than
/// the ceiling `C`
pub fn lock<R, F>(&'static self, f: F) -> R
where
F: FnOnce(&T) -> R,
{
unsafe {
let old_basepri = acquire(&self.locked, C::hw_ceiling());
::compiler_barrier();
let ret = f(&*self.data.get());
::compiler_barrier();
release(&self.locked, old_basepri);
ret
}
}
/// Mutably locks the resource, blocking tasks with priority equal or
/// smaller than the ceiling `C`
pub fn lock_mut<R, F>(&'static self, f: F) -> R
where
F: FnOnce(&mut T) -> R,
{
unsafe {
let old_basepri = acquire(&self.locked, C::hw_ceiling());
::compiler_barrier();
let ret = f(&mut *self.data.get());
::compiler_barrier();
release(&self.locked, old_basepri);
ret
}
}
}
unsafe impl<T, C> Sync for Resource<T, C> {}
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