libcortex_a9: implement OnceLock

libcortex_a9/src/lib.rs

@@ -9,6 +9,7 @@ mod fpu;
 pub mod l2c;
 pub mod mmu;
 pub mod mutex;
+pub mod once_lock;
 pub mod regs;
 pub mod semaphore;
 pub mod sync_channel;

libcortex_a9/src/once_lock.rs

@@ -0,0 +1,36 @@
+use core::{cell::UnsafeCell,
+           sync::atomic::{AtomicBool, Ordering}};
+
+use crate::asm::{enter_critical, exit_critical};
+
+pub struct OnceLock<T> {
+    once: AtomicBool,
+    data: UnsafeCell<Option<T>>,
+}
+
+impl<T> OnceLock<T> {
+    pub const fn new() -> Self {
+        Self {
+            once: AtomicBool::new(false),
+            data: UnsafeCell::new(None),
+        }
+    }
+
+    pub fn set(&self, data: T) -> Result<(), ()> {
+        let irq = unsafe { enter_critical() };
+        let res = self
+            .once
+            .compare_exchange(false, true, Ordering::Relaxed, Ordering::Relaxed)
+            .map_err(|_| ())
+            .map(|_| unsafe { *self.data.get() = Some(data) });
+        unsafe { exit_critical(irq) };
+        res
+    }
+
+    pub fn get(&self) -> Option<&T> {
+        unsafe { &*self.data.get() }.as_ref()
+    }
+}
+
+unsafe impl<T> Sync for OnceLock<T> {}
+unsafe impl<T> Send for OnceLock<T> {}