Support critical_section

Currently untested

Cargo.toml

@@ -21,3 +21,8 @@ exclude = [
 
 [dependencies]
 tock-registers = { version = "0.9.0", default-features = false } # Use it as interface-only library.
+critical-section = "1.1.2"
+
+[features]
+critical-section-single-core = ["critical-section/restore-state-bool"]
+

src/critical_section.rs

@@ -0,0 +1,24 @@
+use critical_section::{set_impl, Impl, RawRestoreState};
+
+use crate::interrupt;
+
+struct SingleCoreCriticalSection;
+set_impl!(SingleCoreCriticalSection);
+
+unsafe impl Impl for SingleCoreCriticalSection {
+    unsafe fn acquire() -> RawRestoreState {
+        let was_active = interrupt::enabled();
+        // NOTE: Fence guarantees are provided by interrupt::disable(), which performs a `compiler_fence(SeqCst)`.
+        interrupt::disable();
+        was_active
+    }
+
+    unsafe fn release(was_active: RawRestoreState) {
+        // Only re-enable interrupts if they were enabled before the critical section.
+        if was_active {
+            // NOTE: Fence guarantees are provided by interrupt::enable(), which performs a
+            // `compiler_fence(SeqCst)`.
+            interrupt::enable()
+        }
+    }
+}

src/interrupt.rs

@@ -0,0 +1,37 @@
+//! Interrupts
+
+#[cfg(target_arch = "aarch64")]
+use crate::registers::{Readable, Writeable, DAIF};
+#[cfg(target_arch = "aarch64")]
+use core::sync::atomic::{compiler_fence, Ordering};
+
+/// Disables all interrupts in the current core.
+#[cfg(target_arch = "aarch64")]
+#[inline]
+pub fn disable() {
+    compiler_fence(Ordering::SeqCst);
+    DAIF.write(DAIF::I::Masked + DAIF::F::Masked);
+    compiler_fence(Ordering::SeqCst);
+}
+
+/// Enables all the interrupts in the current core.
+///
+/// # Safety
+///
+/// - Do not call this function inside a critical section.
+#[cfg(target_arch = "aarch64")]
+#[inline]
+pub unsafe fn enable() {
+    compiler_fence(Ordering::SeqCst);
+    DAIF.write(DAIF::I::Unmasked + DAIF::F::Unmasked);
+    compiler_fence(Ordering::SeqCst);
+}
+
+#[cfg(target_arch = "aarch64")]
+#[inline]
+pub unsafe fn enabled() -> bool {
+    if DAIF.read(DAIF::I) == 0 || DAIF.read(DAIF::F) == 0 {
+        return true;
+    }
+    false
+}

src/lib.rs

@@ -56,4 +56,15 @@
 #![no_std]
 
 pub mod asm;
+pub mod interrupt;
 pub mod registers;
+
+#[cfg(all(target_arch = "aarch64", feature = "critical-section-single-core"))]
+mod critical_section;
+
+/// Used to reexport items for use in macros. Do not use directly.
+/// Not covered by semver guarantees.
+#[doc(hidden)]
+pub mod _export {
+    pub use critical_section;
+}