[PATCH v3 09/13] rust: hrtimer: add `hrtimer::ScopedTimerPointer`

[Andreas Hindborg <a.hindborg@kernel.org>]

Add the trait `ScopedTimerPointer` to allow safe use of stack allocated
timers. Safety is achieved by pinning the stack in place while timers are
running.

Implement the trait for all types that implement `UnsafeTimerPointer`.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/hrtimer.rs | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/rust/kernel/hrtimer.rs b/rust/kernel/hrtimer.rs
index 940390aa2a6c6a222534b545d8d2fbd639a19f64..d8cea8b15a9afc134b10dc5703d88d70323fc943 100644
--- a/rust/kernel/hrtimer.rs
+++ b/rust/kernel/hrtimer.rs
@@ -184,6 +184,39 @@ pub unsafe trait UnsafeTimerPointer: Sync + Sized {
     unsafe fn start(self, expires: Ktime) -> Self::TimerHandle;
 }
 
+/// A trait for stack allocated timers.
+///
+/// # Safety
+///
+/// Implementers must ensure that `start_scoped` does not return until the
+/// timer is dead and the timer handler is not running.
+pub unsafe trait ScopedTimerPointer {
+    /// Start the timer to run after `expires` time units and immediately
+    /// after call `f`. When `f` returns, the timer is cancelled.
+    fn start_scoped<T, F>(self, expires: Ktime, f: F) -> T
+    where
+        F: FnOnce() -> T;
+}
+
+// SAFETY: By the safety requirement of `UnsafeTimerPointer`, dropping the
+// handle returned by [`UnsafeTimerPointer::start`] ensures that the timer is
+// killed.
+unsafe impl<U> ScopedTimerPointer for U
+where
+    U: UnsafeTimerPointer,
+{
+    fn start_scoped<T, F>(self, expires: Ktime, f: F) -> T
+    where
+        F: FnOnce() -> T,
+    {
+        // SAFETY: We drop the timer handle below before returning.
+        let handle = unsafe { UnsafeTimerPointer::start(self, expires) };
+        let t = f();
+        drop(handle);
+        t
+    }
+}
+
 /// Implemented by [`TimerPointer`] implementers to give the C timer callback a
 /// function to call.
 // This is split from `TimerPointer` to make it easier to specify trait bounds.

-- 
2.46.0