This patch allows the use of intrusive `hrtimer` fields in structs that are
managed by an `Arc`.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/time/hrtimer.rs     |  3 +-
 rust/kernel/time/hrtimer/arc.rs | 89 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 91 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index 646cd5bfa6623aec62294f9115fbf6fedc0a426c..e76b864f3435667f56ed607fd3bf029317eb1332 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -98,7 +98,6 @@ unsafe fn raw_get(ptr: *const Self) -> *mut bindings::hrtimer {
     /// # Safety
     ///
     /// `self_ptr` must point to a valid `Self`.
-    #[allow(dead_code)]
     pub(crate) unsafe fn raw_cancel(self_ptr: *const Self) -> bool {
         // SAFETY: timer_ptr points to an allocation of at least `Timer` size.
         let c_timer_ptr = unsafe { Timer::raw_get(self_ptr) };
@@ -294,3 +293,5 @@ unsafe fn raw_get_timer(ptr: *const Self) ->
         }
     }
 }
+
+mod arc;
diff --git a/rust/kernel/time/hrtimer/arc.rs b/rust/kernel/time/hrtimer/arc.rs
new file mode 100644
index 0000000000000000000000000000000000000000..3b256292bc86160a495f7b192b3eda92f2dfcb31
--- /dev/null
+++ b/rust/kernel/time/hrtimer/arc.rs
@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use super::HasTimer;
+use super::RawTimerCallback;
+use super::Timer;
+use super::TimerCallback;
+use super::TimerHandle;
+use super::TimerPointer;
+use crate::sync::Arc;
+use crate::sync::ArcBorrow;
+use crate::time::Ktime;
+
+/// A handle for an `Arc<HasTimer<U>>` returned by a call to
+/// [`TimerPointer::start`].
+pub struct ArcTimerHandle<U>
+where
+    U: HasTimer<U>,
+{
+    pub(crate) inner: Arc<U>,
+}
+
+// SAFETY: We implement drop below, and we cancel the timer in the drop
+// implementation.
+unsafe impl<U> TimerHandle for ArcTimerHandle<U>
+where
+    U: HasTimer<U>,
+{
+    fn cancel(&mut self) -> bool {
+        let self_ptr = Arc::as_ptr(&self.inner);
+
+        // SAFETY: As we obtained `self_ptr` from a valid reference above, it
+        // must point to a valid `U`.
+        let timer_ptr = unsafe { <U as HasTimer<U>>::raw_get_timer(self_ptr) };
+
+        // SAFETY: As `timer_ptr` points into `U` and `U` is valid, `timer_ptr`
+        // must point to a valid `Timer` instance.
+        unsafe { Timer::<U>::raw_cancel(timer_ptr) }
+    }
+}
+
+impl<U> Drop for ArcTimerHandle<U>
+where
+    U: HasTimer<U>,
+{
+    fn drop(&mut self) {
+        self.cancel();
+    }
+}
+
+impl<U> TimerPointer for Arc<U>
+where
+    U: Send + Sync,
+    U: HasTimer<U>,
+    U: for<'a> TimerCallback<CallbackTarget<'a> = Self>,
+{
+    type TimerHandle = ArcTimerHandle<U>;
+
+    fn start(self, expires: Ktime) -> ArcTimerHandle<U> {
+        // SAFETY: Since we generate the pointer passed to `start` from a
+        // valid reference, it is a valid pointer.
+        unsafe { U::start(Arc::as_ptr(&self), expires) };
+
+        ArcTimerHandle { inner: self }
+    }
+}
+
+impl<U> RawTimerCallback for Arc<U>
+where
+    U: HasTimer<U>,
+    U: for<'a> TimerCallback<CallbackTarget<'a> = Self>,
+    U: for<'a> TimerCallback<CallbackTargetParameter<'a> = ArcBorrow<'a, U>>,
+{
+    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
+        // `Timer` is `repr(C)`
+        let timer_ptr = ptr.cast::<super::Timer<U>>();
+
+        // SAFETY: By C API contract `ptr` is the pointer we passed when
+        // queuing the timer, so it is a `Timer<T>` embedded in a `T`.
+        let data_ptr = unsafe { U::timer_container_of(timer_ptr) };
+
+        // SAFETY: `data_ptr` points to the `U` that was used to queue the
+        // timer. This `U` is contained in an `Arc`.
+        let receiver = unsafe { ArcBorrow::from_raw(data_ptr) };
+
+        U::run(receiver);
+
+        bindings::hrtimer_restart_HRTIMER_NORESTART
+    }
+}

-- 
2.46.0