[PATCH v6 04/14] rust: hrtimer: implement `HrTimerPointer` for `Arc`

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

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 31c461ddd2c377101ed6c1c7e009f7dccf458ebc..d0842c7c4c6ddffeef9a79cbf9727819060e4333 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 `HrTimer` size.
         let c_timer_ptr = unsafe { HrTimer::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..4c1812f190bc7a9e0b2fcd5ea2e320f45ba584bc
--- /dev/null
+++ b/rust/kernel/time/hrtimer/arc.rs
@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use super::HasHrTimer;
+use super::HrTimer;
+use super::HrTimerCallback;
+use super::HrTimerHandle;
+use super::HrTimerPointer;
+use super::RawHrTimerCallback;
+use crate::sync::Arc;
+use crate::sync::ArcBorrow;
+use crate::time::Ktime;
+
+/// A handle for an `Arc<HasHrTimer<U>>` returned by a call to
+/// [`HrTimerPointer::start`].
+pub struct ArcHrTimerHandle<U>
+where
+    U: HasHrTimer<U>,
+{
+    pub(crate) inner: Arc<U>,
+}
+
+// SAFETY: We implement drop below, and we cancel the timer in the drop
+// implementation.
+unsafe impl<U> HrTimerHandle for ArcHrTimerHandle<U>
+where
+    U: HasHrTimer<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 HasHrTimer<U>>::raw_get_timer(self_ptr) };
+
+        // SAFETY: As `timer_ptr` points into `U` and `U` is valid, `timer_ptr`
+        // must point to a valid `HrTimer` instance.
+        unsafe { HrTimer::<U>::raw_cancel(timer_ptr) }
+    }
+}
+
+impl<U> Drop for ArcHrTimerHandle<U>
+where
+    U: HasHrTimer<U>,
+{
+    fn drop(&mut self) {
+        self.cancel();
+    }
+}
+
+impl<U> HrTimerPointer for Arc<U>
+where
+    U: Send + Sync,
+    U: HasHrTimer<U>,
+    U: for<'a> HrTimerCallback<CallbackTarget<'a> = Self>,
+{
+    type TimerHandle = ArcHrTimerHandle<U>;
+
+    fn start(self, expires: Ktime) -> ArcHrTimerHandle<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) };
+
+        ArcHrTimerHandle { inner: self }
+    }
+}
+
+impl<U> RawHrTimerCallback for Arc<U>
+where
+    U: HasHrTimer<U>,
+    U: for<'a> HrTimerCallback<CallbackTarget<'a> = Self>,
+    U: for<'a> HrTimerCallback<CallbackTargetParameter<'a> = ArcBorrow<'a, U>>,
+{
+    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
+        // `HrTimer` is `repr(C)`
+        let timer_ptr = ptr.cast::<super::HrTimer<U>>();
+
+        // SAFETY: By C API contract `ptr` is the pointer we passed when
+        // queuing the timer, so it is a `HrTimer<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
+    }
+}

-- 
2.47.0