rust: hrtimer: implement UnsafeHrTimerPointer for Pin<&T>

Allow pinned references to structs that contain a `HrTimer` node to be
scheduled with the `hrtimer` subsystem.

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250309-hrtimer-v3-v6-12-rc2-v12-7-73586e2bd5f1@kernel.org
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Author: Andreas Hindborg

rust/kernel/time/hrtimer.rs

@@ -433,3 +433,5 @@ macro_rules! impl_has_hr_timer {
 
 mod arc;
 pub use arc::ArcHrTimerHandle;
+mod pin;
+pub use pin::PinHrTimerHandle;

rust/kernel/time/hrtimer/pin.rs

@@ -0,0 +1,104 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use super::HasHrTimer;
+use super::HrTimer;
+use super::HrTimerCallback;
+use super::HrTimerHandle;
+use super::RawHrTimerCallback;
+use super::UnsafeHrTimerPointer;
+use crate::time::Ktime;
+use core::pin::Pin;
+
+/// A handle for a `Pin<&HasHrTimer>`. When the handle exists, the timer might be
+/// running.
+pub struct PinHrTimerHandle<'a, T>
+where
+    T: HasHrTimer<T>,
+{
+    pub(crate) inner: Pin<&'a T>,
+}
+
+// SAFETY: We cancel the timer when the handle is dropped. The implementation of
+// the `cancel` method will block if the timer handler is running.
+unsafe impl<'a, T> HrTimerHandle for PinHrTimerHandle<'a, T>
+where
+    T: HasHrTimer<T>,
+{
+    fn cancel(&mut self) -> bool {
+        let self_ptr: *const T = self.inner.get_ref();
+
+        // SAFETY: As we got `self_ptr` from a reference above, it must point to
+        // a valid `T`.
+        let timer_ptr = unsafe { <T as HasHrTimer<T>>::raw_get_timer(self_ptr) };
+
+        // SAFETY: As `timer_ptr` is derived from a reference, it must point to
+        // a valid and initialized `HrTimer`.
+        unsafe { HrTimer::<T>::raw_cancel(timer_ptr) }
+    }
+}
+
+impl<'a, T> Drop for PinHrTimerHandle<'a, T>
+where
+    T: HasHrTimer<T>,
+{
+    fn drop(&mut self) {
+        self.cancel();
+    }
+}
+
+// SAFETY: We capture the lifetime of `Self` when we create a `PinHrTimerHandle`,
+// so `Self` will outlive the handle.
+unsafe impl<'a, T> UnsafeHrTimerPointer for Pin<&'a T>
+where
+    T: Send + Sync,
+    T: HasHrTimer<T>,
+    T: HrTimerCallback<Pointer<'a> = Self>,
+{
+    type TimerHandle = PinHrTimerHandle<'a, T>;
+
+    unsafe fn start(self, expires: Ktime) -> Self::TimerHandle {
+        // Cast to pointer
+        let self_ptr: *const T = self.get_ref();
+
+        // SAFETY:
+        //  - As we derive `self_ptr` from a reference above, it must point to a
+        //    valid `T`.
+        //  - We keep `self` alive by wrapping it in a handle below.
+        unsafe { T::start(self_ptr, expires) };
+
+        PinHrTimerHandle { inner: self }
+    }
+}
+
+impl<'a, T> RawHrTimerCallback for Pin<&'a T>
+where
+    T: HasHrTimer<T>,
+    T: HrTimerCallback<Pointer<'a> = Self>,
+{
+    type CallbackTarget<'b> = Self;
+
+    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
+        // `HrTimer` is `repr(C)`
+        let timer_ptr = ptr as *mut HrTimer<T>;
+
+        // SAFETY: By the safety requirement of this function, `timer_ptr`
+        // points to a `HrTimer<T>` contained in an `T`.
+        let receiver_ptr = unsafe { T::timer_container_of(timer_ptr) };
+
+        // SAFETY:
+        //  - By the safety requirement of this function, `timer_ptr`
+        //    points to a `HrTimer<T>` contained in an `T`.
+        //  - As per the safety requirements of the trait `HrTimerHandle`, the
+        //    `PinHrTimerHandle` associated with this timer is guaranteed to
+        //    be alive until this method returns. That handle borrows the `T`
+        //    behind `receiver_ptr`, thus guaranteeing the validity of
+        //    the reference created below.
+        let receiver_ref = unsafe { &*receiver_ptr };
+
+        // SAFETY: `receiver_ref` only exists as pinned, so it is safe to pin it
+        // here.
+        let receiver_pin = unsafe { Pin::new_unchecked(receiver_ref) };
+
+        T::run(receiver_pin).into_c()
+    }
+}