rust: hrtimer: implement HrTimerPointer for Pin<Box<T>>

Allow `Pin<Box<T>>` to be the target of a timer callback.

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-v12-10-73586e2bd5f1@kernel.org
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Author: Andreas Hindborg

rust/kernel/time/hrtimer.rs

@@ -437,3 +437,6 @@ mod pin;
 pub use pin::PinHrTimerHandle;
 mod pin_mut;
 pub use pin_mut::PinMutHrTimerHandle;
+// `box` is a reserved keyword, so prefix with `t` for timer
+mod tbox;
+pub use tbox::BoxHrTimerHandle;

rust/kernel/time/hrtimer/tbox.rs

@@ -0,0 +1,120 @@
+// 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::prelude::*;
+use crate::time::Ktime;
+use core::ptr::NonNull;
+
+/// A handle for a [`Box<HasHrTimer<T>>`] returned by a call to
+/// [`HrTimerPointer::start`].
+///
+/// # Invariants
+///
+/// - `self.inner` comes from a `Box::into_raw` call.
+pub struct BoxHrTimerHandle<T, A>
+where
+    T: HasHrTimer<T>,
+    A: crate::alloc::Allocator,
+{
+    pub(crate) inner: NonNull<T>,
+    _p: core::marker::PhantomData<A>,
+}
+
+// SAFETY: We implement drop below, and we cancel the timer in the drop
+// implementation.
+unsafe impl<T, A> HrTimerHandle for BoxHrTimerHandle<T, A>
+where
+    T: HasHrTimer<T>,
+    A: crate::alloc::Allocator,
+{
+    fn cancel(&mut self) -> bool {
+        // SAFETY: As we obtained `self.inner` from a valid reference when we
+        // created `self`, it must point to a valid `T`.
+        let timer_ptr = unsafe { <T as HasHrTimer<T>>::raw_get_timer(self.inner.as_ptr()) };
+
+        // SAFETY: As `timer_ptr` points into `T` and `T` is valid, `timer_ptr`
+        // must point to a valid `HrTimer` instance.
+        unsafe { HrTimer::<T>::raw_cancel(timer_ptr) }
+    }
+}
+
+impl<T, A> Drop for BoxHrTimerHandle<T, A>
+where
+    T: HasHrTimer<T>,
+    A: crate::alloc::Allocator,
+{
+    fn drop(&mut self) {
+        self.cancel();
+        // SAFETY: By type invariant, `self.inner` came from a `Box::into_raw`
+        // call.
+        drop(unsafe { Box::<T, A>::from_raw(self.inner.as_ptr()) })
+    }
+}
+
+impl<T, A> HrTimerPointer for Pin<Box<T, A>>
+where
+    T: 'static,
+    T: Send + Sync,
+    T: HasHrTimer<T>,
+    T: for<'a> HrTimerCallback<Pointer<'a> = Pin<Box<T, A>>>,
+    A: crate::alloc::Allocator,
+{
+    type TimerHandle = BoxHrTimerHandle<T, A>;
+
+    fn start(self, expires: Ktime) -> Self::TimerHandle {
+        // SAFETY:
+        //  - We will not move out of this box during timer callback (we pass an
+        //    immutable reference to the callback).
+        //  - `Box::into_raw` is guaranteed to return a valid pointer.
+        let inner =
+            unsafe { NonNull::new_unchecked(Box::into_raw(Pin::into_inner_unchecked(self))) };
+
+        // SAFETY:
+        //  - We keep `self` alive by wrapping it in a handle below.
+        //  - Since we generate the pointer passed to `start` from a valid
+        //    reference, it is a valid pointer.
+        unsafe { T::start(inner.as_ptr(), expires) };
+
+        // INVARIANT: `inner` came from `Box::into_raw` above.
+        BoxHrTimerHandle {
+            inner,
+            _p: core::marker::PhantomData,
+        }
+    }
+}
+
+impl<T, A> RawHrTimerCallback for Pin<Box<T, A>>
+where
+    T: 'static,
+    T: HasHrTimer<T>,
+    T: for<'a> HrTimerCallback<Pointer<'a> = Pin<Box<T, A>>>,
+    A: crate::alloc::Allocator,
+{
+    type CallbackTarget<'a> = Pin<&'a mut T>;
+
+    unsafe extern "C" fn run(ptr: *mut bindings::hrtimer) -> bindings::hrtimer_restart {
+        // `HrTimer` is `repr(C)`
+        let timer_ptr = ptr.cast::<super::HrTimer<T>>();
+
+        // 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 { T::timer_container_of(timer_ptr) };
+
+        // SAFETY:
+        //  - As per the safety requirements of the trait `HrTimerHandle`, the
+        //   `BoxHrTimerHandle` associated with this timer is guaranteed to
+        //   be alive until this method returns. That handle owns the `T`
+        //   behind `data_ptr` thus guaranteeing the validity of
+        //   the reference created below.
+        // - As `data_ptr` comes from a `Pin<Box<T>>`, only pinned references to
+        //   `data_ptr` exist.
+        let data_mut_ref = unsafe { Pin::new_unchecked(&mut *data_ptr) };
+
+        T::run(data_mut_ref).into_c()
+    }
+}