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

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/time/hrtimer.rs      |   2 +
 rust/kernel/time/hrtimer/tbox.rs | 102 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 104 insertions(+)

diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
index 9220a8f559271c7eda8d457dbd52cf9e4800ec14..885d1772f7466725d2a421990f1ae154290ccf60 100644
--- a/rust/kernel/time/hrtimer.rs
+++ b/rust/kernel/time/hrtimer.rs
@@ -396,3 +396,5 @@ unsafe fn raw_get_timer(ptr: *const Self) ->
 mod arc;
 mod pin;
 mod pin_mut;
+// `box` is a reserved keyword, so prefix with `t` for timer
+mod tbox;
diff --git a/rust/kernel/time/hrtimer/tbox.rs b/rust/kernel/time/hrtimer/tbox.rs
new file mode 100644
index 0000000000000000000000000000000000000000..8dec40f7313921cd9f5bf357bb84962480385359
--- /dev/null
+++ b/rust/kernel/time/hrtimer/tbox.rs
@@ -0,0 +1,102 @@
+// 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::mem::ManuallyDrop;
+
+/// A handle for a [`Box<HasHrTimer<U>>`] returned by a call to
+/// [`HrTimerPointer::start`].
+pub struct BoxHrTimerHandle<U, A>
+where
+    U: HasHrTimer<U>,
+    A: crate::alloc::Allocator,
+{
+    pub(crate) inner: *mut U,
+    _p: core::marker::PhantomData<A>,
+}
+
+// SAFETY: We implement drop below, and we cancel the timer in the drop
+// implementation.
+unsafe impl<U, A> HrTimerHandle for BoxHrTimerHandle<U, A>
+where
+    U: HasHrTimer<U>,
+    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 `U`.
+        let timer_ptr = unsafe { <U as HasHrTimer<U>>::raw_get_timer(self.inner) };
+
+        // 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, A> Drop for BoxHrTimerHandle<U, A>
+where
+    U: HasHrTimer<U>,
+    A: crate::alloc::Allocator,
+{
+    fn drop(&mut self) {
+        self.cancel();
+        // SAFETY: `self.inner` came from a `Box::into_raw` call
+        drop(unsafe { Box::<U, A>::from_raw(self.inner) })
+    }
+}
+
+impl<U, A> HrTimerPointer for Pin<Box<U, A>>
+where
+    U: Send + Sync,
+    U: HasHrTimer<U>,
+    U: for<'a> HrTimerCallback<CallbackTarget<'a> = Pin<Box<U, A>>>,
+    U: for<'a> HrTimerCallback<CallbackTargetParameter<'a> = Pin<&'a U>>,
+    A: crate::alloc::Allocator,
+{
+    type TimerHandle = BoxHrTimerHandle<U, A>;
+
+    fn start(self, expires: Ktime) -> Self::TimerHandle {
+        let self_ptr: *const U = <Self as core::ops::Deref>::deref(&self);
+
+        // SAFETY: Since we generate the pointer passed to `start` from a valid
+        // reference, it is a valid pointer.
+        unsafe { U::start(self_ptr, expires) };
+
+        // SAFETY: We will not move out of this box during timer callback (we
+        // pass an immutable reference to the callback).
+        let inner = unsafe { Pin::into_inner_unchecked(self) };
+
+        BoxHrTimerHandle {
+            inner: Box::into_raw(inner),
+            _p: core::marker::PhantomData,
+        }
+    }
+}
+
+impl<U, A> RawHrTimerCallback for Pin<Box<U, A>>
+where
+    U: HasHrTimer<U>,
+    U: for<'a> HrTimerCallback<CallbackTarget<'a> = Pin<Box<U, A>>>,
+    U: for<'a> HrTimerCallback<CallbackTargetParameter<'a> = Pin<&'a U>>,
+    A: crate::alloc::Allocator,
+{
+    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: We called `Box::into_raw` when we queued the timer.
+        let tbox = ManuallyDrop::new(Box::into_pin(unsafe { Box::<U, A>::from_raw(data_ptr) }));
+
+        U::run(tbox.as_ref()).into()
+    }
+}

-- 
2.47.0