[PATCH 4/5] rust: Add `Device::drvdata_borrow_mut`

[Markus Probst <markus.probst@posteo.de>]

This function allows the caller to obtain a mutable reference to the
driver's private data if he has exclusive access.

This will be used in serdev to provide mutable references in callbacks.

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
 rust/kernel/device.rs | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index 2291d85b6849..aa2d87c6f4d3 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -258,6 +258,30 @@ pub unsafe fn drvdata_borrow<T>(&self) -> Pin<&T> {
         //   in `into_foreign()`.
         unsafe { Pin::<KBox<T>>::borrow(ptr.cast()) }
     }
+
+    /// Borrow the driver's private data bound to this [`Device`] mutable.
+    ///
+    /// # Safety
+    ///
+    /// - Must only be called after a preceding call to [`Device::set_drvdata`] and before the
+    ///   device is fully unbound.
+    /// - The type `T` must match the type of the `ForeignOwnable` previously stored by
+    ///   [`Device::set_drvdata`].
+    /// - The caller must have exclusive access to `T`.
+    #[expect(clippy::mut_from_ref)]
+    pub unsafe fn drvdata_borrow_mut<T>(&self) -> Pin<&mut T> {
+        // SAFETY: By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
+        let ptr = unsafe { bindings::dev_get_drvdata(self.as_raw()) };
+
+        // SAFETY:
+        // - By the safety requirements of this function, `ptr` comes from a previous call to
+        //   `into_foreign()`.
+        // - `dev_get_drvdata()` guarantees to return the same pointer given to `dev_set_drvdata()`
+        //   in `into_foreign()`.
+        // - By the safety requirements of this function, `borrow` and `borrow_mut` do not overlap
+        //   on the same object.
+        unsafe { Pin::<KBox<T>>::borrow_mut(ptr.cast()) }
+    }
 }
 
 impl<Ctx: DeviceContext> Device<Ctx> {

-- 
2.55.0