Return the reference to the initialized data in the `devres::register`
function.

This is needed in a following commit (rust: add basic serial device bus
abstractions).

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
 rust/kernel/cpufreq.rs    |  3 ++-
 rust/kernel/devres.rs     | 15 +++++++++++++--
 rust/kernel/drm/driver.rs |  3 ++-
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
index 76faa1ac8501..8cf86bb8e0f4 100644
--- a/rust/kernel/cpufreq.rs
+++ b/rust/kernel/cpufreq.rs
@@ -1051,7 +1051,8 @@ pub fn new_foreign_owned(dev: &Device<Bound>) -> Result
     where
         T: 'static,
     {
-        devres::register(dev, Self::new()?, GFP_KERNEL)
+        devres::register(dev, Self::new()?, GFP_KERNEL)?;
+        Ok(())
     }
 }
 
diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 6afe196be42c..f882bace8601 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -326,15 +326,26 @@ fn register_foreign<P>(dev: &Device<Bound>, data: P) -> Result
 /// }
 ///
 /// fn from_bound_context(dev: &Device<Bound>) -> Result {
-///     devres::register(dev, Registration::new(), GFP_KERNEL)
+///     devres::register(dev, Registration::new(), GFP_KERNEL)?;
+///     Ok(())
 /// }
 /// ```
-pub fn register<T, E>(dev: &Device<Bound>, data: impl PinInit<T, E>, flags: Flags) -> Result
+pub fn register<'a, T, E>(
+    dev: &'a Device<Bound>,
+    data: impl PinInit<T, E>,
+    flags: Flags,
+) -> Result<&'a T>
 where
     T: Send + 'static,
     Error: From<E>,
 {
     let data = KBox::pin_init(data, flags)?;
 
+    let data_ptr = &raw const *data;
+
     register_foreign(dev, data)
+        // SAFETY: `dev` is valid for the lifetime of 'a. As long as there is a reference to
+        // `Device<Bound>`, it is guaranteed that the device is not unbound and data has not been
+        // dropped. Thus `data_ptr` is also valid for the lifetime of 'a.
+        .map(|()| unsafe { &*data_ptr })
 }
diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
index e09f977b5b51..51e0c7e30cc2 100644
--- a/rust/kernel/drm/driver.rs
+++ b/rust/kernel/drm/driver.rs
@@ -145,7 +145,8 @@ pub fn new_foreign_owned(
 
         let reg = Registration::<T>::new(drm, flags)?;
 
-        devres::register(dev, reg, GFP_KERNEL)
+        devres::register(dev, reg, GFP_KERNEL)?;
+        Ok(())
     }
 
     /// Returns a reference to the `Device` instance for this registration.

-- 
2.52.0