[PATCH v5 2/9] rust: device: Enable accessing the FwNode of a Device

[Remo Senekowitsch <remo@buenzli.dev>]

Subsequent patches will add methods for reading properties to FwNode.
The first step to accessing these methods will be to access the "root"
FwNode of a Device.

Add the method `fwnode` to `Device`.

Signed-off-by: Remo Senekowitsch <remo@buenzli.dev>
---
 rust/kernel/device.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index d8619d4485fb4..b4b7056eb80f8 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -186,6 +186,21 @@ unsafe fn printk(&self, klevel: &[u8], msg: fmt::Arguments<'_>) {
         };
     }
 
+    /// Obtain the [`FwNode`](property::FwNode) corresponding to the device.
+    pub fn fwnode(&self) -> Option<&property::FwNode> {
+        // SAFETY: `self` is valid.
+        let fwnode_handle = unsafe { bindings::__dev_fwnode(self.as_raw()) };
+        if fwnode_handle.is_null() {
+            return None;
+        }
+        // SAFETY: `fwnode_handle` is valid. Its lifetime is tied to `&self`. We
+        // return a reference instead of an `ARef<FwNode>` because `dev_fwnode()`
+        // doesn't increment the refcount. It is safe to cast from a
+        // `struct fwnode_handle*` to a `*const FwNode` because `FwNode` is
+        // defined as a `#[repr(transparent)]` wrapper around `fwnode_handle`.
+        Some(unsafe { &*fwnode_handle.cast() })
+    }
+
     /// Checks if property is present or not.
     pub fn property_present(&self, name: &CStr) -> bool {
         // SAFETY: By the invariant of `CStr`, `name` is null-terminated.
-- 
2.49.0