[PATCH v4 1/3] rust: device: implement bus_type_raw()

[Danilo Krummrich <dakr@kernel.org>]

Implement bus_type_raw(), which returns a raw pointer to the device'
struct bus_type.

This is useful for bus devices, to implement the following trait.

	impl TryFrom<&Device> for &pci::Device

With this a caller can try to get the bus specific device from a generic
device in a safe way. try_from() will only succeed if the generic
device' bus type pointer matches the pointer of the bus' type.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/device.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
index 21b343a1dc4d..67a2fc46cf4c 100644
--- a/rust/kernel/device.rs
+++ b/rust/kernel/device.rs
@@ -65,6 +65,16 @@ pub(crate) fn as_raw(&self) -> *mut bindings::device {
         self.0.get()
     }
 
+    /// Returns a raw pointer to the device' bus type.
+    #[expect(unused)]
+    pub(crate) fn bus_type_raw(&self) -> *const bindings::bus_type {
+        // SAFETY:
+        // - By the type invariants, `self.as_raw()` is a valid pointer to a `struct device`.
+        // - `dev->bus` is a pointer to a `const struct bus_type`, which is only ever set at device
+        //    creation.
+        unsafe { (*self.as_raw()).bus }
+    }
+
     /// Convert a raw C `struct device` pointer to a `&'a Device`.
     ///
     /// # Safety
-- 
2.48.1