rust: Add trait to convert a device reference to a bus device reference

Implement the `AsBusDevice` trait for converting a `Device` reference to a
bus device reference for all bus devices.

The `AsBusDevice` trait allows abstractions to provide the bus device in
class device callbacks. It must not be used by drivers and is intended for
bus and class device abstractions only.

Signed-off-by: Markus Probst <markus.probst@posteo.de>
Link: https://patch.msgid.link/20251027200547.1038967-2-markus.probst@posteo.de
[ * Remove unused import.
  * Change visibility of AsBusDevice to public.
  * Fix build for USB.
  * Add impl for I2cClient.
  - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 14 insertions, 1 deletions

diff --git a/rust/kernel/usb.rs b/rust/kernel/usb.rs
index 534e3ded5442..d10b65e9fb6a 100644
--- a/rust/kernel/usb.rs
+++ b/rust/kernel/usb.rs
@@ -15,7 +15,14 @@ use crate::{
     types::{AlwaysRefCounted, Opaque},
     ThisModule,
 };
-use core::{marker::PhantomData, mem::MaybeUninit, ptr::NonNull};
+use core::{
+    marker::PhantomData,
+    mem::{
+        offset_of,
+        MaybeUninit, //
+    },
+    ptr::NonNull,
+};
 
 /// An adapter for the registration of USB drivers.
 pub struct Adapter<T: Driver>(T);
@@ -324,6 +331,12 @@ impl<Ctx: device::DeviceContext> Interface<Ctx> {
     }
 }
 
+// SAFETY: `usb::Interface` is a transparent wrapper of `struct usb_interface`.
+// The offset is guaranteed to point to a valid device field inside `usb::Interface`.
+unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for Interface<Ctx> {
+    const OFFSET: usize = offset_of!(bindings::usb_interface, dev);
+}
+
 // SAFETY: `Interface` is a transparent wrapper of a type that doesn't depend on
 // `Interface`'s generic argument.
 kernel::impl_device_context_deref!(unsafe { Interface });