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, 7 insertions, 0 deletions

diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
index aea1b44d189b..95b056cc1a71 100644
--- a/rust/kernel/i2c.rs
+++ b/rust/kernel/i2c.rs
@@ -24,6 +24,7 @@ use crate::{
 
 use core::{
     marker::PhantomData,
+    mem::offset_of,
     ptr::{
         from_ref,
         NonNull, //
@@ -476,6 +477,12 @@ impl<Ctx: device::DeviceContext> I2cClient<Ctx> {
     }
 }
 
+// SAFETY: `I2cClient` is a transparent wrapper of `struct i2c_client`.
+// The offset is guaranteed to point to a valid device field inside `I2cClient`.
+unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for I2cClient<Ctx> {
+    const OFFSET: usize = offset_of!(bindings::i2c_client, dev);
+}
+
 // SAFETY: `I2cClient` is a transparent wrapper of a type that doesn't depend on
 // `I2cClient`'s generic argument.
 kernel::impl_device_context_deref!(unsafe { I2cClient });