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/pci.rs b/rust/kernel/pci.rs
index 410b79d46632..82e128431f08 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -24,6 +24,7 @@ use crate::{
 };
 use core::{
     marker::PhantomData,
+    mem::offset_of,
     ptr::{
         addr_of_mut,
         NonNull, //
@@ -443,6 +444,12 @@ impl Device<device::Core> {
     }
 }
 
+// SAFETY: `pci::Device` is a transparent wrapper of `struct pci_dev`.
+// The offset is guaranteed to point to a valid device field inside `pci::Device`.
+unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for Device<Ctx> {
+    const OFFSET: usize = offset_of!(bindings::pci_dev, dev);
+}
+
 // SAFETY: `Device` is a transparent wrapper of a type that doesn't depend on `Device`'s generic
 // argument.
 kernel::impl_device_context_deref!(unsafe { Device });