[PATCH v21 3/4] rust: faux: Allow retrieving a bound Device

[Lyude Paul]

When writing up some rust code that used faux devices for unit testing, I
noticed that we never actually added the Bound device context to
faux::Registration's AsRef<device::Device> implementation. This being said:
the Registration object itself is proof that a driver is bound to the
device - so this should be safe.

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
Reviewed-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Acked-by: Danilo Krummrich <dakr@xxxxxxxxxx>

---
V18:
- Add notes from Danilo to safety comment.
V21:
- Quote all code in comments with backticks.

 rust/kernel/faux.rs | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/rust/kernel/faux.rs b/rust/kernel/faux.rs
index 43b4974f48cd2..36c92ae2943c0 100644
--- a/rust/kernel/faux.rs
+++ b/rust/kernel/faux.rs
@@ -25,7 +25,8 @@
 ///
 /// # Invariants
 ///
-/// `self.0` always holds a valid pointer to an initialized and registered [`struct faux_device`].
+/// - `self.0` always holds a valid pointer to an initialized and registered [`struct faux_device`].
+/// - This object is proof that the object described by this `Registration` is bound to a device.
 ///
 /// [`struct faux_device`]: srctree/include/linux/device/faux.h
 pub struct Registration(NonNull<bindings::faux_device>);
@@ -59,10 +60,17 @@ fn as_raw(&self) -> *mut bindings::faux_device {
     }
 }
 
-impl AsRef<device::Device> for Registration {
-    fn as_ref(&self) -> &device::Device {
-        // SAFETY: The underlying `device` in `faux_device` is guaranteed by the C API to be
-        // a valid initialized `device`.
+impl AsRef<device::Device<device::Bound>> for Registration {
+    fn as_ref(&self) -> &device::Device<device::Bound> {
+        // SAFETY:
+        // - The underlying `device` in `faux_device` is guaranteed by the C API to be a valid
+        //   initialized `device`.
+        // - `faux_match()` always returns 1, and probe runs synchronously
+        //   (PROBE_FORCE_SYNCHRONOUS).
+        // - `suppress_bind_attrs = true` on faux_driver prevents userspace-triggered unbind via
+        //   sysfs.
+        // - `mem::forget(Registration)` is not a problem; if the `Registration` is leaked, the faux
+        //   device stays bound forever.
         unsafe { device::Device::from_raw(addr_of_mut!((*self.as_raw()).dev)) }
     }
 }
-- 
2.54.0