[PATCH v3] i2c: rust: fix I2cAdapter refcounts double increment

[Nicolás Antinori]

When `I2cAdapter::get` executes, it first calls
`bindings::i2c_get_adapter()` which increments the device and module
reference counts. It then takes a reference to the raw pointer and
converts it to an `ARef` via `.into()`.

The implementation of `From<&T> for ARef<T>` where `T: AlwaysRefCounted`
unconditionally calls `T::inc_ref()`. This leads to a second increment
to the reference counts.

Since the returned `ARef` will only release a single reference when
dropped via `dec_ref()`, this leaks one device and module reference count
on every call.

This fix was suggested by sashiko.dev.

Link: https://sashiko.dev/#/patchset/20260521190937.248904-1-nico.antinori.7@xxxxxxxxx
Signed-off-by: Nicolás Antinori <nico.antinori.7@xxxxxxxxx>
---
v3:
 - Remove unnecessary `.as_ptr()` and `NonNull` rewrap.
 - Add SAFETY comment for `ARef::from_raw`.
 - Small change in commit message to make it clearer.
v2:
 - Add missing Link tag to commit message.
 - https://lore.kernel.org/rust-for-linux/20260526194734.14378-1-nico.antinori.7@xxxxxxxxx
v1: https://lore.kernel.org/rust-for-linux/20260524181151.24988-1-nico.antinori.7@xxxxxxxxx

 rust/kernel/i2c.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/i2c.rs b/rust/kernel/i2c.rs
index 7b908f0c5a58..c084a45b1916 100644
--- a/rust/kernel/i2c.rs
+++ b/rust/kernel/i2c.rs
@@ -405,7 +405,9 @@ pub fn get(index: i32) -> Result<ARef<Self>> {

         // SAFETY: `adapter` is non-null and points to a live `i2c_adapter`.
         // `I2cAdapter` is #[repr(transparent)], so this cast is valid.
-        Ok(unsafe { (&*adapter.as_ptr().cast::<I2cAdapter<device::Normal>>()).into() })
+        // `i2c_get_adapter` returned the adapter with an incremented refcount, which we pass to
+        // the `ARef`.
+        Ok(unsafe { ARef::from_raw(adapter.cast::<I2cAdapter<device::Normal>>()) })
     }
 }

--
2.53.0