rust: net::phy implement AsRef<kernel::device::Device> trait

Implement AsRef<kernel::device::Device> trait for Device. A PHY driver
needs a reference to device::Device to call the firmware API.
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

rust/kernel/net/phy.rs

@@ -7,8 +7,7 @@
 //! C headers: [`include/linux/phy.h`](srctree/include/linux/phy.h).
 
 use crate::{error::*, prelude::*, types::Opaque};
-
-use core::marker::PhantomData;
+use core::{marker::PhantomData, ptr::addr_of_mut};
 
 /// PHY state machine states.
 ///
@@ -58,8 +57,9 @@ pub enum DuplexMode {
 ///
 /// # Invariants
 ///
-/// Referencing a `phy_device` using this struct asserts that you are in
-/// a context where all methods defined on this struct are safe to call.
+/// - Referencing a `phy_device` using this struct asserts that you are in
+///   a context where all methods defined on this struct are safe to call.
+/// - This struct always has a valid `self.0.mdio.dev`.
 ///
 /// [`struct phy_device`]: srctree/include/linux/phy.h
 // During the calls to most functions in [`Driver`], the C side (`PHYLIB`) holds a lock that is
@@ -76,9 +76,11 @@ impl Device {
     ///
     /// # Safety
     ///
-    /// For the duration of 'a, the pointer must point at a valid `phy_device`,
-    /// and the caller must be in a context where all methods defined on this struct
-    /// are safe to call.
+    /// For the duration of `'a`,
+    /// - the pointer must point at a valid `phy_device`, and the caller
+    ///   must be in a context where all methods defined on this struct
+    ///   are safe to call.
+    /// - `(*ptr).mdio.dev` must be a valid.
     unsafe fn from_raw<'a>(ptr: *mut bindings::phy_device) -> &'a mut Self {
         // CAST: `Self` is a `repr(transparent)` wrapper around `bindings::phy_device`.
         let ptr = ptr.cast::<Self>();
@@ -302,6 +304,14 @@ pub fn genphy_read_abilities(&mut self) -> Result {
     }
 }
 
+impl AsRef<kernel::device::Device> for Device {
+    fn as_ref(&self) -> &kernel::device::Device {
+        let phydev = self.0.get();
+        // SAFETY: The struct invariant ensures that `mdio.dev` is valid.
+        unsafe { kernel::device::Device::as_ref(addr_of_mut!((*phydev).mdio.dev)) }
+    }
+}
+
 /// Defines certain other features this PHY supports (like interrupts).
 ///
 /// These flag values are used in [`Driver::FLAGS`].