rust: miscdevice: Provide accessor to pull out miscdevice::this_device

There are situations where a pointer to a `struct device` will become
necessary (e.g. for calling into dev_*() functions).  This accessor
allows callers to pull this out from the `struct miscdevice`.

Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Lee Jones <lee@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241210-miscdevice-file-param-v3-3-b2a79b666dc5@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

rust/kernel/miscdevice.rs

@@ -10,6 +10,7 @@
 
 use crate::{
     bindings,
+    device::Device,
     error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
     fs::File,
     prelude::*,
@@ -85,6 +86,16 @@ impl<T: MiscDevice> MiscDeviceRegistration<T> {
     pub fn as_raw(&self) -> *mut bindings::miscdevice {
         self.inner.get()
     }
+
+    /// Access the `this_device` field.
+    pub fn device(&self) -> &Device {
+        // SAFETY: This can only be called after a successful register(), which always
+        // initialises `this_device` with a valid device. Furthermore, the signature of this
+        // function tells the borrow-checker that the `&Device` reference must not outlive the
+        // `&MiscDeviceRegistration<T>` used to obtain it, so the last use of the reference must be
+        // before the underlying `struct miscdevice` is destroyed.
+        unsafe { Device::as_ref((*self.as_raw()).this_device) }
+    }
 }
 
 #[pinned_drop]