rust: types: add `Opaque::ffi_init`

This function allows to easily initialize `Opaque` with the pin-init
API. `Opaque::ffi_init` takes a closure and returns a pin-initializer.
This pin-initiailizer calls the given closure with a pointer to the
inner `T`.

Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-14-y86-dev@protonmail.com
[ Fixed typo. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init.rs

@@ -177,6 +177,14 @@
 //! }
 //! ```
 //!
+//! For the special case where initializing a field is a single FFI-function call that cannot fail,
+//! there exist the helper function [`Opaque::ffi_init`]. This function initialize a single
+//! [`Opaque`] field by just delegating to the supplied closure. You can use these in combination
+//! with [`pin_init!`].
+//!
+//! For more information on how to use [`pin_init_from_closure()`], take a look at the uses inside
+//! the `kernel` crate. The [`sync`] module is a good starting point.
+//!
 //! [`sync`]: kernel::sync
 //! [pinning]: https://doc.rust-lang.org/std/pin/index.html
 //! [structurally pinned fields]:
@@ -187,6 +195,7 @@
 //! [`impl PinInit<T, E>`]: PinInit
 //! [`impl Init<T, E>`]: Init
 //! [`Opaque`]: kernel::types::Opaque
+//! [`Opaque::ffi_init`]: kernel::types::Opaque::ffi_init
 //! [`pin_data`]: ::macros::pin_data
 
 use crate::{

rust/kernel/types.rs

@@ -2,6 +2,7 @@
 
 //! Kernel types.
 
+use crate::init::{self, PinInit};
 use alloc::boxed::Box;
 use core::{
     cell::UnsafeCell,
@@ -234,6 +235,25 @@ impl<T> Opaque<T> {
         Self(MaybeUninit::uninit())
     }
 
+    /// Creates a pin-initializer from the given initializer closure.
+    ///
+    /// The returned initializer calls the given closure with the pointer to the inner `T` of this
+    /// `Opaque`. Since this memory is uninitialized, the closure is not allowed to read from it.
+    ///
+    /// This function is safe, because the `T` inside of an `Opaque` is allowed to be
+    /// uninitialized. Additionally, access to the inner `T` requires `unsafe`, so the caller needs
+    /// to verify at that point that the inner value is valid.
+    pub fn ffi_init(init_func: impl FnOnce(*mut T)) -> impl PinInit<Self> {
+        // SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully
+        // initialize the `T`.
+        unsafe {
+            init::pin_init_from_closure::<_, ::core::convert::Infallible>(move |slot| {
+                init_func(Self::raw_get(slot));
+                Ok(())
+            })
+        }
+    }
+
     /// Returns a raw pointer to the opaque data.
     pub fn get(&self) -> *mut T {
         UnsafeCell::raw_get(self.0.as_ptr())