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Date:   Sat, 08 Apr 2023 12:26:22 +0000
To:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Alice Ryhl <alice@ryhl.io>, Andreas Hindborg <nmi@metaspace.dk>
From:   Benno Lossin <y86-dev@protonmail.com>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev, Benno Lossin <y86-dev@protonmail.com>,
        Alice Ryhl <aliceryhl@google.com>,
        Andreas Hindborg <a.hindborg@samsung.com>
Subject: [PATCH v7 13/15] rust: types: add `Opaque::ffi_init`
Message-ID: <20230408122429.1103522-14-y86-dev@protonmail.com>
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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 <y86-dev@protonmail.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Andreas Hindborg <a.hindborg@samsung.com>
---
 rust/kernel/init.rs  |  9 +++++++++
 rust/kernel/types.rs | 20 ++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index ffd539e2f5ef..a1298c8bbda0 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -177,6 +177,14 @@
 //! }
 //! ```
 //!
+//! For the special case where initializing a field is a single FFI-functi=
on call that cannot fail,
+//! there exist the helper function [`Opaque::ffi_init`]. This function in=
itialize a single
+//! [`Opaque`] field by just delegating to the supplied closure. You can u=
se 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::{
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index ff2b2fac951d..3a46ec1a00cd 100644
--- a/rust/kernel/types.rs
+++ b/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 a=
llowed to read from it.
+    ///
+    /// This function is safe, because the `T` inside of an `Opaque` is al=
lowed to be
+    /// uninitialized. Additionally, access to the inner `T` requires `uns=
afe`, 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_f=
unc` to not fully
+        // initiailize 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())
--
2.39.2