linux-kernel - [PATCH v3 3/4] rust: transmute: Migrate AsBytes/FromBytes to ffi crate for bindgen

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Message-ID: <20251226-transmute-v3-3-c69a81bf8621@google.com>
Date: Fri, 26 Dec 2025 21:08:22 +0000
From: Matthew Maurer <mmaurer@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Matthew Maurer <mmaurer@...gle.com>
Subject: [PATCH v3 3/4] rust: transmute: Migrate AsBytes/FromBytes to ffi
 crate for bindgen

When referencing a trait from inside the `uapi` or `bindings` crates,
the `kernel` crate is not available as the it depends on them. To break
this cycle, move the `transmute` module containing the two traits to the
`ffi` crate which is already used to export similar traits to `uapi` and
`bindings`.

Re-export `transmute` in its original position in the kernel to avoid
any breakage. Normal users are expected to continue using it from
`kernel::transmute`.

Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
---
 rust/Makefile                     | 6 +++---
 rust/{ffi.rs => ffi/lib.rs}       | 5 +++++
 rust/{kernel => ffi}/transmute.rs | 0
 rust/kernel/lib.rs                | 2 +-
 scripts/generate_rust_analyzer.py | 2 +-
 5 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/rust/Makefile b/rust/Makefile
index 5d357dce1704d15e43effc528be8f5a4d74d3d8d..5c2cb78f7c258aa87d8128593159be1f5945252f 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -207,7 +207,7 @@ rustdoc-compiler_builtins: $(src)/compiler_builtins.rs rustdoc-core FORCE
 	+$(call if_changed,rustdoc)
 
 rustdoc-ffi: private is-kernel-object := y
-rustdoc-ffi: $(src)/ffi.rs rustdoc-core FORCE
+rustdoc-ffi: $(src)/ffi/lib.rs rustdoc-core FORCE
 	+$(call if_changed,rustdoc)
 
 rustdoc-pin_init_internal: private rustdoc_host = yes
@@ -249,7 +249,7 @@ quiet_cmd_rustc_test_library = $(RUSTC_OR_CLIPPY_QUIET) TL $<
 rusttestlib-build_error: $(src)/build_error.rs FORCE
 	+$(call if_changed,rustc_test_library)
 
-rusttestlib-ffi: $(src)/ffi.rs FORCE
+rusttestlib-ffi: $(src)/ffi/lib.rs FORCE
 	+$(call if_changed,rustc_test_library)
 
 rusttestlib-proc_macro2: private rustc_target_flags = $(proc_macro2-flags)
@@ -657,7 +657,7 @@ $(obj)/build_error.o: $(src)/build_error.rs $(obj)/compiler_builtins.o FORCE
 	+$(call if_changed_rule,rustc_library)
 
 $(obj)/ffi.o: private skip_gendwarfksyms = 1
-$(obj)/ffi.o: $(src)/ffi.rs $(obj)/compiler_builtins.o FORCE
+$(obj)/ffi.o: $(src)/ffi/lib.rs $(obj)/compiler_builtins.o FORCE
 	+$(call if_changed_rule,rustc_library)
 
 $(obj)/bindings.o: private rustc_target_flags = --extern ffi --extern pin_init
diff --git a/rust/ffi.rs b/rust/ffi/lib.rs
similarity index 87%
rename from rust/ffi.rs
rename to rust/ffi/lib.rs
index f961e9728f590fd2c52d4c03a1f715d654051d04..14052362f091a609bc505fe6eca77fe998fe2321 100644
--- a/rust/ffi.rs
+++ b/rust/ffi/lib.rs
@@ -10,6 +10,11 @@
 
 #![no_std]
 
+#[doc(hidden)]
+// This lives here to make it accessible to `bindings`, similar to the other `ffi` types.
+// User code should access it through `kernel::transmute`.
+pub mod transmute;
+
 macro_rules! alias {
     ($($name:ident = $ty:ty;)*) => {$(
         #[allow(non_camel_case_types, missing_docs)]
diff --git a/rust/kernel/transmute.rs b/rust/ffi/transmute.rs
similarity index 100%
rename from rust/kernel/transmute.rs
rename to rust/ffi/transmute.rs
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index f812cf12004286962985a068665443dc22c389a2..4aa54dd83319ef16bd4baa1964114f1e6549942b 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -63,6 +63,7 @@
 extern crate self as kernel;
 
 pub use ffi;
+pub use ffi::transmute;
 
 pub mod acpi;
 pub mod alloc;
@@ -146,7 +147,6 @@
 pub mod task;
 pub mod time;
 pub mod tracepoint;
-pub mod transmute;
 pub mod types;
 pub mod uaccess;
 #[cfg(CONFIG_USB = "y")]
diff --git a/scripts/generate_rust_analyzer.py b/scripts/generate_rust_analyzer.py
index 147d0cc940681426771db865bc2462e7029a6d7d..843d081eacaca8edeeac5978bd8107a498008186 100755
--- a/scripts/generate_rust_analyzer.py
+++ b/scripts/generate_rust_analyzer.py
@@ -137,7 +137,7 @@ def generate_crates(srctree, objtree, sysroot_src, external_src, cfgs, core_edit
 
     append_crate(
         "ffi",
-        srctree / "rust" / "ffi.rs",
+        srctree / "rust" / "ffi" / "lib.rs",
         ["core", "compiler_builtins"],
     )
 

-- 
2.52.0.351.gbe84eed79e-goog