The kernel includes a large number of static inline functions that are
defined in header files. One example is the crypto_shash_descsize()
function which is defined in hash.h as

static inline unsigned int crypto_shash_descsize(struct crypto_shash *tfm)
{
	return tfm->descsize;
}

bindgen is currently unable to generate bindings to these functions as
they are not publically exposed (they are static after all).

The Rust code currently uses rust_helper_* functions, such as
rust_helper_alloc_pages() for example to call the static inline
functions. But this is a hassle as someone needs to write a C helper
function.

Instead we can use the bindgen wrap-static-fns feature. The feature
is marked as experimental, but has recently been promoted to
non-experimental (dependig on your version of bindgen).

By supporting wrap-static-fns we automatically generate a C file called
extern.c that exposes the static inline functions, for example like this

```
unsigned int crypto_shash_descsize__extern(struct crypto_shash *tfm) { return crypto_shash_descsize(tfm); }
```

The nice part is that this is auto-generated.

We then also get a bindings_generate_static.rs file with the Rust
binding, like this

```
extern "C" {
    #[link_name = "crypto_shash_descsize__extern"]
    pub fn crypto_shash_descsize(tfm: *mut crypto_shash) -> core::ffi::c_uint;
}
```

So now we can use the static inline functions just like normal
functions.

There are a bunch of static inline functions that don't work though, because
the C compiler fails to build extern.c:
 * functions with inline asm generate "operand probably does not match constraints"
   errors (rip_rel_ptr() for example)
 * functions with bit masks (u32_encode_bits() and friends) result in
   "call to ‘__bad_mask’ declared with attribute error: bad bitfield mask"
   errors

As well as that any static inline function that calls a function that has been
kconfig-ed out will fail to link as the function being called isn't built
(mdio45_ethtool_gset_npage for example)

Due to these failures we use a allow-list system (where functions must
be manually enabled).

Link: https://github.com/rust-lang/rust-bindgen/discussions/2405
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
---
 rust/.gitignore               |  2 ++
 rust/Makefile                 | 37 ++++++++++++++++++++++++++++++++---
 rust/bindgen_static_functions |  6 ++++++
 rust/bindings/lib.rs          |  4 ++++
 rust/exports.c                |  1 +
 5 files changed, 47 insertions(+), 3 deletions(-)
 create mode 100644 rust/bindgen_static_functions

diff --git a/rust/.gitignore b/rust/.gitignore
index d3829ffab80b..741a18023801 100644
--- a/rust/.gitignore
+++ b/rust/.gitignore
@@ -1,10 +1,12 @@
 # SPDX-License-Identifier: GPL-2.0
 
 bindings_generated.rs
+bindings_generated_static.rs
 bindings_helpers_generated.rs
 doctests_kernel_generated.rs
 doctests_kernel_generated_kunit.c
 uapi_generated.rs
 exports_*_generated.h
+extern.c
 doc/
 test/
diff --git a/rust/Makefile b/rust/Makefile
index 6daaa4dc21db..431edc73e893 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -10,14 +10,17 @@ always-$(CONFIG_RUST) += exports_core_generated.h
 # for Rust only, thus there is no header nor prototypes.
 obj-$(CONFIG_RUST) += helpers/helpers.o
 CFLAGS_REMOVE_helpers/helpers.o = -Wmissing-prototypes -Wmissing-declarations
+CFLAGS_REMOVE_extern.o = -Wmissing-prototypes -Wmissing-declarations -Wdiscarded-qualifiers
 
 always-$(CONFIG_RUST) += libmacros.so
 no-clean-files += libmacros.so
 
-always-$(CONFIG_RUST) += bindings/bindings_generated.rs bindings/bindings_helpers_generated.rs
-obj-$(CONFIG_RUST) += bindings.o kernel.o
+always-$(CONFIG_RUST) += bindings/bindings_generated.rs bindings/bindings_helpers_generated.rs \
+	bindings/bindings_generated_static.rs
+obj-$(CONFIG_RUST) += bindings.o kernel.o extern.o
 always-$(CONFIG_RUST) += exports_helpers_generated.h \
-    exports_bindings_generated.h exports_kernel_generated.h
+    exports_bindings_generated.h exports_bindings_static_generated.h \
+    exports_kernel_generated.h
 
 always-$(CONFIG_RUST) += uapi/uapi_generated.rs
 obj-$(CONFIG_RUST) += uapi.o
@@ -274,6 +277,21 @@ quiet_cmd_bindgen = BINDGEN $@
 		-o $@ -- $(bindgen_c_flags_final) -DMODULE \
 		$(bindgen_target_cflags) $(bindgen_target_extra)
 
+quiet_cmd_bindgen_static = BINDGEN $@
+      cmd_bindgen_static = \
+	$(BINDGEN) $< $(bindgen_target_flags) \
+		--use-core --with-derive-default --ctypes-prefix core::ffi --no-layout-tests \
+		--no-debug '.*' --enable-function-attribute-detection \
+		--experimental --wrap-static-fns \
+		--wrap-static-fns-path $(src)/extern.c \
+		$(bindgen_static_functions) \
+		-o $@ -- $(bindgen_c_flags_final) -DMODULE \
+		$(bindgen_target_cflags) $(bindgen_target_extra)
+
+$(src)/extern.c: $(obj)/bindings/bindings_generated_static.rs \
+	$(src)/bindings/bindings_helper.h
+	@sed -i 's|rust/bindings|bindings|g' $@
+
 $(obj)/bindings/bindings_generated.rs: private bindgen_target_flags = \
     $(shell grep -Ev '^#|^$$' $(src)/bindgen_parameters)
 $(obj)/bindings/bindings_generated.rs: private bindgen_target_extra = ; \
@@ -282,6 +300,15 @@ $(obj)/bindings/bindings_generated.rs: $(src)/bindings/bindings_helper.h \
     $(src)/bindgen_parameters FORCE
 	$(call if_changed_dep,bindgen)
 
+$(obj)/bindings/bindings_generated_static.rs: private bindgen_target_flags = \
+    $(shell grep -Ev '^#|^$$' $(src)/bindgen_parameters)
+$(obj)/bindings/bindings_generated_static.rs: private bindgen_static_functions = \
+    $(shell grep -Ev '^#|^$$' $(src)/bindgen_static_functions)
+$(obj)/bindings/bindings_generated_static.rs: $(src)/bindings/bindings_helper.h \
+	$(src)/bindgen_static_functions \
+    $(src)/bindgen_parameters FORCE
+	$(call if_changed_dep,bindgen_static)
+
 $(obj)/uapi/uapi_generated.rs: private bindgen_target_flags = \
     $(shell grep -Ev '^#|^$$' $(src)/bindgen_parameters)
 $(obj)/uapi/uapi_generated.rs: $(src)/uapi/uapi_helper.h \
@@ -325,6 +352,9 @@ $(obj)/exports_helpers_generated.h: $(obj)/helpers/helpers.o FORCE
 $(obj)/exports_bindings_generated.h: $(obj)/bindings.o FORCE
 	$(call if_changed,exports)
 
+$(obj)/exports_bindings_static_generated.h: $(obj)/extern.o FORCE
+	$(call if_changed,exports)
+
 $(obj)/exports_kernel_generated.h: $(obj)/kernel.o FORCE
 	$(call if_changed,exports)
 
@@ -400,6 +430,7 @@ $(obj)/build_error.o: $(src)/build_error.rs $(obj)/compiler_builtins.o FORCE
 $(obj)/bindings.o: $(src)/bindings/lib.rs \
     $(obj)/compiler_builtins.o \
     $(obj)/bindings/bindings_generated.rs \
+    $(obj)/bindings/bindings_generated_static.rs \
     $(obj)/bindings/bindings_helpers_generated.rs FORCE
 	+$(call if_changed_rule,rustc_library)
 
diff --git a/rust/bindgen_static_functions b/rust/bindgen_static_functions
new file mode 100644
index 000000000000..1f24360daeb3
--- /dev/null
+++ b/rust/bindgen_static_functions
@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0
+
+# Don't generate structs
+--blocklist-type '.*'
+
+--allowlist-function blk_mq_rq_to_pdu
diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs
index d6da3011281a..c991bcf607ec 100644
--- a/rust/bindings/lib.rs
+++ b/rust/bindings/lib.rs
@@ -34,6 +34,10 @@ mod bindings_raw {
         env!("OBJTREE"),
         "/rust/bindings/bindings_generated.rs"
     ));
+    include!(concat!(
+        env!("OBJTREE"),
+        "/rust/bindings/bindings_generated_static.rs"
+    ));
 }
 
 // When both a directly exposed symbol and a helper exists for the same function,
diff --git a/rust/exports.c b/rust/exports.c
index 587f0e776aba..288958d2ebea 100644
--- a/rust/exports.c
+++ b/rust/exports.c
@@ -18,6 +18,7 @@
 #include "exports_core_generated.h"
 #include "exports_helpers_generated.h"
 #include "exports_bindings_generated.h"
+#include "exports_bindings_static_generated.h"
 #include "exports_kernel_generated.h"
 
 // For modules using `rust/build_error.rs`.
-- 
2.47.0