panic_qr: use new #[export] macro

This validates at compile time that the signatures match what is in the
header file. It highlights one annoyance with the compile-time check,
which is that it can only be used with functions marked unsafe.

If the function is not unsafe, then this error is emitted:

error[E0308]: `if` and `else` have incompatible types
   --> <linux>/drivers/gpu/drm/drm_panic_qr.rs:987:19
    |
986 | #[export]
    | --------- expected because of this
987 | pub extern "C" fn drm_panic_qr_max_data_size(version: u8, url_len: usize) -> usize {
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected unsafe fn, found safe fn
    |
    = note: expected fn item `unsafe extern "C" fn(_, _) -> _ {kernel::bindings::drm_panic_qr_max_data_size}`
               found fn item `extern "C" fn(_, _) -> _ {drm_panic_qr_max_data_size}`

The signature declarations are moved to a header file so it can be
included in the Rust bindings helper, and the extern keyword is removed
as it is unnecessary.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250303-export-macro-v3-5-41fbad85a27f@google.com
[ Fixed `rustfmt`. Moved on top the unsafe requirement comment to follow
  the usual style, and slightly reworded it for clarity. Formatted
  bindings helper comment. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

drivers/gpu/drm/drm_panic.c

@@ -486,11 +486,6 @@ static void drm_panic_qr_exit(void)
 	stream.workspace = NULL;
 }
 
-extern size_t drm_panic_qr_max_data_size(u8 version, size_t url_len);
-
-extern u8 drm_panic_qr_generate(const char *url, u8 *data, size_t data_len, size_t data_size,
-				u8 *tmp, size_t tmp_size);
-
 static int drm_panic_get_qr_code_url(u8 **qr_image)
 {
 	struct kmsg_dump_iter iter;

drivers/gpu/drm/drm_panic_qr.rs

@@ -27,7 +27,7 @@
 //! * <https://github.com/bjguillot/qr>
 
 use core::cmp;
-use kernel::str::CStr;
+use kernel::{prelude::*, str::CStr};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Ord, PartialOrd)]
 struct Version(usize);
@@ -929,7 +929,7 @@ impl QrImage<'_> {
 /// * `tmp` must be valid for reading and writing for `tmp_size` bytes.
 ///
 /// They must remain valid for the duration of the function call.
-#[no_mangle]
+#[export]
 pub unsafe extern "C" fn drm_panic_qr_generate(
     url: *const kernel::ffi::c_char,
     data: *mut u8,
@@ -980,8 +980,13 @@ pub unsafe extern "C" fn drm_panic_qr_generate(
 /// * If `url_len` > 0, remove the 2 segments header/length and also count the
 ///   conversion to numeric segments.
 /// * If `url_len` = 0, only removes 3 bytes for 1 binary segment.
-#[no_mangle]
-pub extern "C" fn drm_panic_qr_max_data_size(version: u8, url_len: usize) -> usize {
+///
+/// # Safety
+///
+/// Always safe to call.
+// Required to be unsafe due to the `#[export]` annotation.
+#[export]
+pub unsafe extern "C" fn drm_panic_qr_max_data_size(version: u8, url_len: usize) -> usize {
     #[expect(clippy::manual_range_contains)]
     if version < 1 || version > 40 {
         return 0;

include/drm/drm_panic.h

@@ -163,4 +163,11 @@ static inline void drm_panic_unlock(struct drm_device *dev, unsigned long flags)
 
 #endif
 
+#if defined(CONFIG_DRM_PANIC_SCREEN_QR_CODE)
+size_t drm_panic_qr_max_data_size(u8 version, size_t url_len);
+
+u8 drm_panic_qr_generate(const char *url, u8 *data, size_t data_len, size_t data_size,
+			 u8 *tmp, size_t tmp_size);
+#endif
+
 #endif /* __DRM_PANIC_H__ */

rust/bindings/bindings_helper.h

@@ -37,6 +37,11 @@
 #include <linux/workqueue.h>
 #include <trace/events/rust_sample.h>
 
+#if defined(CONFIG_DRM_PANIC_SCREEN_QR_CODE)
+// Used by `#[export]` in `drivers/gpu/drm/drm_panic_qr.rs`.
+#include <drm/drm_panic.h>
+#endif
+
 /* `bindgen` gets confused at certain things. */
 const size_t RUST_CONST_HELPER_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;
 const size_t RUST_CONST_HELPER_PAGE_SIZE = PAGE_SIZE;