kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 48 insertions, 0 deletions

diff --git a/drivers/soc/apple/mailbox.h b/drivers/soc/apple/mailbox.h
new file mode 100644
index 000000000..f73a8913d
--- /dev/null
+++ b/drivers/soc/apple/mailbox.h
@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0-only OR MIT */
+/*
+ * Apple mailbox message format
+ *
+ * Copyright The Asahi Linux Contributors
+ */
+
+#ifndef _APPLE_MAILBOX_H_
+#define _APPLE_MAILBOX_H_
+
+#include <linux/device.h>
+#include <linux/types.h>
+
+/* encodes a single 96bit message sent over the single channel */
+struct apple_mbox_msg {
+	u64 msg0;
+	u32 msg1;
+};
+
+struct apple_mbox {
+	struct device *dev;
+	void __iomem *regs;
+	const struct apple_mbox_hw *hw;
+	bool active;
+
+	int irq_recv_not_empty;
+	int irq_send_empty;
+
+	spinlock_t rx_lock;
+	spinlock_t tx_lock;
+
+	struct completion tx_empty;
+
+	/** Receive callback for incoming messages */
+	void (*rx)(struct apple_mbox *mbox, struct apple_mbox_msg msg, void *cookie);
+	void *cookie;
+};
+
+struct apple_mbox *apple_mbox_get(struct device *dev, int index);
+struct apple_mbox *apple_mbox_get_byname(struct device *dev, const char *name);
+
+int apple_mbox_start(struct apple_mbox *mbox);
+void apple_mbox_stop(struct apple_mbox *mbox);
+int apple_mbox_poll(struct apple_mbox *mbox);
+int apple_mbox_send(struct apple_mbox *mbox, struct apple_mbox_msg msg,
+		    bool atomic);
+
+#endif