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, 142 insertions, 0 deletions

diff --git a/arch/powerpc/boot/ugecon.c b/arch/powerpc/boot/ugecon.c
new file mode 100644
index 000000000..938a38bd4
--- /dev/null
+++ b/arch/powerpc/boot/ugecon.c
@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * arch/powerpc/boot/ugecon.c
+ *
+ * USB Gecko bootwrapper console.
+ * Copyright (C) 2008-2009 The GameCube Linux Team
+ * Copyright (C) 2008,2009 Albert Herranz
+ */
+
+#include <stddef.h>
+#include "stdio.h"
+#include "types.h"
+#include "io.h"
+#include "ops.h"
+
+
+#define EXI_CLK_32MHZ           5
+
+#define EXI_CSR                 0x00
+#define   EXI_CSR_CLKMASK       (0x7<<4)
+#define     EXI_CSR_CLK_32MHZ   (EXI_CLK_32MHZ<<4)
+#define   EXI_CSR_CSMASK        (0x7<<7)
+#define     EXI_CSR_CS_0        (0x1<<7)  /* Chip Select 001 */
+
+#define EXI_CR                  0x0c
+#define   EXI_CR_TSTART         (1<<0)
+#define   EXI_CR_WRITE		(1<<2)
+#define   EXI_CR_READ_WRITE     (2<<2)
+#define   EXI_CR_TLEN(len)      (((len)-1)<<4)
+
+#define EXI_DATA                0x10
+
+
+/* virtual address base for input/output, retrieved from device tree */
+static void *ug_io_base;
+
+
+static u32 ug_io_transaction(u32 in)
+{
+	u32 *csr_reg = ug_io_base + EXI_CSR;
+	u32 *data_reg = ug_io_base + EXI_DATA;
+	u32 *cr_reg = ug_io_base + EXI_CR;
+	u32 csr, data, cr;
+
+	/* select */
+	csr = EXI_CSR_CLK_32MHZ | EXI_CSR_CS_0;
+	out_be32(csr_reg, csr);
+
+	/* read/write */
+	data = in;
+	out_be32(data_reg, data);
+	cr = EXI_CR_TLEN(2) | EXI_CR_READ_WRITE | EXI_CR_TSTART;
+	out_be32(cr_reg, cr);
+
+	while (in_be32(cr_reg) & EXI_CR_TSTART)
+		barrier();
+
+	/* deselect */
+	out_be32(csr_reg, 0);
+
+	data = in_be32(data_reg);
+	return data;
+}
+
+static int ug_is_txfifo_ready(void)
+{
+	return ug_io_transaction(0xc0000000) & 0x04000000;
+}
+
+static void ug_raw_putc(char ch)
+{
+	ug_io_transaction(0xb0000000 | (ch << 20));
+}
+
+static void ug_putc(char ch)
+{
+	int count = 16;
+
+	if (!ug_io_base)
+		return;
+
+	while (!ug_is_txfifo_ready() && count--)
+		barrier();
+	if (count >= 0)
+		ug_raw_putc(ch);
+}
+
+void ug_console_write(const char *buf, int len)
+{
+	char *b = (char *)buf;
+
+	while (len--) {
+		if (*b == '\n')
+			ug_putc('\r');
+		ug_putc(*b++);
+	}
+}
+
+static int ug_is_adapter_present(void)
+{
+	if (!ug_io_base)
+		return 0;
+	return ug_io_transaction(0x90000000) == 0x04700000;
+}
+
+static void *ug_grab_exi_io_base(void)
+{
+	u32 v;
+	void *devp;
+
+	devp = find_node_by_compatible(NULL, "nintendo,flipper-exi");
+	if (devp == NULL)
+		goto err_out;
+	if (getprop(devp, "virtual-reg", &v, sizeof(v)) != sizeof(v))
+		goto err_out;
+
+	return (void *)v;
+
+err_out:
+	return NULL;
+}
+
+void *ug_probe(void)
+{
+	void *exi_io_base;
+	int i;
+
+	exi_io_base = ug_grab_exi_io_base();
+	if (!exi_io_base)
+		return NULL;
+
+	/* look for a usbgecko on memcard slots A and B */
+	for (i = 0; i < 2; i++) {
+		ug_io_base = exi_io_base + 0x14 * i;
+		if (ug_is_adapter_present())
+			break;
+	}
+	if (i == 2)
+		ug_io_base = NULL;
+	return ug_io_base;
+}
+