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, 143 insertions, 0 deletions

diff --git a/drivers/video/fbdev/controlfb.h b/drivers/video/fbdev/controlfb.h
new file mode 100644
index 000000000..e779cca51
--- /dev/null
+++ b/drivers/video/fbdev/controlfb.h
@@ -0,0 +1,143 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * controlfb_hw.h: Constants of all sorts for controlfb
+ *
+ * Copyright (C) 1998 Daniel Jacobowitz <dan@debian.org>
+ *
+ * Based on an awful lot of code, including:
+ *
+ * control.c: Console support for PowerMac "control" display adaptor.
+ * Copyright (C) 1996 Paul Mackerras.
+ *
+ * The so far unpublished platinumfb.c
+ * Copyright (C) 1998 Jon Howell
+ */
+
+/*
+ * Structure of the registers for the RADACAL colormap device.
+ */
+struct cmap_regs {
+	unsigned char addr;	/* index for both cmap and misc registers */
+	char pad1[15];
+	unsigned char crsr;	/* cursor palette */
+	char pad2[15];
+	unsigned char dat;	/* RADACAL misc register data */
+	char pad3[15];
+	unsigned char lut;	/* cmap data */
+	char pad4[15];
+};
+
+/*
+ * Structure of the registers for the "control" display adaptor.
+ */
+#define PAD(x)	char x[12]
+
+struct preg {			/* padded register */
+	unsigned r;
+	char pad[12];
+};
+
+struct control_regs {
+	struct preg vcount;	/* vertical counter */
+	/* Vertical parameters are in units of 1/2 scan line */
+	struct preg vswin;	/* between vsblank and vssync */
+	struct preg vsblank;	/* vert start blank */
+	struct preg veblank;	/* vert end blank (display start) */
+	struct preg vewin;	/* between vesync and veblank */
+	struct preg vesync;	/* vert end sync */
+	struct preg vssync;	/* vert start sync */
+	struct preg vperiod;	/* vert period */
+	struct preg piped;	/* pipe delay hardware cursor */
+	/* Horizontal params are in units of 2 pixels */
+	struct preg hperiod;	/* horiz period - 2 */
+	struct preg hsblank;	/* horiz start blank */
+	struct preg heblank;	/* horiz end blank */
+	struct preg hesync;	/* horiz end sync */
+	struct preg hssync;	/* horiz start sync */
+	struct preg heq;	/* half horiz sync len */
+	struct preg hlfln;	/* half horiz period */
+	struct preg hserr;	/* horiz period - horiz sync len */
+	struct preg cnttst;
+	struct preg ctrl;	/* display control */
+	struct preg start_addr;	/* start address: 5 lsbs zero */
+	struct preg pitch;	/* addrs diff between scan lines */
+	struct preg mon_sense;	/* monitor sense bits */
+	struct preg vram_attr;	/* enable vram banks */
+	struct preg mode;
+	struct preg rfrcnt;	/* refresh count */
+	struct preg intr_ena;	/* interrupt enable */
+	struct preg intr_stat;	/* interrupt status */
+	struct preg res[5];
+};
+
+struct control_regints {
+	/* Vertical parameters are in units of 1/2 scan line */
+	unsigned vswin;	/* between vsblank and vssync */
+	unsigned vsblank;	/* vert start blank */
+	unsigned veblank;	/* vert end blank (display start) */
+	unsigned vewin;	/* between vesync and veblank */
+	unsigned vesync;	/* vert end sync */
+	unsigned vssync;	/* vert start sync */
+	unsigned vperiod;	/* vert period */
+	unsigned piped;		/* pipe delay hardware cursor */
+	/* Horizontal params are in units of 2 pixels */
+	/* Except, apparently, for hres > 1024 (or == 1280?) */
+	unsigned hperiod;	/* horiz period - 2 */
+	unsigned hsblank;	/* horiz start blank */
+	unsigned heblank;	/* horiz end blank */
+	unsigned hesync;	/* horiz end sync */
+	unsigned hssync;	/* horiz start sync */
+	unsigned heq;		/* half horiz sync len */
+	unsigned hlfln;		/* half horiz period */
+	unsigned hserr;		/* horiz period - horiz sync len */
+};
+	
+/*
+ * Dot clock rate is
+ * 3.9064MHz * 2**clock_params[2] * clock_params[1] / clock_params[0].
+ */
+struct control_regvals {
+	unsigned regs[16];		/* for vswin .. hserr */
+	unsigned char mode;
+	unsigned char radacal_ctrl;
+	unsigned char clock_params[3];
+};
+
+#define CTRLFB_OFF 16	/* position of pixel 0 in frame buffer */
+
+
+/*
+ * Best cmode supported by control
+ */
+struct max_cmodes {
+	int m[2];	/* 0: 2MB vram, 1: 4MB vram */
+};
+
+/*
+ * Video modes supported by macmodes.c
+ */
+static struct max_cmodes control_mac_modes[] = {
+	{{-1,-1}},	/* 512x384, 60Hz interlaced (NTSC) */
+	{{-1,-1}},	/* 512x384, 60Hz */
+	{{-1,-1}},	/* 640x480, 50Hz interlaced (PAL) */
+	{{-1,-1}},	/* 640x480, 60Hz interlaced (NTSC) */
+	{{ 2, 2}},	/* 640x480, 60Hz (VGA) */
+	{{ 2, 2}},	/* 640x480, 67Hz */
+	{{-1,-1}},	/* 640x870, 75Hz (portrait) */
+	{{-1,-1}},	/* 768x576, 50Hz (PAL full frame) */
+	{{ 2, 2}},	/* 800x600, 56Hz */
+	{{ 2, 2}},	/* 800x600, 60Hz */
+	{{ 2, 2}},	/* 800x600, 72Hz */
+	{{ 2, 2}},	/* 800x600, 75Hz */
+	{{ 1, 2}},	/* 832x624, 75Hz */
+	{{ 1, 2}},	/* 1024x768, 60Hz */
+	{{ 1, 2}},	/* 1024x768, 70Hz (or 72Hz?) */
+	{{ 1, 2}},	/* 1024x768, 75Hz (VESA) */
+	{{ 1, 2}},	/* 1024x768, 75Hz */
+	{{ 1, 2}},	/* 1152x870, 75Hz */
+	{{ 0, 1}},	/* 1280x960, 75Hz */
+	{{ 0, 1}},	/* 1280x1024, 75Hz */
+	{{ 1, 2}},	/* 1152x768, 60Hz */
+	{{ 0, 1}},	/* 1600x1024, 60Hz */
+};
+