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, 240 insertions, 0 deletions

diff --git a/drivers/video/fbdev/bt431.h b/drivers/video/fbdev/bt431.h
new file mode 100644
index 000000000..3929602f5
--- /dev/null
+++ b/drivers/video/fbdev/bt431.h
@@ -0,0 +1,240 @@
+/*
+ *	linux/drivers/video/bt431.h
+ *
+ *	Copyright 2003  Thiemo Seufer <seufer@csv.ica.uni-stuttgart.de>
+ *	Copyright 2016  Maciej W. Rozycki <macro@linux-mips.org>
+ *
+ *	This file is subject to the terms and conditions of the GNU General
+ *	Public License. See the file COPYING in the main directory of this
+ *	archive for more details.
+ */
+#include <linux/types.h>
+
+#define BT431_CURSOR_SIZE	64
+
+/*
+ * Bt431 cursor generator registers, 32-bit aligned.
+ * Two twin Bt431 are used on the DECstation's PMAG-AA.
+ */
+struct bt431_regs {
+	volatile u16 addr_lo;
+	u16 pad0;
+	volatile u16 addr_hi;
+	u16 pad1;
+	volatile u16 addr_cmap;
+	u16 pad2;
+	volatile u16 addr_reg;
+	u16 pad3;
+};
+
+static inline u16 bt431_set_value(u8 val)
+{
+	return ((val << 8) | (val & 0xff)) & 0xffff;
+}
+
+static inline u8 bt431_get_value(u16 val)
+{
+	return val & 0xff;
+}
+
+/*
+ * Additional registers addressed indirectly.
+ */
+#define BT431_REG_CMD		0x0000
+#define BT431_REG_CXLO		0x0001
+#define BT431_REG_CXHI		0x0002
+#define BT431_REG_CYLO		0x0003
+#define BT431_REG_CYHI		0x0004
+#define BT431_REG_WXLO		0x0005
+#define BT431_REG_WXHI		0x0006
+#define BT431_REG_WYLO		0x0007
+#define BT431_REG_WYHI		0x0008
+#define BT431_REG_WWLO		0x0009
+#define BT431_REG_WWHI		0x000a
+#define BT431_REG_WHLO		0x000b
+#define BT431_REG_WHHI		0x000c
+
+#define BT431_REG_CRAM_BASE	0x0000
+#define BT431_REG_CRAM_END	0x01ff
+
+/*
+ * Command register.
+ */
+#define BT431_CMD_CURS_ENABLE	0x40
+#define BT431_CMD_XHAIR_ENABLE	0x20
+#define BT431_CMD_OR_CURSORS	0x10
+#define BT431_CMD_XOR_CURSORS	0x00
+#define BT431_CMD_1_1_MUX	0x00
+#define BT431_CMD_4_1_MUX	0x04
+#define BT431_CMD_5_1_MUX	0x08
+#define BT431_CMD_xxx_MUX	0x0c
+#define BT431_CMD_THICK_1	0x00
+#define BT431_CMD_THICK_3	0x01
+#define BT431_CMD_THICK_5	0x02
+#define BT431_CMD_THICK_7	0x03
+
+static inline void bt431_select_reg(struct bt431_regs *regs, int ir)
+{
+	/*
+	 * The compiler splits the write in two bytes without these
+	 * helper variables.
+	 */
+	volatile u16 *lo = &(regs->addr_lo);
+	volatile u16 *hi = &(regs->addr_hi);
+
+	mb();
+	*lo = bt431_set_value(ir & 0xff);
+	wmb();
+	*hi = bt431_set_value((ir >> 8) & 0xff);
+}
+
+/* Autoincrement read/write. */
+static inline u8 bt431_read_reg_inc(struct bt431_regs *regs)
+{
+	/*
+	 * The compiler splits the write in two bytes without the
+	 * helper variable.
+	 */
+	volatile u16 *r = &(regs->addr_reg);
+
+	mb();
+	return bt431_get_value(*r);
+}
+
+static inline void bt431_write_reg_inc(struct bt431_regs *regs, u8 value)
+{
+	/*
+	 * The compiler splits the write in two bytes without the
+	 * helper variable.
+	 */
+	volatile u16 *r = &(regs->addr_reg);
+
+	mb();
+	*r = bt431_set_value(value);
+}
+
+static inline u8 bt431_read_reg(struct bt431_regs *regs, int ir)
+{
+	bt431_select_reg(regs, ir);
+	return bt431_read_reg_inc(regs);
+}
+
+static inline void bt431_write_reg(struct bt431_regs *regs, int ir, u8 value)
+{
+	bt431_select_reg(regs, ir);
+	bt431_write_reg_inc(regs, value);
+}
+
+/* Autoincremented read/write for the cursor map. */
+static inline u16 bt431_read_cmap_inc(struct bt431_regs *regs)
+{
+	/*
+	 * The compiler splits the write in two bytes without the
+	 * helper variable.
+	 */
+	volatile u16 *r = &(regs->addr_cmap);
+
+	mb();
+	return *r;
+}
+
+static inline void bt431_write_cmap_inc(struct bt431_regs *regs, u16 value)
+{
+	/*
+	 * The compiler splits the write in two bytes without the
+	 * helper variable.
+	 */
+	volatile u16 *r = &(regs->addr_cmap);
+
+	mb();
+	*r = value;
+}
+
+static inline u16 bt431_read_cmap(struct bt431_regs *regs, int cr)
+{
+	bt431_select_reg(regs, cr);
+	return bt431_read_cmap_inc(regs);
+}
+
+static inline void bt431_write_cmap(struct bt431_regs *regs, int cr, u16 value)
+{
+	bt431_select_reg(regs, cr);
+	bt431_write_cmap_inc(regs, value);
+}
+
+static inline void bt431_enable_cursor(struct bt431_regs *regs)
+{
+	bt431_write_reg(regs, BT431_REG_CMD,
+			BT431_CMD_CURS_ENABLE | BT431_CMD_OR_CURSORS
+			| BT431_CMD_4_1_MUX | BT431_CMD_THICK_1);
+}
+
+static inline void bt431_erase_cursor(struct bt431_regs *regs)
+{
+	bt431_write_reg(regs, BT431_REG_CMD, BT431_CMD_4_1_MUX);
+}
+
+static inline void bt431_position_cursor(struct bt431_regs *regs, u16 x, u16 y)
+{
+	/*
+	 * Magic from the MACH sources.
+	 *
+	 * Cx = x + D + H - P
+	 *  P = 37 if 1:1, 52 if 4:1, 57 if 5:1
+	 *  D = pixel skew between outdata and external data
+	 *  H = pixels between HSYNCH falling and active video
+	 *
+	 * Cy = y + V - 32
+	 *  V = scanlines between HSYNCH falling, two or more
+	 *      clocks after VSYNCH falling, and active video
+	 */
+	x += 412 - 52;
+	y += 68 - 32;
+
+	/* Use autoincrement. */
+	bt431_select_reg(regs, BT431_REG_CXLO);
+	bt431_write_reg_inc(regs, x & 0xff); /* BT431_REG_CXLO */
+	bt431_write_reg_inc(regs, (x >> 8) & 0x0f); /* BT431_REG_CXHI */
+	bt431_write_reg_inc(regs, y & 0xff); /* BT431_REG_CYLO */
+	bt431_write_reg_inc(regs, (y >> 8) & 0x0f); /* BT431_REG_CYHI */
+}
+
+static inline void bt431_set_cursor(struct bt431_regs *regs,
+				    const char *data, const char *mask,
+				    u16 rop, u16 width, u16 height)
+{
+	u16 x, y;
+	int i;
+
+	i = 0;
+	width = DIV_ROUND_UP(width, 8);
+	bt431_select_reg(regs, BT431_REG_CRAM_BASE);
+	for (y = 0; y < BT431_CURSOR_SIZE; y++)
+		for (x = 0; x < BT431_CURSOR_SIZE / 8; x++) {
+			u16 val = 0;
+
+			if (y < height && x < width) {
+				val = mask[i];
+				if (rop == ROP_XOR)
+					val = (val << 8) | (val ^ data[i]);
+				else
+					val = (val << 8) | (val & data[i]);
+				i++;
+			}
+			bt431_write_cmap_inc(regs, val);
+		}
+}
+
+static inline void bt431_init_cursor(struct bt431_regs *regs)
+{
+	/* no crosshair window */
+	bt431_select_reg(regs, BT431_REG_WXLO);
+	bt431_write_reg_inc(regs, 0x00); /* BT431_REG_WXLO */
+	bt431_write_reg_inc(regs, 0x00); /* BT431_REG_WXHI */
+	bt431_write_reg_inc(regs, 0x00); /* BT431_REG_WYLO */
+	bt431_write_reg_inc(regs, 0x00); /* BT431_REG_WYHI */
+	bt431_write_reg_inc(regs, 0x00); /* BT431_REG_WWLO */
+	bt431_write_reg_inc(regs, 0x00); /* BT431_REG_WWHI */
+	bt431_write_reg_inc(regs, 0x00); /* BT431_REG_WHLO */
+	bt431_write_reg_inc(regs, 0x00); /* BT431_REG_WHHI */
+}