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, 259 insertions, 0 deletions

diff --git a/include/drm/drm_fixed.h b/include/drm/drm_fixed.h
new file mode 100644
index 000000000..21d822aee
--- /dev/null
+++ b/include/drm/drm_fixed.h
@@ -0,0 +1,259 @@
+/*
+ * Copyright 2009 Red Hat Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE COPYRIGHT HOLDER(S) OR AUTHOR(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * Authors: Dave Airlie
+ *          Christian König
+ */
+#ifndef DRM_FIXED_H
+#define DRM_FIXED_H
+
+#include <linux/math64.h>
+#include <linux/types.h>
+#include <linux/wordpart.h>
+
+typedef union dfixed {
+	u32 full;
+} fixed20_12;
+
+
+#define dfixed_const(A) (u32)(((A) << 12))/*  + ((B + 0.000122)*4096)) */
+#define dfixed_const_half(A) (u32)(((A) << 12) + 2048)
+#define dfixed_const_666(A) (u32)(((A) << 12) + 2731)
+#define dfixed_const_8(A) (u32)(((A) << 12) + 3277)
+#define dfixed_mul(A, B) ((u64)((u64)(A).full * (B).full + 2048) >> 12)
+#define dfixed_init(A) { .full = dfixed_const((A)) }
+#define dfixed_init_half(A) { .full = dfixed_const_half((A)) }
+#define dfixed_trunc(A) ((A).full >> 12)
+#define dfixed_frac(A) ((A).full & ((1 << 12) - 1))
+
+static inline u32 dfixed_floor(fixed20_12 A)
+{
+	u32 non_frac = dfixed_trunc(A);
+
+	return dfixed_const(non_frac);
+}
+
+static inline u32 dfixed_ceil(fixed20_12 A)
+{
+	u32 non_frac = dfixed_trunc(A);
+
+	if (A.full > dfixed_const(non_frac))
+		return dfixed_const(non_frac + 1);
+	else
+		return dfixed_const(non_frac);
+}
+
+static inline u32 dfixed_div(fixed20_12 A, fixed20_12 B)
+{
+	u64 tmp = ((u64)A.full << 13);
+
+	do_div(tmp, B.full);
+	tmp += 1;
+	tmp /= 2;
+	return lower_32_bits(tmp);
+}
+
+#define DRM_FIXED_POINT		32
+#define DRM_FIXED_ONE		(1ULL << DRM_FIXED_POINT)
+#define DRM_FIXED_DECIMAL_MASK	(DRM_FIXED_ONE - 1)
+#define DRM_FIXED_DIGITS_MASK	(~DRM_FIXED_DECIMAL_MASK)
+#define DRM_FIXED_EPSILON	1LL
+#define DRM_FIXED_ALMOST_ONE	(DRM_FIXED_ONE - DRM_FIXED_EPSILON)
+
+/**
+ * drm_sm2fixp() - convert signed-magnitude to fixed point
+ * @a: 1.31.32 signed-magnitude fixed point
+ *
+ * Convert a 1.31.32 signed-magnitude fixed point to 32.32
+ * 2s-complement fixed point
+ *
+ * @return s64 2s-complement fixed point
+ */
+static inline s64 drm_sm2fixp(__u64 a)
+{
+	if ((a & (1LL << 63))) {
+		return -(a & 0x7fffffffffffffffll);
+	} else {
+		return a;
+	}
+}
+
+static inline s64 drm_int2fixp(int a)
+{
+	return ((s64)a) << DRM_FIXED_POINT;
+}
+
+static inline int drm_fixp2int(s64 a)
+{
+	return ((s64)a) >> DRM_FIXED_POINT;
+}
+
+static inline int drm_fixp2int_round(s64 a)
+{
+	return drm_fixp2int(a + DRM_FIXED_ONE / 2);
+}
+
+static inline int drm_fixp2int_ceil(s64 a)
+{
+	if (a >= 0)
+		return drm_fixp2int(a + DRM_FIXED_ALMOST_ONE);
+	else
+		return drm_fixp2int(a - DRM_FIXED_ALMOST_ONE);
+}
+
+static inline unsigned drm_fixp_msbset(s64 a)
+{
+	unsigned shift, sign = (a >> 63) & 1;
+
+	for (shift = 62; shift > 0; --shift)
+		if (((a >> shift) & 1) != sign)
+			return shift;
+
+	return 0;
+}
+
+static inline s64 drm_fixp_mul(s64 a, s64 b)
+{
+	unsigned shift = drm_fixp_msbset(a) + drm_fixp_msbset(b);
+	s64 result;
+
+	if (shift > 61) {
+		shift = shift - 61;
+		a >>= (shift >> 1) + (shift & 1);
+		b >>= shift >> 1;
+	} else
+		shift = 0;
+
+	result = a * b;
+
+	if (shift > DRM_FIXED_POINT)
+		return result << (shift - DRM_FIXED_POINT);
+
+	if (shift < DRM_FIXED_POINT)
+		return result >> (DRM_FIXED_POINT - shift);
+
+	return result;
+}
+
+static inline s64 drm_fixp_div(s64 a, s64 b)
+{
+	unsigned shift = 62 - drm_fixp_msbset(a);
+	s64 result;
+
+	a <<= shift;
+
+	if (shift < DRM_FIXED_POINT)
+		b >>= (DRM_FIXED_POINT - shift);
+
+	result = div64_s64(a, b);
+
+	if (shift > DRM_FIXED_POINT)
+		return result >> (shift - DRM_FIXED_POINT);
+
+	return result;
+}
+
+static inline s64 drm_fixp_from_fraction(s64 a, s64 b)
+{
+	s64 res;
+	bool a_neg = a < 0;
+	bool b_neg = b < 0;
+	u64 a_abs = a_neg ? -a : a;
+	u64 b_abs = b_neg ? -b : b;
+	u64 rem;
+
+	/* determine integer part */
+	u64 res_abs  = div64_u64_rem(a_abs, b_abs, &rem);
+
+	/* determine fractional part */
+	{
+		u32 i = DRM_FIXED_POINT;
+
+		do {
+			rem <<= 1;
+			res_abs <<= 1;
+			if (rem >= b_abs) {
+				res_abs |= 1;
+				rem -= b_abs;
+			}
+		} while (--i != 0);
+	}
+
+	/* round up LSB */
+	{
+		u64 summand = (rem << 1) >= b_abs;
+
+		res_abs += summand;
+	}
+
+	res = (s64) res_abs;
+	if (a_neg ^ b_neg)
+		res = -res;
+	return res;
+}
+
+static inline s64 drm_fixp_exp(s64 x)
+{
+	s64 tolerance = div64_s64(DRM_FIXED_ONE, 1000000);
+	s64 sum = DRM_FIXED_ONE, term, y = x;
+	u64 count = 1;
+
+	if (x < 0)
+		y = -1 * x;
+
+	term = y;
+
+	while (term >= tolerance) {
+		sum = sum + term;
+		count = count + 1;
+		term = drm_fixp_mul(term, div64_s64(y, count));
+	}
+
+	if (x < 0)
+		sum = drm_fixp_div(DRM_FIXED_ONE, sum);
+
+	return sum;
+}
+
+static inline int fxp_q4_from_int(int val_int)
+{
+	return val_int << 4;
+}
+
+static inline int fxp_q4_to_int(int val_q4)
+{
+	return val_q4 >> 4;
+}
+
+static inline int fxp_q4_to_int_roundup(int val_q4)
+{
+	return (val_q4 + 0xf) >> 4;
+}
+
+static inline int fxp_q4_to_frac(int val_q4)
+{
+	return val_q4 & 0xf;
+}
+
+#define FXP_Q4_FMT		"%d.%04d"
+#define FXP_Q4_ARGS(val_q4)	fxp_q4_to_int(val_q4), (fxp_q4_to_frac(val_q4) * 625)
+
+#endif