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/cpufreq/speedstep-lib.h b/drivers/cpufreq/speedstep-lib.h
new file mode 100644
index 000000000..48329647d
--- /dev/null
+++ b/drivers/cpufreq/speedstep-lib.h
@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * (C) 2002 - 2003 Dominik Brodowski <linux@brodo.de>
+ *
+ *  Library for common functions for Intel SpeedStep v.1 and v.2 support
+ *
+ *  BIG FAT DISCLAIMER: Work in progress code. Possibly *dangerous*
+ */
+
+
+
+/* processors */
+enum speedstep_processor {
+	SPEEDSTEP_CPU_PIII_C_EARLY = 0x00000001,  /* Coppermine core */
+	SPEEDSTEP_CPU_PIII_C	   = 0x00000002,  /* Coppermine core */
+	SPEEDSTEP_CPU_PIII_T	   = 0x00000003,  /* Tualatin core */
+	SPEEDSTEP_CPU_P4M	   = 0x00000004,  /* P4-M  */
+/* the following processors are not speedstep-capable and are not auto-detected
+ * in speedstep_detect_processor(). However, their speed can be detected using
+ * the speedstep_get_frequency() call. */
+	SPEEDSTEP_CPU_PM	   = 0xFFFFFF03,  /* Pentium M  */
+	SPEEDSTEP_CPU_P4D	   = 0xFFFFFF04,  /* desktop P4  */
+	SPEEDSTEP_CPU_PCORE	   = 0xFFFFFF05,  /* Core */
+};
+
+/* speedstep states -- only two of them */
+
+#define SPEEDSTEP_HIGH	0x00000000
+#define SPEEDSTEP_LOW	0x00000001
+
+
+/* detect a speedstep-capable processor */
+extern enum speedstep_processor speedstep_detect_processor(void);
+
+/* detect the current speed (in khz) of the processor */
+extern unsigned int speedstep_get_frequency(enum speedstep_processor processor);
+
+
+/* detect the low and high speeds of the processor. The callback
+ * set_state"'s first argument is either SPEEDSTEP_HIGH or
+ * SPEEDSTEP_LOW; the second argument is zero so that no
+ * cpufreq_notify_transition calls are initiated.
+ */
+extern int speedstep_get_freqs(enum speedstep_processor processor,
+			       unsigned int *low_speed,
+			       unsigned int *high_speed,
+			       unsigned int *transition_latency,
+			       void (*set_state)(unsigned int state));