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, 162 insertions, 0 deletions

diff --git a/arch/m68k/coldfire/pit.c b/arch/m68k/coldfire/pit.c
new file mode 100644
index 000000000..de4eaf8db
--- /dev/null
+++ b/arch/m68k/coldfire/pit.c
@@ -0,0 +1,162 @@
+// SPDX-License-Identifier: GPL-2.0
+/***************************************************************************/
+
+/*
+ *	pit.c -- Freescale ColdFire PIT timer. Currently this type of
+ *	         hardware timer only exists in the Freescale ColdFire
+ *		 5270/5271, 5282 and 5208 CPUs. No doubt newer ColdFire
+ *		 family members will probably use it too.
+ *
+ *	Copyright (C) 1999-2008, Greg Ungerer (gerg@snapgear.com)
+ *	Copyright (C) 2001-2004, SnapGear Inc. (www.snapgear.com)
+ */
+
+/***************************************************************************/
+
+#include <linux/kernel.h>
+#include <linux/sched.h>
+#include <linux/param.h>
+#include <linux/init.h>
+#include <linux/interrupt.h>
+#include <linux/irq.h>
+#include <linux/clockchips.h>
+#include <asm/machdep.h>
+#include <asm/io.h>
+#include <asm/coldfire.h>
+#include <asm/mcfpit.h>
+#include <asm/mcfsim.h>
+
+/***************************************************************************/
+
+/*
+ *	By default use timer1 as the system clock timer.
+ */
+#define	FREQ	((MCF_CLK / 2) / 64)
+#define	TA(a)	(MCFPIT_BASE1 + (a))
+#define PIT_CYCLES_PER_JIFFY (FREQ / HZ)
+
+static u32 pit_cnt;
+
+/*
+ * Initialize the PIT timer.
+ *
+ * This is also called after resume to bring the PIT into operation again.
+ */
+
+static int cf_pit_set_periodic(struct clock_event_device *evt)
+{
+	mcf_write16(MCFPIT_PCSR_DISABLE, TA(MCFPIT_PCSR));
+	mcf_write16(PIT_CYCLES_PER_JIFFY, TA(MCFPIT_PMR));
+	mcf_write16(MCFPIT_PCSR_EN | MCFPIT_PCSR_PIE |
+		     MCFPIT_PCSR_OVW | MCFPIT_PCSR_RLD |
+		     MCFPIT_PCSR_CLK64, TA(MCFPIT_PCSR));
+	return 0;
+}
+
+static int cf_pit_set_oneshot(struct clock_event_device *evt)
+{
+	mcf_write16(MCFPIT_PCSR_DISABLE, TA(MCFPIT_PCSR));
+	mcf_write16(MCFPIT_PCSR_EN | MCFPIT_PCSR_PIE |
+		     MCFPIT_PCSR_OVW | MCFPIT_PCSR_CLK64, TA(MCFPIT_PCSR));
+	return 0;
+}
+
+static int cf_pit_shutdown(struct clock_event_device *evt)
+{
+	mcf_write16(MCFPIT_PCSR_DISABLE, TA(MCFPIT_PCSR));
+	return 0;
+}
+
+/*
+ * Program the next event in oneshot mode
+ *
+ * Delta is given in PIT ticks
+ */
+static int cf_pit_next_event(unsigned long delta,
+		struct clock_event_device *evt)
+{
+	mcf_write16(delta, TA(MCFPIT_PMR));
+	return 0;
+}
+
+struct clock_event_device cf_pit_clockevent = {
+	.name			= "pit",
+	.features		= CLOCK_EVT_FEAT_PERIODIC |
+				  CLOCK_EVT_FEAT_ONESHOT,
+	.set_state_shutdown	= cf_pit_shutdown,
+	.set_state_periodic	= cf_pit_set_periodic,
+	.set_state_oneshot	= cf_pit_set_oneshot,
+	.set_next_event		= cf_pit_next_event,
+	.shift			= 32,
+	.irq			= MCF_IRQ_PIT1,
+};
+
+
+
+/***************************************************************************/
+
+static irqreturn_t pit_tick(int irq, void *dummy)
+{
+	struct clock_event_device *evt = &cf_pit_clockevent;
+	u16 pcsr;
+
+	/* Reset the ColdFire timer */
+	pcsr = mcf_read16(TA(MCFPIT_PCSR));
+	mcf_write16(pcsr | MCFPIT_PCSR_PIF, TA(MCFPIT_PCSR));
+
+	pit_cnt += PIT_CYCLES_PER_JIFFY;
+	evt->event_handler(evt);
+	return IRQ_HANDLED;
+}
+
+/***************************************************************************/
+
+static u64 pit_read_clk(struct clocksource *cs)
+{
+	unsigned long flags;
+	u32 cycles;
+	u16 pcntr;
+
+	local_irq_save(flags);
+	pcntr = mcf_read16(TA(MCFPIT_PCNTR));
+	cycles = pit_cnt;
+	local_irq_restore(flags);
+
+	return cycles + PIT_CYCLES_PER_JIFFY - pcntr;
+}
+
+/***************************************************************************/
+
+static struct clocksource pit_clk = {
+	.name	= "pit",
+	.rating	= 100,
+	.read	= pit_read_clk,
+	.mask	= CLOCKSOURCE_MASK(32),
+};
+
+/***************************************************************************/
+
+void hw_timer_init(void)
+{
+	int ret;
+
+	cf_pit_clockevent.cpumask = cpumask_of(smp_processor_id());
+	cf_pit_clockevent.mult = div_sc(FREQ, NSEC_PER_SEC, 32);
+	cf_pit_clockevent.max_delta_ns =
+		clockevent_delta2ns(0xFFFF, &cf_pit_clockevent);
+	cf_pit_clockevent.max_delta_ticks = 0xFFFF;
+	cf_pit_clockevent.min_delta_ns =
+		clockevent_delta2ns(0x3f, &cf_pit_clockevent);
+	cf_pit_clockevent.min_delta_ticks = 0x3f;
+	clockevents_register_device(&cf_pit_clockevent);
+
+	ret = request_irq(MCF_IRQ_PIT1, pit_tick, IRQF_TIMER, "timer", NULL);
+	if (ret) {
+		pr_err("Failed to request irq %d (timer): %pe\n", MCF_IRQ_PIT1,
+		       ERR_PTR(ret));
+	}
+
+	clocksource_register_hz(&pit_clk, FREQ);
+}
+
+/***************************************************************************/