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>

3 files changed, 122 insertions, 0 deletions

diff --git a/drivers/clk/imgtec/Kconfig b/drivers/clk/imgtec/Kconfig
new file mode 100644
index 000000000..c965fd091
--- /dev/null
+++ b/drivers/clk/imgtec/Kconfig
@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-2.0-only
+config COMMON_CLK_BOSTON
+	bool "Clock driver for MIPS Boston boards"
+	depends on MIPS || COMPILE_TEST
+	select MFD_SYSCON
+	help
+	  Enable this to support the system & CPU clocks on the MIPS Boston
+	  development board from Imagination Technologies. These are simple
+	  fixed rate clocks whose rate is determined by reading a platform
+	  provided register.
diff --git a/drivers/clk/imgtec/Makefile b/drivers/clk/imgtec/Makefile
new file mode 100644
index 000000000..d2e81426c
--- /dev/null
+++ b/drivers/clk/imgtec/Makefile
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+obj-$(CONFIG_COMMON_CLK_BOSTON)		+= clk-boston.o
diff --git a/drivers/clk/imgtec/clk-boston.c b/drivers/clk/imgtec/clk-boston.c
new file mode 100644
index 000000000..f28c3ab93
--- /dev/null
+++ b/drivers/clk/imgtec/clk-boston.c
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2016-2017 Imagination Technologies
+ * Author: Paul Burton <paul.burton@mips.com>
+ */
+
+#define pr_fmt(fmt) "clk-boston: " fmt
+
+#include <linux/clk-provider.h>
+#include <linux/kernel.h>
+#include <linux/of.h>
+#include <linux/regmap.h>
+#include <linux/slab.h>
+#include <linux/mfd/syscon.h>
+
+#include <dt-bindings/clock/boston-clock.h>
+
+#define BOSTON_PLAT_MMCMDIV		0x30
+# define BOSTON_PLAT_MMCMDIV_CLK0DIV	(0xff << 0)
+# define BOSTON_PLAT_MMCMDIV_INPUT	(0xff << 8)
+# define BOSTON_PLAT_MMCMDIV_MUL	(0xff << 16)
+# define BOSTON_PLAT_MMCMDIV_CLK1DIV	(0xff << 24)
+
+#define BOSTON_CLK_COUNT 3
+
+static u32 ext_field(u32 val, u32 mask)
+{
+	return (val & mask) >> (ffs(mask) - 1);
+}
+
+static void __init clk_boston_setup(struct device_node *np)
+{
+	unsigned long in_freq, cpu_freq, sys_freq;
+	uint mmcmdiv, mul, cpu_div, sys_div;
+	struct clk_hw_onecell_data *onecell;
+	struct regmap *regmap;
+	struct clk_hw *hw;
+	int err;
+
+	regmap = syscon_node_to_regmap(np->parent);
+	if (IS_ERR(regmap)) {
+		pr_err("failed to find regmap\n");
+		return;
+	}
+
+	err = regmap_read(regmap, BOSTON_PLAT_MMCMDIV, &mmcmdiv);
+	if (err) {
+		pr_err("failed to read mmcm_div register: %d\n", err);
+		return;
+	}
+
+	in_freq = ext_field(mmcmdiv, BOSTON_PLAT_MMCMDIV_INPUT) * 1000000;
+	mul = ext_field(mmcmdiv, BOSTON_PLAT_MMCMDIV_MUL);
+
+	sys_div = ext_field(mmcmdiv, BOSTON_PLAT_MMCMDIV_CLK0DIV);
+	sys_freq = mult_frac(in_freq, mul, sys_div);
+
+	cpu_div = ext_field(mmcmdiv, BOSTON_PLAT_MMCMDIV_CLK1DIV);
+	cpu_freq = mult_frac(in_freq, mul, cpu_div);
+
+	onecell = kzalloc_flex(*onecell, hws, BOSTON_CLK_COUNT);
+	if (!onecell)
+		return;
+
+	onecell->num = BOSTON_CLK_COUNT;
+
+	hw = clk_hw_register_fixed_rate(NULL, "input", NULL, 0, in_freq);
+	if (IS_ERR(hw)) {
+		pr_err("failed to register input clock: %pe\n", hw);
+		goto fail_input;
+	}
+	onecell->hws[BOSTON_CLK_INPUT] = hw;
+
+	hw = clk_hw_register_fixed_rate(NULL, "sys", "input", 0, sys_freq);
+	if (IS_ERR(hw)) {
+		pr_err("failed to register sys clock: %pe\n", hw);
+		goto fail_sys;
+	}
+	onecell->hws[BOSTON_CLK_SYS] = hw;
+
+	hw = clk_hw_register_fixed_rate(NULL, "cpu", "input", 0, cpu_freq);
+	if (IS_ERR(hw)) {
+		pr_err("failed to register cpu clock: %pe\n", hw);
+		goto fail_cpu;
+	}
+	onecell->hws[BOSTON_CLK_CPU] = hw;
+
+	err = of_clk_add_hw_provider(np, of_clk_hw_onecell_get, onecell);
+	if (err) {
+		pr_err("failed to add DT provider: %d\n", err);
+		goto fail_clk_add;
+	}
+
+	return;
+
+fail_clk_add:
+	clk_hw_unregister_fixed_rate(onecell->hws[BOSTON_CLK_CPU]);
+fail_cpu:
+	clk_hw_unregister_fixed_rate(onecell->hws[BOSTON_CLK_SYS]);
+fail_sys:
+	clk_hw_unregister_fixed_rate(onecell->hws[BOSTON_CLK_INPUT]);
+fail_input:
+	kfree(onecell);
+}
+
+/*
+ * Use CLK_OF_DECLARE so that this driver is probed early enough to provide the
+ * CPU frequency for use with the GIC or cop0 counters/timers.
+ */
+CLK_OF_DECLARE(clk_boston, "img,boston-clock", clk_boston_setup);