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, 137 insertions, 0 deletions

diff --git a/drivers/leds/leds-syscon.c b/drivers/leds/leds-syscon.c
new file mode 100644
index 000000000..d633ad519
--- /dev/null
+++ b/drivers/leds/leds-syscon.c
@@ -0,0 +1,137 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Generic Syscon LEDs Driver
+ *
+ * Copyright (c) 2014, Linaro Limited
+ * Author: Linus Walleij <linus.walleij@linaro.org>
+ */
+#include <linux/io.h>
+#include <linux/init.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/stat.h>
+#include <linux/slab.h>
+#include <linux/mfd/syscon.h>
+#include <linux/regmap.h>
+#include <linux/leds.h>
+
+/**
+ * struct syscon_led - state container for syscon based LEDs
+ * @cdev: LED class device for this LED
+ * @map: regmap to access the syscon device backing this LED
+ * @offset: the offset into the syscon regmap for the LED register
+ * @mask: the bit in the register corresponding to the LED
+ * @state: current state of the LED
+ */
+struct syscon_led {
+	struct led_classdev cdev;
+	struct regmap *map;
+	u32 offset;
+	u32 mask;
+	bool state;
+};
+
+static void syscon_led_set(struct led_classdev *led_cdev,
+	enum led_brightness value)
+{
+	struct syscon_led *sled =
+		container_of(led_cdev, struct syscon_led, cdev);
+	u32 val;
+	int ret;
+
+	if (value == LED_OFF) {
+		val = 0;
+		sled->state = false;
+	} else {
+		val = sled->mask;
+		sled->state = true;
+	}
+
+	ret = regmap_update_bits(sled->map, sled->offset, sled->mask, val);
+	if (ret < 0)
+		dev_err(sled->cdev.dev, "error updating LED status\n");
+}
+
+static int syscon_led_probe(struct platform_device *pdev)
+{
+	struct led_init_data init_data = {};
+	struct device *dev = &pdev->dev;
+	struct device_node *np = dev_of_node(dev);
+	struct device *parent;
+	struct regmap *map;
+	struct syscon_led *sled;
+	enum led_default_state state;
+	u32 value;
+	int ret;
+
+	parent = dev->parent;
+	if (!parent) {
+		dev_err(dev, "no parent for syscon LED\n");
+		return -ENODEV;
+	}
+	map = syscon_node_to_regmap(dev_of_node(parent));
+	if (IS_ERR(map)) {
+		dev_err(dev, "no regmap for syscon LED parent\n");
+		return PTR_ERR(map);
+	}
+
+	sled = devm_kzalloc(dev, sizeof(*sled), GFP_KERNEL);
+	if (!sled)
+		return -ENOMEM;
+
+	sled->map = map;
+
+	if (of_property_read_u32(np, "reg", &sled->offset) &&
+	    of_property_read_u32(np, "offset", &sled->offset))
+		return -EINVAL;
+	if (of_property_read_u32(np, "mask", &sled->mask))
+		return -EINVAL;
+
+	init_data.fwnode = of_fwnode_handle(np);
+
+	state = led_init_default_state_get(init_data.fwnode);
+	switch (state) {
+	case LEDS_DEFSTATE_ON:
+		ret = regmap_update_bits(map, sled->offset, sled->mask, sled->mask);
+		if (ret < 0)
+			return ret;
+		sled->state = true;
+		break;
+	case LEDS_DEFSTATE_KEEP:
+		ret = regmap_read(map, sled->offset, &value);
+		if (ret < 0)
+			return ret;
+		sled->state = !!(value & sled->mask);
+		break;
+	default:
+		ret = regmap_update_bits(map, sled->offset, sled->mask, 0);
+		if (ret < 0)
+			return ret;
+		sled->state = false;
+	}
+	sled->cdev.brightness_set = syscon_led_set;
+
+	ret = devm_led_classdev_register_ext(dev, &sled->cdev, &init_data);
+	if (ret < 0)
+		return ret;
+
+	platform_set_drvdata(pdev, sled);
+	dev_info(dev, "registered LED %s\n", sled->cdev.name);
+
+	return 0;
+}
+
+static const struct of_device_id of_syscon_leds_match[] = {
+	{ .compatible = "register-bit-led", },
+	{},
+};
+
+static struct platform_driver syscon_led_driver = {
+	.probe		= syscon_led_probe,
+	.driver		= {
+		.name	= "leds-syscon",
+		.of_match_table = of_syscon_leds_match,
+		.suppress_bind_attrs = true,
+	},
+};
+builtin_platform_driver(syscon_led_driver);