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, 192 insertions, 0 deletions

diff --git a/drivers/input/misc/max7360-rotary.c b/drivers/input/misc/max7360-rotary.c
new file mode 100644
index 000000000..385831ef3
--- /dev/null
+++ b/drivers/input/misc/max7360-rotary.c
@@ -0,0 +1,192 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2025 Bootlin
+ *
+ * Author: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
+ */
+
+#include <linux/bitfield.h>
+#include <linux/device/devres.h>
+#include <linux/dev_printk.h>
+#include <linux/init.h>
+#include <linux/input.h>
+#include <linux/interrupt.h>
+#include <linux/mfd/max7360.h>
+#include <linux/property.h>
+#include <linux/platform_device.h>
+#include <linux/pm_wakeirq.h>
+#include <linux/regmap.h>
+#include <linux/types.h>
+
+#define MAX7360_ROTARY_DEFAULT_STEPS 24
+
+struct max7360_rotary {
+	struct input_dev *input;
+	struct regmap *regmap;
+	unsigned int debounce_ms;
+
+	unsigned int pos;
+
+	u32 steps;
+	u32 axis;
+	bool relative_axis;
+	bool rollover;
+};
+
+static void max7360_rotary_report_event(struct max7360_rotary *max7360_rotary, int steps)
+{
+	if (max7360_rotary->relative_axis) {
+		input_report_rel(max7360_rotary->input, max7360_rotary->axis, steps);
+	} else {
+		int pos = max7360_rotary->pos;
+		int maxval = max7360_rotary->steps;
+
+		/*
+		 * Add steps to the position.
+		 * Make sure added steps are always in ]-maxval; maxval[
+		 * interval, so (pos + maxval) is always >= 0.
+		 * Then set back pos to the [0; maxval[ interval.
+		 */
+		pos += steps % maxval;
+		if (max7360_rotary->rollover)
+			pos = (pos + maxval) % maxval;
+		else
+			pos = clamp(pos, 0, maxval - 1);
+
+		max7360_rotary->pos = pos;
+		input_report_abs(max7360_rotary->input, max7360_rotary->axis, max7360_rotary->pos);
+	}
+
+	input_sync(max7360_rotary->input);
+}
+
+static irqreturn_t max7360_rotary_irq(int irq, void *data)
+{
+	struct max7360_rotary *max7360_rotary = data;
+	struct device *dev = max7360_rotary->input->dev.parent;
+	unsigned int val;
+	int error;
+
+	error = regmap_read(max7360_rotary->regmap, MAX7360_REG_RTR_CNT, &val);
+	if (error < 0) {
+		dev_err(dev, "Failed to read rotary counter\n");
+		return IRQ_NONE;
+	}
+
+	if (val == 0)
+		return IRQ_NONE;
+
+	max7360_rotary_report_event(max7360_rotary, sign_extend32(val, 7));
+
+	return IRQ_HANDLED;
+}
+
+static int max7360_rotary_hw_init(struct max7360_rotary *max7360_rotary)
+{
+	struct device *dev = max7360_rotary->input->dev.parent;
+	int val;
+	int error;
+
+	val = FIELD_PREP(MAX7360_ROT_DEBOUNCE, max7360_rotary->debounce_ms) |
+	      FIELD_PREP(MAX7360_ROT_INTCNT, 1) | MAX7360_ROT_INTCNT_DLY;
+	error = regmap_write(max7360_rotary->regmap, MAX7360_REG_RTRCFG, val);
+	if (error)
+		dev_err(dev, "Failed to set max7360 rotary encoder configuration\n");
+
+	return error;
+}
+
+static int max7360_rotary_probe(struct platform_device *pdev)
+{
+	struct max7360_rotary *max7360_rotary;
+	struct device *dev = &pdev->dev;
+	struct input_dev *input;
+	struct regmap *regmap;
+	int irq;
+	int error;
+
+	regmap = dev_get_regmap(dev->parent, NULL);
+	if (!regmap)
+		return dev_err_probe(dev, -ENODEV, "Could not get parent regmap\n");
+
+	irq = fwnode_irq_get_byname(dev_fwnode(dev->parent), "inti");
+	if (irq < 0)
+		return dev_err_probe(dev, irq, "Failed to get IRQ\n");
+
+	max7360_rotary = devm_kzalloc(dev, sizeof(*max7360_rotary), GFP_KERNEL);
+	if (!max7360_rotary)
+		return -ENOMEM;
+
+	max7360_rotary->regmap = regmap;
+
+	device_property_read_u32(dev->parent, "linux,axis", &max7360_rotary->axis);
+	max7360_rotary->rollover = device_property_read_bool(dev->parent,
+							     "rotary-encoder,rollover");
+	max7360_rotary->relative_axis =
+		device_property_read_bool(dev->parent, "rotary-encoder,relative-axis");
+
+	error = device_property_read_u32(dev->parent, "rotary-encoder,steps",
+					 &max7360_rotary->steps);
+	if (error)
+		max7360_rotary->steps = MAX7360_ROTARY_DEFAULT_STEPS;
+
+	device_property_read_u32(dev->parent, "rotary-debounce-delay-ms",
+				 &max7360_rotary->debounce_ms);
+	if (max7360_rotary->debounce_ms > MAX7360_ROT_DEBOUNCE_MAX)
+		return dev_err_probe(dev, -EINVAL, "Invalid debounce timing: %u\n",
+				     max7360_rotary->debounce_ms);
+
+	input = devm_input_allocate_device(dev);
+	if (!input)
+		return -ENOMEM;
+
+	max7360_rotary->input = input;
+
+	input->id.bustype = BUS_I2C;
+	input->name = pdev->name;
+
+	if (max7360_rotary->relative_axis)
+		input_set_capability(input, EV_REL, max7360_rotary->axis);
+	else
+		input_set_abs_params(input, max7360_rotary->axis, 0, max7360_rotary->steps, 0, 1);
+
+	error = devm_request_threaded_irq(dev, irq, NULL, max7360_rotary_irq,
+					  IRQF_ONESHOT | IRQF_SHARED,
+					  "max7360-rotary", max7360_rotary);
+	if (error)
+		return dev_err_probe(dev, error, "Failed to register interrupt\n");
+
+	error = input_register_device(input);
+	if (error)
+		return dev_err_probe(dev, error, "Could not register input device\n");
+
+	error = max7360_rotary_hw_init(max7360_rotary);
+	if (error)
+		return dev_err_probe(dev, error, "Failed to initialize max7360 rotary\n");
+
+	device_init_wakeup(dev, true);
+	error = dev_pm_set_wake_irq(dev, irq);
+	if (error)
+		dev_warn(dev, "Failed to set up wakeup irq: %d\n", error);
+
+	return 0;
+}
+
+static void max7360_rotary_remove(struct platform_device *pdev)
+{
+	dev_pm_clear_wake_irq(&pdev->dev);
+	device_init_wakeup(&pdev->dev, false);
+}
+
+static struct platform_driver max7360_rotary_driver = {
+	.driver = {
+		.name	= "max7360-rotary",
+	},
+	.probe		= max7360_rotary_probe,
+	.remove		= max7360_rotary_remove,
+};
+module_platform_driver(max7360_rotary_driver);
+
+MODULE_DESCRIPTION("MAX7360 Rotary driver");
+MODULE_AUTHOR("Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>");
+MODULE_LICENSE("GPL");