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, 226 insertions, 0 deletions

diff --git a/drivers/input/misc/mma8450.c b/drivers/input/misc/mma8450.c
new file mode 100644
index 000000000..403659a08
--- /dev/null
+++ b/drivers/input/misc/mma8450.c
@@ -0,0 +1,226 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Driver for Freescale's 3-Axis Accelerometer MMA8450
+ *
+ *  Copyright (C) 2011 Freescale Semiconductor, Inc. All Rights Reserved.
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/delay.h>
+#include <linux/i2c.h>
+#include <linux/input.h>
+
+#define MMA8450_DRV_NAME	"mma8450"
+
+#define MODE_CHANGE_DELAY_MS	100
+#define POLL_INTERVAL		100
+#define POLL_INTERVAL_MAX	500
+
+/* register definitions */
+#define MMA8450_STATUS		0x00
+#define MMA8450_STATUS_ZXYDR	0x08
+
+#define MMA8450_OUT_X8		0x01
+#define MMA8450_OUT_Y8		0x02
+#define MMA8450_OUT_Z8		0x03
+
+#define MMA8450_OUT_X_LSB	0x05
+#define MMA8450_OUT_X_MSB	0x06
+#define MMA8450_OUT_Y_LSB	0x07
+#define MMA8450_OUT_Y_MSB	0x08
+#define MMA8450_OUT_Z_LSB	0x09
+#define MMA8450_OUT_Z_MSB	0x0a
+
+#define MMA8450_XYZ_DATA_CFG	0x16
+
+#define MMA8450_CTRL_REG1	0x38
+#define MMA8450_CTRL_REG2	0x39
+#define MMA8450_ID		0xc6
+#define MMA8450_WHO_AM_I	0x0f
+
+static int mma8450_read(struct i2c_client *c, unsigned int off)
+{
+	int ret;
+
+	ret = i2c_smbus_read_byte_data(c, off);
+	if (ret < 0)
+		dev_err(&c->dev,
+			"failed to read register 0x%02x, error %d\n",
+			off, ret);
+
+	return ret;
+}
+
+static int mma8450_write(struct i2c_client *c, unsigned int off, u8 v)
+{
+	int error;
+
+	error = i2c_smbus_write_byte_data(c, off, v);
+	if (error < 0) {
+		dev_err(&c->dev,
+			"failed to write to register 0x%02x, error %d\n",
+			off, error);
+		return error;
+	}
+
+	return 0;
+}
+
+static int mma8450_read_block(struct i2c_client *c, unsigned int off,
+			      u8 *buf, size_t size)
+{
+	int err;
+
+	err = i2c_smbus_read_i2c_block_data(c, off, size, buf);
+	if (err < 0) {
+		dev_err(&c->dev,
+			"failed to read block data at 0x%02x, error %d\n",
+			MMA8450_OUT_X_LSB, err);
+		return err;
+	}
+
+	return 0;
+}
+
+static void mma8450_poll(struct input_dev *input)
+{
+	struct i2c_client *c = input_get_drvdata(input);
+	int x, y, z;
+	int ret;
+	u8 buf[6];
+
+	ret = mma8450_read(c, MMA8450_STATUS);
+	if (ret < 0)
+		return;
+
+	if (!(ret & MMA8450_STATUS_ZXYDR))
+		return;
+
+	ret = mma8450_read_block(c, MMA8450_OUT_X_LSB, buf, sizeof(buf));
+	if (ret < 0)
+		return;
+
+	x = ((int)(s8)buf[1] << 4) | (buf[0] & 0xf);
+	y = ((int)(s8)buf[3] << 4) | (buf[2] & 0xf);
+	z = ((int)(s8)buf[5] << 4) | (buf[4] & 0xf);
+
+	input_report_abs(input, ABS_X, x);
+	input_report_abs(input, ABS_Y, y);
+	input_report_abs(input, ABS_Z, z);
+	input_sync(input);
+}
+
+/* Initialize the MMA8450 chip */
+static int mma8450_open(struct input_dev *input)
+{
+	struct i2c_client *c = input_get_drvdata(input);
+	int err;
+
+	/* enable all events from X/Y/Z, no FIFO */
+	err = mma8450_write(c, MMA8450_XYZ_DATA_CFG, 0x07);
+	if (err)
+		return err;
+
+	/*
+	 * Sleep mode poll rate - 50Hz
+	 * System output data rate - 400Hz
+	 * Full scale selection - Active, +/- 2G
+	 */
+	err = mma8450_write(c, MMA8450_CTRL_REG1, 0x01);
+	if (err)
+		return err;
+
+	msleep(MODE_CHANGE_DELAY_MS);
+	return 0;
+}
+
+static void mma8450_close(struct input_dev *input)
+{
+	struct i2c_client *c = input_get_drvdata(input);
+
+	mma8450_write(c, MMA8450_CTRL_REG1, 0x00);
+	mma8450_write(c, MMA8450_CTRL_REG2, 0x01);
+}
+
+/*
+ * I2C init/probing/exit functions
+ */
+static int mma8450_probe(struct i2c_client *c)
+{
+	struct i2c_adapter *adapter = c->adapter;
+	struct input_dev *input;
+	int err, client_id;
+
+	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE |
+						I2C_FUNC_SMBUS_BYTE_DATA))
+		return dev_err_probe(&c->dev, -EINVAL,
+				     "I2C adapter doesn't support SMBUS BYTE");
+
+	client_id = i2c_smbus_read_byte_data(c, MMA8450_WHO_AM_I);
+	if (client_id != MMA8450_ID)
+		return dev_err_probe(&c->dev, -EINVAL,
+				     "unexpected chip ID 0x%x (vs 0x%x)\n",
+				     client_id, MMA8450_ID);
+
+	input = devm_input_allocate_device(&c->dev);
+	if (!input)
+		return -ENOMEM;
+
+	input_set_drvdata(input, c);
+
+	input->name = MMA8450_DRV_NAME;
+	input->id.bustype = BUS_I2C;
+
+	input->open = mma8450_open;
+	input->close = mma8450_close;
+
+	input_set_abs_params(input, ABS_X, -2048, 2047, 32, 32);
+	input_set_abs_params(input, ABS_Y, -2048, 2047, 32, 32);
+	input_set_abs_params(input, ABS_Z, -2048, 2047, 32, 32);
+
+	err = input_setup_polling(input, mma8450_poll);
+	if (err) {
+		dev_err(&c->dev, "failed to set up polling\n");
+		return err;
+	}
+
+	input_set_poll_interval(input, POLL_INTERVAL);
+	input_set_max_poll_interval(input, POLL_INTERVAL_MAX);
+
+	err = input_register_device(input);
+	if (err) {
+		dev_err(&c->dev, "failed to register input device\n");
+		return err;
+	}
+
+	return 0;
+}
+
+static const struct i2c_device_id mma8450_id[] = {
+	{ .name = MMA8450_DRV_NAME },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, mma8450_id);
+
+static const struct of_device_id mma8450_dt_ids[] = {
+	{ .compatible = "fsl,mma8450", },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, mma8450_dt_ids);
+
+static struct i2c_driver mma8450_driver = {
+	.driver = {
+		.name	= MMA8450_DRV_NAME,
+		.of_match_table = mma8450_dt_ids,
+	},
+	.probe		= mma8450_probe,
+	.id_table	= mma8450_id,
+};
+
+module_i2c_driver(mma8450_driver);
+
+MODULE_AUTHOR("Freescale Semiconductor, Inc.");
+MODULE_DESCRIPTION("MMA8450 3-Axis Accelerometer Driver");
+MODULE_LICENSE("GPL");