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, 184 insertions, 0 deletions

diff --git a/drivers/input/touchscreen/hampshire.c b/drivers/input/touchscreen/hampshire.c
new file mode 100644
index 000000000..394ae8c88
--- /dev/null
+++ b/drivers/input/touchscreen/hampshire.c
@@ -0,0 +1,184 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Hampshire serial touchscreen driver
+ *
+ * Copyright (c) 2010 Adam Bennett
+ * Based on the dynapro driver (c) Tias Guns
+ */
+
+
+/*
+ * 2010/04/08 Adam Bennett <abennett72@gmail.com>
+ *   Copied dynapro.c and edited for Hampshire 4-byte protocol
+ */
+
+#include <linux/errno.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/input.h>
+#include <linux/serio.h>
+
+#define DRIVER_DESC	"Hampshire serial touchscreen driver"
+
+MODULE_AUTHOR("Adam Bennett <abennett72@gmail.com>");
+MODULE_DESCRIPTION(DRIVER_DESC);
+MODULE_LICENSE("GPL");
+
+/*
+ * Definitions & global arrays.
+ */
+
+#define HAMPSHIRE_FORMAT_TOUCH_BIT 0x40
+#define HAMPSHIRE_FORMAT_LENGTH 4
+#define HAMPSHIRE_RESPONSE_BEGIN_BYTE 0x80
+
+#define HAMPSHIRE_MIN_XC 0
+#define HAMPSHIRE_MAX_XC 0x1000
+#define HAMPSHIRE_MIN_YC 0
+#define HAMPSHIRE_MAX_YC 0x1000
+
+#define HAMPSHIRE_GET_XC(data) (((data[3] & 0x0c) >> 2) | (data[1] << 2) | ((data[0] & 0x38) << 6))
+#define HAMPSHIRE_GET_YC(data) ((data[3] & 0x03) | (data[2] << 2) | ((data[0] & 0x07) << 9))
+#define HAMPSHIRE_GET_TOUCHED(data) (HAMPSHIRE_FORMAT_TOUCH_BIT & data[0])
+
+/*
+ * Per-touchscreen data.
+ */
+
+struct hampshire {
+	struct input_dev *dev;
+	struct serio *serio;
+	int idx;
+	unsigned char data[HAMPSHIRE_FORMAT_LENGTH];
+	char phys[32];
+};
+
+static void hampshire_process_data(struct hampshire *phampshire)
+{
+	struct input_dev *dev = phampshire->dev;
+
+	if (HAMPSHIRE_FORMAT_LENGTH == ++phampshire->idx) {
+		input_report_abs(dev, ABS_X, HAMPSHIRE_GET_XC(phampshire->data));
+		input_report_abs(dev, ABS_Y, HAMPSHIRE_GET_YC(phampshire->data));
+		input_report_key(dev, BTN_TOUCH,
+				 HAMPSHIRE_GET_TOUCHED(phampshire->data));
+		input_sync(dev);
+
+		phampshire->idx = 0;
+	}
+}
+
+static irqreturn_t hampshire_interrupt(struct serio *serio,
+		unsigned char data, unsigned int flags)
+{
+	struct hampshire *phampshire = serio_get_drvdata(serio);
+
+	phampshire->data[phampshire->idx] = data;
+
+	if (HAMPSHIRE_RESPONSE_BEGIN_BYTE & phampshire->data[0])
+		hampshire_process_data(phampshire);
+	else
+		dev_dbg(&serio->dev, "unknown/unsynchronized data: %x\n",
+			phampshire->data[0]);
+
+	return IRQ_HANDLED;
+}
+
+static void hampshire_disconnect(struct serio *serio)
+{
+	struct hampshire *phampshire = serio_get_drvdata(serio);
+
+	input_get_device(phampshire->dev);
+	input_unregister_device(phampshire->dev);
+	serio_close(serio);
+	serio_set_drvdata(serio, NULL);
+	input_put_device(phampshire->dev);
+	kfree(phampshire);
+}
+
+/*
+ * hampshire_connect() is the routine that is called when someone adds a
+ * new serio device that supports hampshire protocol and registers it as
+ * an input device. This is usually accomplished using inputattach.
+ */
+
+static int hampshire_connect(struct serio *serio, struct serio_driver *drv)
+{
+	struct hampshire *phampshire;
+	struct input_dev *input_dev;
+	int err;
+
+	phampshire = kzalloc_obj(*phampshire);
+	input_dev = input_allocate_device();
+	if (!phampshire || !input_dev) {
+		err = -ENOMEM;
+		goto fail1;
+	}
+
+	phampshire->serio = serio;
+	phampshire->dev = input_dev;
+	scnprintf(phampshire->phys, sizeof(phampshire->phys),
+		  "%s/input0", serio->phys);
+
+	input_dev->name = "Hampshire Serial TouchScreen";
+	input_dev->phys = phampshire->phys;
+	input_dev->id.bustype = BUS_RS232;
+	input_dev->id.vendor = SERIO_HAMPSHIRE;
+	input_dev->id.product = 0;
+	input_dev->id.version = 0x0001;
+	input_dev->dev.parent = &serio->dev;
+	input_dev->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_ABS);
+	input_dev->keybit[BIT_WORD(BTN_TOUCH)] = BIT_MASK(BTN_TOUCH);
+	input_set_abs_params(phampshire->dev, ABS_X,
+			     HAMPSHIRE_MIN_XC, HAMPSHIRE_MAX_XC, 0, 0);
+	input_set_abs_params(phampshire->dev, ABS_Y,
+			     HAMPSHIRE_MIN_YC, HAMPSHIRE_MAX_YC, 0, 0);
+
+	serio_set_drvdata(serio, phampshire);
+
+	err = serio_open(serio, drv);
+	if (err)
+		goto fail2;
+
+	err = input_register_device(phampshire->dev);
+	if (err)
+		goto fail3;
+
+	return 0;
+
+ fail3:	serio_close(serio);
+ fail2:	serio_set_drvdata(serio, NULL);
+ fail1:	input_free_device(input_dev);
+	kfree(phampshire);
+	return err;
+}
+
+/*
+ * The serio driver structure.
+ */
+
+static const struct serio_device_id hampshire_serio_ids[] = {
+	{
+		.type	= SERIO_RS232,
+		.proto	= SERIO_HAMPSHIRE,
+		.id	= SERIO_ANY,
+		.extra	= SERIO_ANY,
+	},
+	{ 0 }
+};
+
+MODULE_DEVICE_TABLE(serio, hampshire_serio_ids);
+
+static struct serio_driver hampshire_drv = {
+	.driver		= {
+		.name	= "hampshire",
+	},
+	.description	= DRIVER_DESC,
+	.id_table	= hampshire_serio_ids,
+	.interrupt	= hampshire_interrupt,
+	.connect	= hampshire_connect,
+	.disconnect	= hampshire_disconnect,
+};
+
+module_serio_driver(hampshire_drv);