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, 244 insertions, 0 deletions

diff --git a/drivers/input/joystick/twidjoy.c b/drivers/input/joystick/twidjoy.c
new file mode 100644
index 000000000..fb452552c
--- /dev/null
+++ b/drivers/input/joystick/twidjoy.c
@@ -0,0 +1,244 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Copyright (c) 2001 Arndt Schoenewald
+ *  Copyright (c) 2000-2001 Vojtech Pavlik
+ *  Copyright (c) 2000 Mark Fletcher
+ *
+ *  Sponsored by Quelltext AG (http://www.quelltext-ag.de), Dortmund, Germany
+ */
+
+/*
+ * Driver to use Handykey's Twiddler (the first edition, i.e. the one with
+ * the RS232 interface) as a joystick under Linux
+ *
+ * The Twiddler is a one-handed chording keyboard featuring twelve buttons on
+ * the front, six buttons on the top, and a built-in tilt sensor. The buttons
+ * on the front, which are grouped as four rows of three buttons, are pressed
+ * by the four fingers (this implies only one button per row can be held down
+ * at the same time) and the buttons on the top are for the thumb. The tilt
+ * sensor delivers X and Y axis data depending on how the Twiddler is held.
+ * Additional information can be found at http://www.handykey.com.
+ *
+ * This driver does not use the Twiddler for its intended purpose, i.e. as
+ * a chording keyboard, but as a joystick: pressing and releasing a button
+ * immediately sends a corresponding button event, and tilting it generates
+ * corresponding ABS_X and ABS_Y events. This turns the Twiddler into a game
+ * controller with amazing 18 buttons :-)
+ *
+ * Note: The Twiddler2 (the successor of the Twiddler that connects directly
+ * to the PS/2 keyboard and mouse ports) is NOT supported by this driver!
+ *
+ * For questions or feedback regarding this driver module please contact:
+ * Arndt Schoenewald <arndt@quelltext.com>
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/input.h>
+#include <linux/serio.h>
+
+#define DRIVER_DESC	"Handykey Twiddler keyboard as a joystick driver"
+
+MODULE_DESCRIPTION(DRIVER_DESC);
+MODULE_LICENSE("GPL");
+
+/*
+ * Constants.
+ */
+
+#define TWIDJOY_MAX_LENGTH 5
+
+static struct twidjoy_button_spec {
+	int bitshift;
+	int bitmask;
+	int buttons[3];
+}
+twidjoy_buttons[] = {
+	{  0, 3, { BTN_A,      BTN_B,     BTN_C    } },
+	{  2, 3, { BTN_X,      BTN_Y,     BTN_Z    } },
+	{  4, 3, { BTN_TL,     BTN_TR,    BTN_TR2  } },
+	{  6, 3, { BTN_SELECT, BTN_START, BTN_MODE } },
+	{  8, 1, { BTN_BASE5                       } },
+	{  9, 1, { BTN_BASE                        } },
+	{ 10, 1, { BTN_BASE3                       } },
+	{ 11, 1, { BTN_BASE4                       } },
+	{ 12, 1, { BTN_BASE2                       } },
+	{ 13, 1, { BTN_BASE6                       } },
+	{ 0,  0, { 0                               } }
+};
+
+/*
+ * Per-Twiddler data.
+ */
+
+struct twidjoy {
+	struct input_dev *dev;
+	int idx;
+	unsigned char data[TWIDJOY_MAX_LENGTH];
+	char phys[32];
+};
+
+/*
+ * twidjoy_process_packet() decodes packets the driver receives from the
+ * Twiddler. It updates the data accordingly.
+ */
+
+static void twidjoy_process_packet(struct twidjoy *twidjoy)
+{
+	struct input_dev *dev = twidjoy->dev;
+	unsigned char *data = twidjoy->data;
+	struct twidjoy_button_spec *bp;
+	int button_bits, abs_x, abs_y;
+
+	button_bits = ((data[1] & 0x7f) << 7) | (data[0] & 0x7f);
+
+	for (bp = twidjoy_buttons; bp->bitmask; bp++) {
+		int value = (button_bits & (bp->bitmask << bp->bitshift)) >> bp->bitshift;
+		int i;
+
+		for (i = 0; i < bp->bitmask; i++)
+			input_report_key(dev, bp->buttons[i], i+1 == value);
+	}
+
+	abs_x = ((data[4] & 0x07) << 5) | ((data[3] & 0x7C) >> 2);
+	if (data[4] & 0x08) abs_x -= 256;
+
+	abs_y = ((data[3] & 0x01) << 7) | ((data[2] & 0x7F) >> 0);
+	if (data[3] & 0x02) abs_y -= 256;
+
+	input_report_abs(dev, ABS_X, -abs_x);
+	input_report_abs(dev, ABS_Y, +abs_y);
+
+	input_sync(dev);
+}
+
+/*
+ * twidjoy_interrupt() is called by the low level driver when characters
+ * are ready for us. We then buffer them for further processing, or call the
+ * packet processing routine.
+ */
+
+static irqreturn_t twidjoy_interrupt(struct serio *serio, unsigned char data, unsigned int flags)
+{
+	struct twidjoy *twidjoy = serio_get_drvdata(serio);
+
+	/* All Twiddler packets are 5 bytes. The fact that the first byte
+	 * has a MSB of 0 and all other bytes have a MSB of 1 can be used
+	 * to check and regain sync. */
+
+	if ((data & 0x80) == 0)
+		twidjoy->idx = 0;	/* this byte starts a new packet */
+	else if (twidjoy->idx == 0)
+		return IRQ_HANDLED;	/* wrong MSB -- ignore this byte */
+
+	if (twidjoy->idx < TWIDJOY_MAX_LENGTH)
+		twidjoy->data[twidjoy->idx++] = data;
+
+	if (twidjoy->idx == TWIDJOY_MAX_LENGTH) {
+		twidjoy_process_packet(twidjoy);
+		twidjoy->idx = 0;
+	}
+
+	return IRQ_HANDLED;
+}
+
+/*
+ * twidjoy_disconnect() is the opposite of twidjoy_connect()
+ */
+
+static void twidjoy_disconnect(struct serio *serio)
+{
+	struct twidjoy *twidjoy = serio_get_drvdata(serio);
+
+	serio_close(serio);
+	serio_set_drvdata(serio, NULL);
+	input_unregister_device(twidjoy->dev);
+	kfree(twidjoy);
+}
+
+/*
+ * twidjoy_connect() is the routine that is called when someone adds a
+ * new serio device. It looks for the Twiddler, and if found, registers
+ * it as an input device.
+ */
+
+static int twidjoy_connect(struct serio *serio, struct serio_driver *drv)
+{
+	struct twidjoy_button_spec *bp;
+	struct twidjoy *twidjoy;
+	struct input_dev *input_dev;
+	int err = -ENOMEM;
+	int i;
+
+	twidjoy = kzalloc_obj(*twidjoy);
+	input_dev = input_allocate_device();
+	if (!twidjoy || !input_dev)
+		goto fail1;
+
+	twidjoy->dev = input_dev;
+	snprintf(twidjoy->phys, sizeof(twidjoy->phys), "%s/input0", serio->phys);
+
+	input_dev->name = "Handykey Twiddler";
+	input_dev->phys = twidjoy->phys;
+	input_dev->id.bustype = BUS_RS232;
+	input_dev->id.vendor = SERIO_TWIDJOY;
+	input_dev->id.product = 0x0001;
+	input_dev->id.version = 0x0100;
+	input_dev->dev.parent = &serio->dev;
+
+	input_dev->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_ABS);
+	input_set_abs_params(input_dev, ABS_X, -50, 50, 4, 4);
+	input_set_abs_params(input_dev, ABS_Y, -50, 50, 4, 4);
+
+	for (bp = twidjoy_buttons; bp->bitmask; bp++)
+		for (i = 0; i < bp->bitmask; i++)
+			set_bit(bp->buttons[i], input_dev->keybit);
+
+	serio_set_drvdata(serio, twidjoy);
+
+	err = serio_open(serio, drv);
+	if (err)
+		goto fail2;
+
+	err = input_register_device(twidjoy->dev);
+	if (err)
+		goto fail3;
+
+	return 0;
+
+ fail3:	serio_close(serio);
+ fail2:	serio_set_drvdata(serio, NULL);
+ fail1:	input_free_device(input_dev);
+	kfree(twidjoy);
+	return err;
+}
+
+/*
+ * The serio driver structure.
+ */
+
+static const struct serio_device_id twidjoy_serio_ids[] = {
+	{
+		.type	= SERIO_RS232,
+		.proto	= SERIO_TWIDJOY,
+		.id	= SERIO_ANY,
+		.extra	= SERIO_ANY,
+	},
+	{ 0 }
+};
+
+MODULE_DEVICE_TABLE(serio, twidjoy_serio_ids);
+
+static struct serio_driver twidjoy_drv = {
+	.driver		= {
+		.name	= "twidjoy",
+	},
+	.description	= DRIVER_DESC,
+	.id_table	= twidjoy_serio_ids,
+	.interrupt	= twidjoy_interrupt,
+	.connect	= twidjoy_connect,
+	.disconnect	= twidjoy_disconnect,
+};
+
+module_serio_driver(twidjoy_drv);