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, 156 insertions, 0 deletions

diff --git a/drivers/input/keyboard/dlink-dir685-touchkeys.c b/drivers/input/keyboard/dlink-dir685-touchkeys.c
new file mode 100644
index 000000000..bf94367f6
--- /dev/null
+++ b/drivers/input/keyboard/dlink-dir685-touchkeys.c
@@ -0,0 +1,156 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * D-Link DIR-685 router I2C-based Touchkeys input driver
+ * Copyright (C) 2017 Linus Walleij <linus.walleij@linaro.org>
+ *
+ * This is a one-off touchkey controller based on the Cypress Semiconductor
+ * CY8C214 MCU with some firmware in its internal 8KB flash. The circuit
+ * board inside the router is named E119921
+ */
+
+#include <linux/module.h>
+#include <linux/i2c.h>
+#include <linux/interrupt.h>
+#include <linux/delay.h>
+#include <linux/input.h>
+#include <linux/slab.h>
+#include <linux/string_choices.h>
+#include <linux/bitops.h>
+
+struct dir685_touchkeys {
+	struct device		*dev;
+	struct i2c_client	*client;
+	struct input_dev	*input;
+	unsigned long		cur_key;
+	u16			codes[7];
+};
+
+static irqreturn_t dir685_tk_irq_thread(int irq, void *data)
+{
+	struct dir685_touchkeys *tk = data;
+	const int num_bits = min_t(int, ARRAY_SIZE(tk->codes), 16);
+	unsigned long changed;
+	u8 buf[6];
+	unsigned long key;
+	int i;
+	int err;
+
+	memset(buf, 0, sizeof(buf));
+	err = i2c_master_recv(tk->client, buf, sizeof(buf));
+	if (err != sizeof(buf)) {
+		dev_err(tk->dev, "short read %d\n", err);
+		return IRQ_HANDLED;
+	}
+
+	dev_dbg(tk->dev, "IN: %*ph\n", (int)sizeof(buf), buf);
+	key = be16_to_cpup((__be16 *) &buf[4]);
+
+	/* Figure out if any bits went high or low since last message */
+	changed = tk->cur_key ^ key;
+	for_each_set_bit(i, &changed, num_bits) {
+		dev_dbg(tk->dev, "key %d is %s\n", i,
+			str_down_up(test_bit(i, &key)));
+		input_report_key(tk->input, tk->codes[i], test_bit(i, &key));
+	}
+
+	/* Store currently down keys */
+	tk->cur_key = key;
+	input_sync(tk->input);
+
+	return IRQ_HANDLED;
+}
+
+static int dir685_tk_probe(struct i2c_client *client)
+{
+	static const u8 bl_data[] = { 0xa7, 0x40 };
+	struct device *dev = &client->dev;
+	struct dir685_touchkeys *tk;
+	int err;
+	int i;
+
+	tk = devm_kzalloc(&client->dev, sizeof(*tk), GFP_KERNEL);
+	if (!tk)
+		return -ENOMEM;
+
+	tk->input = devm_input_allocate_device(dev);
+	if (!tk->input)
+		return -ENOMEM;
+
+	tk->client = client;
+	tk->dev = dev;
+
+	tk->input->keycodesize = sizeof(u16);
+	tk->input->keycodemax = ARRAY_SIZE(tk->codes);
+	tk->input->keycode = tk->codes;
+	tk->codes[0] = KEY_UP;
+	tk->codes[1] = KEY_DOWN;
+	tk->codes[2] = KEY_LEFT;
+	tk->codes[3] = KEY_RIGHT;
+	tk->codes[4] = KEY_ENTER;
+	tk->codes[5] = KEY_WPS_BUTTON;
+	/*
+	 * This key appears in the vendor driver, but I have
+	 * not been able to activate it.
+	 */
+	tk->codes[6] = KEY_RESERVED;
+
+	__set_bit(EV_KEY, tk->input->evbit);
+	for (i = 0; i < ARRAY_SIZE(tk->codes); i++)
+		__set_bit(tk->codes[i], tk->input->keybit);
+	__clear_bit(KEY_RESERVED, tk->input->keybit);
+
+	tk->input->name = "D-Link DIR-685 touchkeys";
+	tk->input->id.bustype = BUS_I2C;
+
+	err = input_register_device(tk->input);
+	if (err)
+		return err;
+
+	/* Set the brightness to max level */
+	err = i2c_master_send(client, bl_data, sizeof(bl_data));
+	if (err != sizeof(bl_data))
+		dev_warn(tk->dev, "error setting brightness level\n");
+
+	if (!client->irq) {
+		dev_err(dev, "no IRQ on the I2C device\n");
+		return -ENODEV;
+	}
+	err = devm_request_threaded_irq(dev, client->irq,
+					NULL, dir685_tk_irq_thread,
+					IRQF_ONESHOT,
+					"dir685-tk", tk);
+	if (err) {
+		dev_err(dev, "can't request IRQ\n");
+		return err;
+	}
+
+	return 0;
+}
+
+static const struct i2c_device_id dir685_tk_id[] = {
+	{ .name = "dir685tk" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, dir685_tk_id);
+
+#ifdef CONFIG_OF
+static const struct of_device_id dir685_tk_of_match[] = {
+	{ .compatible = "dlink,dir685-touchkeys" },
+	{},
+};
+MODULE_DEVICE_TABLE(of, dir685_tk_of_match);
+#endif
+
+static struct i2c_driver dir685_tk_i2c_driver = {
+	.driver = {
+		.name	= "dlink-dir685-touchkeys",
+		.of_match_table = of_match_ptr(dir685_tk_of_match),
+	},
+	.probe		= dir685_tk_probe,
+	.id_table	= dir685_tk_id,
+};
+module_i2c_driver(dir685_tk_i2c_driver);
+
+MODULE_AUTHOR("Linus Walleij <linus.walleij@linaro.org>");
+MODULE_DESCRIPTION("D-Link DIR-685 touchkeys driver");
+MODULE_LICENSE("GPL");