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, 158 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-em3027.c b/drivers/rtc/rtc-em3027.c
new file mode 100644
index 000000000..d555e5d59
--- /dev/null
+++ b/drivers/rtc/rtc-em3027.c
@@ -0,0 +1,158 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * An rtc/i2c driver for the EM Microelectronic EM3027
+ * Copyright 2011 CompuLab, Ltd.
+ *
+ * Author: Mike Rapoport <mike@compulab.co.il>
+ *
+ * Based on rtc-ds1672.c by Alessandro Zummo <a.zummo@towertech.it>
+ */
+
+#include <linux/i2c.h>
+#include <linux/rtc.h>
+#include <linux/bcd.h>
+#include <linux/module.h>
+#include <linux/of.h>
+
+/* Registers */
+#define EM3027_REG_ON_OFF_CTRL	0x00
+#define EM3027_REG_IRQ_CTRL	0x01
+#define EM3027_REG_IRQ_FLAGS	0x02
+#define EM3027_REG_STATUS	0x03
+#define EM3027_REG_RST_CTRL	0x04
+
+#define EM3027_REG_WATCH_SEC	0x08
+#define EM3027_REG_WATCH_MIN	0x09
+#define EM3027_REG_WATCH_HOUR	0x0a
+#define EM3027_REG_WATCH_DATE	0x0b
+#define EM3027_REG_WATCH_DAY	0x0c
+#define EM3027_REG_WATCH_MON	0x0d
+#define EM3027_REG_WATCH_YEAR	0x0e
+
+#define EM3027_REG_ALARM_SEC	0x10
+#define EM3027_REG_ALARM_MIN	0x11
+#define EM3027_REG_ALARM_HOUR	0x12
+#define EM3027_REG_ALARM_DATE	0x13
+#define EM3027_REG_ALARM_DAY	0x14
+#define EM3027_REG_ALARM_MON	0x15
+#define EM3027_REG_ALARM_YEAR	0x16
+
+static struct i2c_driver em3027_driver;
+
+static int em3027_get_time(struct device *dev, struct rtc_time *tm)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+
+	unsigned char addr = EM3027_REG_WATCH_SEC;
+	unsigned char buf[7];
+
+	struct i2c_msg msgs[] = {
+		{/* setup read addr */
+			.addr = client->addr,
+			.len = 1,
+			.buf = &addr
+		},
+		{/* read time/date */
+			.addr = client->addr,
+			.flags = I2C_M_RD,
+			.len = 7,
+			.buf = buf
+		},
+	};
+
+	/* read time/date registers */
+	if ((i2c_transfer(client->adapter, &msgs[0], 2)) != 2) {
+		dev_err(&client->dev, "%s: read error\n", __func__);
+		return -EIO;
+	}
+
+	tm->tm_sec	= bcd2bin(buf[0]);
+	tm->tm_min	= bcd2bin(buf[1]);
+	tm->tm_hour	= bcd2bin(buf[2]);
+	tm->tm_mday	= bcd2bin(buf[3]);
+	tm->tm_wday	= bcd2bin(buf[4]);
+	tm->tm_mon	= bcd2bin(buf[5]) - 1;
+	tm->tm_year	= bcd2bin(buf[6]) + 100;
+
+	return 0;
+}
+
+static int em3027_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	unsigned char buf[8];
+
+	struct i2c_msg msg = {
+		.addr = client->addr,
+		.len = 8,
+		.buf = buf,	/* write time/date */
+	};
+
+	buf[0] = EM3027_REG_WATCH_SEC;
+	buf[1] = bin2bcd(tm->tm_sec);
+	buf[2] = bin2bcd(tm->tm_min);
+	buf[3] = bin2bcd(tm->tm_hour);
+	buf[4] = bin2bcd(tm->tm_mday);
+	buf[5] = bin2bcd(tm->tm_wday);
+	buf[6] = bin2bcd(tm->tm_mon + 1);
+	buf[7] = bin2bcd(tm->tm_year % 100);
+
+	/* write time/date registers */
+	if ((i2c_transfer(client->adapter, &msg, 1)) != 1) {
+		dev_err(&client->dev, "%s: write error\n", __func__);
+		return -EIO;
+	}
+
+	return 0;
+}
+
+static const struct rtc_class_ops em3027_rtc_ops = {
+	.read_time = em3027_get_time,
+	.set_time = em3027_set_time,
+};
+
+static int em3027_probe(struct i2c_client *client)
+{
+	struct rtc_device *rtc;
+
+	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
+		return -ENODEV;
+
+	rtc = devm_rtc_device_register(&client->dev, em3027_driver.driver.name,
+				  &em3027_rtc_ops, THIS_MODULE);
+	if (IS_ERR(rtc))
+		return PTR_ERR(rtc);
+
+	i2c_set_clientdata(client, rtc);
+
+	return 0;
+}
+
+static const struct i2c_device_id em3027_id[] = {
+	{ .name = "em3027" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, em3027_id);
+
+#ifdef CONFIG_OF
+static const struct of_device_id em3027_of_match[] = {
+	{ .compatible = "emmicro,em3027", },
+	{}
+};
+MODULE_DEVICE_TABLE(of, em3027_of_match);
+#endif
+
+static struct i2c_driver em3027_driver = {
+	.driver = {
+		   .name = "rtc-em3027",
+		   .of_match_table = of_match_ptr(em3027_of_match),
+	},
+	.probe = em3027_probe,
+	.id_table = em3027_id,
+};
+
+module_i2c_driver(em3027_driver);
+
+MODULE_AUTHOR("Mike Rapoport <mike@compulab.co.il>");
+MODULE_DESCRIPTION("EM Microelectronic EM3027 RTC driver");
+MODULE_LICENSE("GPL");