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, 235 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-max6900.c b/drivers/rtc/rtc-max6900.c
new file mode 100644
index 000000000..8ef6d0fcd
--- /dev/null
+++ b/drivers/rtc/rtc-max6900.c
@@ -0,0 +1,235 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * rtc class driver for the Maxim MAX6900 chip
+ *
+ * Copyright (c) 2007 MontaVista, Software, Inc.
+ *
+ * Author: Dale Farnsworth <dale@farnsworth.org>
+ *
+ * based on previously existing rtc class drivers
+ */
+
+#include <linux/module.h>
+#include <linux/i2c.h>
+#include <linux/bcd.h>
+#include <linux/rtc.h>
+#include <linux/delay.h>
+
+/*
+ * register indices
+ */
+#define MAX6900_REG_SC			0	/* seconds      00-59 */
+#define MAX6900_REG_MN			1	/* minutes      00-59 */
+#define MAX6900_REG_HR			2	/* hours        00-23 */
+#define MAX6900_REG_DT			3	/* day of month 00-31 */
+#define MAX6900_REG_MO			4	/* month        01-12 */
+#define MAX6900_REG_DW			5	/* day of week   1-7  */
+#define MAX6900_REG_YR			6	/* year         00-99 */
+#define MAX6900_REG_CT			7	/* control */
+						/* register 8 is undocumented */
+#define MAX6900_REG_CENTURY		9	/* century */
+#define MAX6900_REG_LEN			10
+
+#define MAX6900_BURST_LEN		8	/* can burst r/w first 8 regs */
+
+#define MAX6900_REG_CT_WP		(1 << 7)	/* Write Protect */
+
+/*
+ * register read/write commands
+ */
+#define MAX6900_REG_CONTROL_WRITE	0x8e
+#define MAX6900_REG_CENTURY_WRITE	0x92
+#define MAX6900_REG_CENTURY_READ	0x93
+#define MAX6900_REG_RESERVED_READ	0x96
+#define MAX6900_REG_BURST_WRITE		0xbe
+#define MAX6900_REG_BURST_READ		0xbf
+
+#define MAX6900_IDLE_TIME_AFTER_WRITE	3	/* specification says 2.5 mS */
+
+static struct i2c_driver max6900_driver;
+
+static int max6900_i2c_read_regs(struct i2c_client *client, u8 *buf)
+{
+	u8 reg_burst_read[1] = { MAX6900_REG_BURST_READ };
+	u8 reg_century_read[1] = { MAX6900_REG_CENTURY_READ };
+	struct i2c_msg msgs[4] = {
+		{
+		 .addr = client->addr,
+		 .flags = 0,	/* write */
+		 .len = sizeof(reg_burst_read),
+		 .buf = reg_burst_read}
+		,
+		{
+		 .addr = client->addr,
+		 .flags = I2C_M_RD,
+		 .len = MAX6900_BURST_LEN,
+		 .buf = buf}
+		,
+		{
+		 .addr = client->addr,
+		 .flags = 0,	/* write */
+		 .len = sizeof(reg_century_read),
+		 .buf = reg_century_read}
+		,
+		{
+		 .addr = client->addr,
+		 .flags = I2C_M_RD,
+		 .len = sizeof(buf[MAX6900_REG_CENTURY]),
+		 .buf = &buf[MAX6900_REG_CENTURY]
+		 }
+	};
+	int rc;
+
+	rc = i2c_transfer(client->adapter, msgs, ARRAY_SIZE(msgs));
+	if (rc != ARRAY_SIZE(msgs)) {
+		dev_err(&client->dev, "%s: register read failed\n", __func__);
+		return -EIO;
+	}
+	return 0;
+}
+
+static int max6900_i2c_write_regs(struct i2c_client *client, u8 const *buf)
+{
+	u8 i2c_century_buf[1 + 1] = { MAX6900_REG_CENTURY_WRITE };
+	struct i2c_msg century_msgs[1] = {
+		{
+		 .addr = client->addr,
+		 .flags = 0,	/* write */
+		 .len = sizeof(i2c_century_buf),
+		 .buf = i2c_century_buf}
+	};
+	u8 i2c_burst_buf[MAX6900_BURST_LEN + 1] = { MAX6900_REG_BURST_WRITE };
+	struct i2c_msg burst_msgs[1] = {
+		{
+		 .addr = client->addr,
+		 .flags = 0,	/* write */
+		 .len = sizeof(i2c_burst_buf),
+		 .buf = i2c_burst_buf}
+	};
+	int rc;
+
+	/*
+	 * We have to make separate calls to i2c_transfer because of
+	 * the need to delay after each write to the chip.  Also,
+	 * we write the century byte first, since we set the write-protect
+	 * bit as part of the burst write.
+	 */
+	i2c_century_buf[1] = buf[MAX6900_REG_CENTURY];
+
+	rc = i2c_transfer(client->adapter, century_msgs,
+			  ARRAY_SIZE(century_msgs));
+	if (rc != ARRAY_SIZE(century_msgs))
+		goto write_failed;
+
+	msleep(MAX6900_IDLE_TIME_AFTER_WRITE);
+
+	memcpy(&i2c_burst_buf[1], buf, MAX6900_BURST_LEN);
+
+	rc = i2c_transfer(client->adapter, burst_msgs, ARRAY_SIZE(burst_msgs));
+	if (rc != ARRAY_SIZE(burst_msgs))
+		goto write_failed;
+	msleep(MAX6900_IDLE_TIME_AFTER_WRITE);
+
+	return 0;
+
+ write_failed:
+	dev_err(&client->dev, "%s: register write failed\n", __func__);
+	return -EIO;
+}
+
+static int max6900_rtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	int rc;
+	u8 regs[MAX6900_REG_LEN];
+
+	rc = max6900_i2c_read_regs(client, regs);
+	if (rc < 0)
+		return rc;
+
+	tm->tm_sec = bcd2bin(regs[MAX6900_REG_SC]);
+	tm->tm_min = bcd2bin(regs[MAX6900_REG_MN]);
+	tm->tm_hour = bcd2bin(regs[MAX6900_REG_HR] & 0x3f);
+	tm->tm_mday = bcd2bin(regs[MAX6900_REG_DT]);
+	tm->tm_mon = bcd2bin(regs[MAX6900_REG_MO]) - 1;
+	tm->tm_year = bcd2bin(regs[MAX6900_REG_YR]) +
+		      bcd2bin(regs[MAX6900_REG_CENTURY]) * 100 - 1900;
+	tm->tm_wday = bcd2bin(regs[MAX6900_REG_DW]);
+
+	return 0;
+}
+
+static int max6900_i2c_clear_write_protect(struct i2c_client *client)
+{
+	return i2c_smbus_write_byte_data(client, MAX6900_REG_CONTROL_WRITE, 0);
+}
+
+static int max6900_rtc_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	u8 regs[MAX6900_REG_LEN];
+	int rc;
+
+	rc = max6900_i2c_clear_write_protect(client);
+	if (rc < 0)
+		return rc;
+
+	regs[MAX6900_REG_SC] = bin2bcd(tm->tm_sec);
+	regs[MAX6900_REG_MN] = bin2bcd(tm->tm_min);
+	regs[MAX6900_REG_HR] = bin2bcd(tm->tm_hour);
+	regs[MAX6900_REG_DT] = bin2bcd(tm->tm_mday);
+	regs[MAX6900_REG_MO] = bin2bcd(tm->tm_mon + 1);
+	regs[MAX6900_REG_DW] = bin2bcd(tm->tm_wday);
+	regs[MAX6900_REG_YR] = bin2bcd(tm->tm_year % 100);
+	regs[MAX6900_REG_CENTURY] = bin2bcd((tm->tm_year + 1900) / 100);
+	/* set write protect */
+	regs[MAX6900_REG_CT] = MAX6900_REG_CT_WP;
+
+	rc = max6900_i2c_write_regs(client, regs);
+	if (rc < 0)
+		return rc;
+
+	return 0;
+}
+
+static const struct rtc_class_ops max6900_rtc_ops = {
+	.read_time = max6900_rtc_read_time,
+	.set_time = max6900_rtc_set_time,
+};
+
+static int max6900_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, max6900_driver.driver.name,
+					&max6900_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 max6900_id[] = {
+	{ .name = "max6900" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, max6900_id);
+
+static struct i2c_driver max6900_driver = {
+	.driver = {
+		   .name = "rtc-max6900",
+		   },
+	.probe = max6900_probe,
+	.id_table = max6900_id,
+};
+
+module_i2c_driver(max6900_driver);
+
+MODULE_DESCRIPTION("Maxim MAX6900 RTC driver");
+MODULE_AUTHOR("Dale Farnsworth <dale@farnsworth.org>");
+MODULE_LICENSE("GPL");