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, 292 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-rx4581.c b/drivers/rtc/rtc-rx4581.c
new file mode 100644
index 000000000..c092e0452
--- /dev/null
+++ b/drivers/rtc/rtc-rx4581.c
@@ -0,0 +1,292 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* drivers/rtc/rtc-rx4581.c
+ *
+ * written by Torben Hohn <torbenh@linutronix.de>
+ *
+ * Based on:
+ * drivers/rtc/rtc-max6902.c
+ *
+ * Copyright (C) 2006 8D Technologies inc.
+ * Copyright (C) 2004 Compulab Ltd.
+ *
+ * Driver for MAX6902 spi RTC
+ *
+ * and based on:
+ * drivers/rtc/rtc-rx8581.c
+ *
+ * An I2C driver for the Epson RX8581 RTC
+ *
+ * Author: Martyn Welch <martyn.welch@ge.com>
+ * Copyright 2008 GE Intelligent Platforms Embedded Systems, Inc.
+ *
+ * Based on: rtc-pcf8563.c (An I2C driver for the Philips PCF8563 RTC)
+ * Copyright 2005-06 Tower Technologies
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/platform_device.h>
+#include <linux/init.h>
+#include <linux/rtc.h>
+#include <linux/spi/spi.h>
+#include <linux/bcd.h>
+
+#define RX4581_REG_SC		0x00 /* Second in BCD */
+#define RX4581_REG_MN		0x01 /* Minute in BCD */
+#define RX4581_REG_HR		0x02 /* Hour in BCD */
+#define RX4581_REG_DW		0x03 /* Day of Week */
+#define RX4581_REG_DM		0x04 /* Day of Month in BCD */
+#define RX4581_REG_MO		0x05 /* Month in BCD */
+#define RX4581_REG_YR		0x06 /* Year in BCD */
+#define RX4581_REG_RAM		0x07 /* RAM */
+#define RX4581_REG_AMN		0x08 /* Alarm Min in BCD*/
+#define RX4581_REG_AHR		0x09 /* Alarm Hour in BCD */
+#define RX4581_REG_ADM		0x0A
+#define RX4581_REG_ADW		0x0A
+#define RX4581_REG_TMR0		0x0B
+#define RX4581_REG_TMR1		0x0C
+#define RX4581_REG_EXT		0x0D /* Extension Register */
+#define RX4581_REG_FLAG		0x0E /* Flag Register */
+#define RX4581_REG_CTRL		0x0F /* Control Register */
+
+
+/* Flag Register bit definitions */
+#define RX4581_FLAG_UF		0x20 /* Update */
+#define RX4581_FLAG_TF		0x10 /* Timer */
+#define RX4581_FLAG_AF		0x08 /* Alarm */
+#define RX4581_FLAG_VLF		0x02 /* Voltage Low */
+
+/* Control Register bit definitions */
+#define RX4581_CTRL_UIE		0x20 /* Update Interrupt Enable */
+#define RX4581_CTRL_TIE		0x10 /* Timer Interrupt Enable */
+#define RX4581_CTRL_AIE		0x08 /* Alarm Interrupt Enable */
+#define RX4581_CTRL_STOP	0x02 /* STOP bit */
+#define RX4581_CTRL_RESET	0x01 /* RESET bit */
+
+static int rx4581_set_reg(struct device *dev, unsigned char address,
+				unsigned char data)
+{
+	struct spi_device *spi = to_spi_device(dev);
+	unsigned char buf[2];
+
+	/* high nibble must be '0' to write */
+	buf[0] = address & 0x0f;
+	buf[1] = data;
+
+	return spi_write_then_read(spi, buf, 2, NULL, 0);
+}
+
+static int rx4581_get_reg(struct device *dev, unsigned char address,
+				unsigned char *data)
+{
+	struct spi_device *spi = to_spi_device(dev);
+
+	/* Set MSB to indicate read */
+	*data = address | 0x80;
+
+	return spi_write_then_read(spi, data, 1, data, 1);
+}
+
+/*
+ * In the routines that deal directly with the rx8581 hardware, we use
+ * rtc_time -- month 0-11, hour 0-23, yr = calendar year-epoch.
+ */
+static int rx4581_get_datetime(struct device *dev, struct rtc_time *tm)
+{
+	struct spi_device *spi = to_spi_device(dev);
+	unsigned char date[7];
+	unsigned char data;
+	int err;
+
+	/* First we ensure that the "update flag" is not set, we read the
+	 * time and date then re-read the "update flag". If the update flag
+	 * has been set, we know that the time has changed during the read so
+	 * we repeat the whole process again.
+	 */
+	err = rx4581_get_reg(dev, RX4581_REG_FLAG, &data);
+	if (err != 0) {
+		dev_err(dev, "Unable to read device flags\n");
+		return -EIO;
+	}
+
+	do {
+		/* If update flag set, clear it */
+		if (data & RX4581_FLAG_UF) {
+			err = rx4581_set_reg(dev,
+				RX4581_REG_FLAG, (data & ~RX4581_FLAG_UF));
+			if (err != 0) {
+				dev_err(dev, "Unable to write device "
+					"flags\n");
+				return -EIO;
+			}
+		}
+
+		/* Now read time and date */
+		date[0] = 0x80;
+		err = spi_write_then_read(spi, date, 1, date, 7);
+		if (err < 0) {
+			dev_err(dev, "Unable to read date\n");
+			return -EIO;
+		}
+
+		/* Check flag register */
+		err = rx4581_get_reg(dev, RX4581_REG_FLAG, &data);
+		if (err != 0) {
+			dev_err(dev, "Unable to read device flags\n");
+			return -EIO;
+		}
+	} while (data & RX4581_FLAG_UF);
+
+	if (data & RX4581_FLAG_VLF)
+		dev_info(dev,
+			"low voltage detected, date/time is not reliable.\n");
+
+	dev_dbg(dev,
+		"%s: raw data is sec=%02x, min=%02x, hr=%02x, "
+		"wday=%02x, mday=%02x, mon=%02x, year=%02x\n",
+		__func__,
+		date[0], date[1], date[2], date[3], date[4], date[5], date[6]);
+
+	tm->tm_sec = bcd2bin(date[RX4581_REG_SC] & 0x7F);
+	tm->tm_min = bcd2bin(date[RX4581_REG_MN] & 0x7F);
+	tm->tm_hour = bcd2bin(date[RX4581_REG_HR] & 0x3F); /* rtc hr 0-23 */
+	tm->tm_wday = ilog2(date[RX4581_REG_DW] & 0x7F);
+	tm->tm_mday = bcd2bin(date[RX4581_REG_DM] & 0x3F);
+	tm->tm_mon = bcd2bin(date[RX4581_REG_MO] & 0x1F) - 1; /* rtc mn 1-12 */
+	tm->tm_year = bcd2bin(date[RX4581_REG_YR]);
+	if (tm->tm_year < 70)
+		tm->tm_year += 100;	/* assume we are in 1970...2069 */
+
+
+	dev_dbg(dev, "%s: tm is secs=%d, mins=%d, hours=%d, "
+		"mday=%d, mon=%d, year=%d, wday=%d\n",
+		__func__,
+		tm->tm_sec, tm->tm_min, tm->tm_hour,
+		tm->tm_mday, tm->tm_mon, tm->tm_year, tm->tm_wday);
+
+	return 0;
+}
+
+static int rx4581_set_datetime(struct device *dev, struct rtc_time *tm)
+{
+	struct spi_device *spi = to_spi_device(dev);
+	int err;
+	unsigned char buf[8], data;
+
+	dev_dbg(dev, "%s: secs=%d, mins=%d, hours=%d, "
+		"mday=%d, mon=%d, year=%d, wday=%d\n",
+		__func__,
+		tm->tm_sec, tm->tm_min, tm->tm_hour,
+		tm->tm_mday, tm->tm_mon, tm->tm_year, tm->tm_wday);
+
+	buf[0] = 0x00;
+	/* hours, minutes and seconds */
+	buf[RX4581_REG_SC+1] = bin2bcd(tm->tm_sec);
+	buf[RX4581_REG_MN+1] = bin2bcd(tm->tm_min);
+	buf[RX4581_REG_HR+1] = bin2bcd(tm->tm_hour);
+
+	buf[RX4581_REG_DM+1] = bin2bcd(tm->tm_mday);
+
+	/* month, 1 - 12 */
+	buf[RX4581_REG_MO+1] = bin2bcd(tm->tm_mon + 1);
+
+	/* year and century */
+	buf[RX4581_REG_YR+1] = bin2bcd(tm->tm_year % 100);
+	buf[RX4581_REG_DW+1] = (0x1 << tm->tm_wday);
+
+	/* Stop the clock */
+	err = rx4581_get_reg(dev, RX4581_REG_CTRL, &data);
+	if (err != 0) {
+		dev_err(dev, "Unable to read control register\n");
+		return -EIO;
+	}
+
+	err = rx4581_set_reg(dev, RX4581_REG_CTRL,
+		(data | RX4581_CTRL_STOP));
+	if (err != 0) {
+		dev_err(dev, "Unable to write control register\n");
+		return -EIO;
+	}
+
+	/* write register's data */
+	err = spi_write_then_read(spi, buf, 8, NULL, 0);
+	if (err != 0) {
+		dev_err(dev, "Unable to write to date registers\n");
+		return -EIO;
+	}
+
+	/* get VLF and clear it */
+	err = rx4581_get_reg(dev, RX4581_REG_FLAG, &data);
+	if (err != 0) {
+		dev_err(dev, "Unable to read flag register\n");
+		return -EIO;
+	}
+
+	err = rx4581_set_reg(dev, RX4581_REG_FLAG,
+		(data & ~(RX4581_FLAG_VLF)));
+	if (err != 0) {
+		dev_err(dev, "Unable to write flag register\n");
+		return -EIO;
+	}
+
+	/* Restart the clock */
+	err = rx4581_get_reg(dev, RX4581_REG_CTRL, &data);
+	if (err != 0) {
+		dev_err(dev, "Unable to read control register\n");
+		return -EIO;
+	}
+
+	err = rx4581_set_reg(dev, RX4581_REG_CTRL,
+		(data & ~(RX4581_CTRL_STOP)));
+	if (err != 0) {
+		dev_err(dev, "Unable to write control register\n");
+		return -EIO;
+	}
+
+	return 0;
+}
+
+static const struct rtc_class_ops rx4581_rtc_ops = {
+	.read_time	= rx4581_get_datetime,
+	.set_time	= rx4581_set_datetime,
+};
+
+static int rx4581_probe(struct spi_device *spi)
+{
+	struct rtc_device *rtc;
+	unsigned char tmp;
+	int res;
+
+	res = rx4581_get_reg(&spi->dev, RX4581_REG_SC, &tmp);
+	if (res != 0)
+		return res;
+
+	rtc = devm_rtc_device_register(&spi->dev, "rx4581",
+				&rx4581_rtc_ops, THIS_MODULE);
+	if (IS_ERR(rtc))
+		return PTR_ERR(rtc);
+
+	spi_set_drvdata(spi, rtc);
+	return 0;
+}
+
+static const struct spi_device_id rx4581_id[] = {
+	{ "rx4581", 0 },
+	{ }
+};
+MODULE_DEVICE_TABLE(spi, rx4581_id);
+
+static struct spi_driver rx4581_driver = {
+	.driver = {
+		.name	= "rtc-rx4581",
+	},
+	.probe	= rx4581_probe,
+	.id_table = rx4581_id,
+};
+
+module_spi_driver(rx4581_driver);
+
+MODULE_DESCRIPTION("rx4581 spi RTC driver");
+MODULE_AUTHOR("Torben Hohn");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("spi:rtc-rx4581");