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, 452 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
new file mode 100644
index 000000000..e12f0f806
--- /dev/null
+++ b/drivers/rtc/rtc-mcp795.c
@@ -0,0 +1,452 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * SPI Driver for Microchip MCP795 RTC
+ *
+ * Copyright (C) Josef Gajdusek <atx@atx.name>
+ *
+ * based on other Linux RTC drivers
+ *
+ * Device datasheet:
+ * https://ww1.microchip.com/downloads/en/DeviceDoc/22280A.pdf
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/device.h>
+#include <linux/printk.h>
+#include <linux/spi/spi.h>
+#include <linux/rtc.h>
+#include <linux/of.h>
+#include <linux/bcd.h>
+#include <linux/delay.h>
+
+/* MCP795 Instructions, see datasheet table 3-1 */
+#define MCP795_EEREAD	0x03
+#define MCP795_EEWRITE	0x02
+#define MCP795_EEWRDI	0x04
+#define MCP795_EEWREN	0x06
+#define MCP795_SRREAD	0x05
+#define MCP795_SRWRITE	0x01
+#define MCP795_READ	0x13
+#define MCP795_WRITE	0x12
+#define MCP795_UNLOCK	0x14
+#define MCP795_IDWRITE	0x32
+#define MCP795_IDREAD	0x33
+#define MCP795_CLRWDT	0x44
+#define MCP795_CLRRAM	0x54
+
+/* MCP795 RTCC registers, see datasheet table 4-1 */
+#define MCP795_REG_SECONDS	0x01
+#define MCP795_REG_DAY		0x04
+#define MCP795_REG_MONTH	0x06
+#define MCP795_REG_CONTROL	0x08
+#define MCP795_REG_ALM0_SECONDS	0x0C
+#define MCP795_REG_ALM0_DAY	0x0F
+
+#define MCP795_ST_BIT		BIT(7)
+#define MCP795_24_BIT		BIT(6)
+#define MCP795_LP_BIT		BIT(5)
+#define MCP795_EXTOSC_BIT	BIT(3)
+#define MCP795_OSCON_BIT	BIT(5)
+#define MCP795_ALM0_BIT		BIT(4)
+#define MCP795_ALM1_BIT		BIT(5)
+#define MCP795_ALM0IF_BIT	BIT(3)
+#define MCP795_ALM0C0_BIT	BIT(4)
+#define MCP795_ALM0C1_BIT	BIT(5)
+#define MCP795_ALM0C2_BIT	BIT(6)
+
+#define SEC_PER_DAY		(24 * 60 * 60)
+
+static int mcp795_rtcc_read(struct device *dev, u8 addr, u8 *buf, u8 count)
+{
+	struct spi_device *spi = to_spi_device(dev);
+	int ret;
+	u8 tx[2];
+
+	tx[0] = MCP795_READ;
+	tx[1] = addr;
+	ret = spi_write_then_read(spi, tx, sizeof(tx), buf, count);
+
+	if (ret)
+		dev_err(dev, "Failed reading %d bytes from address %x.\n",
+					count, addr);
+
+	return ret;
+}
+
+static int mcp795_rtcc_write(struct device *dev, u8 addr, u8 *data, u8 count)
+{
+	struct spi_device *spi = to_spi_device(dev);
+	int ret;
+	u8 tx[257];
+
+	tx[0] = MCP795_WRITE;
+	tx[1] = addr;
+	memcpy(&tx[2], data, count);
+
+	ret = spi_write(spi, tx, 2 + count);
+
+	if (ret)
+		dev_err(dev, "Failed to write %d bytes to address %x.\n",
+					count, addr);
+
+	return ret;
+}
+
+static int mcp795_rtcc_set_bits(struct device *dev, u8 addr, u8 mask, u8 state)
+{
+	int ret;
+	u8 tmp;
+
+	ret = mcp795_rtcc_read(dev, addr, &tmp, 1);
+	if (ret)
+		return ret;
+
+	if ((tmp & mask) != state) {
+		tmp = (tmp & ~mask) | state;
+		ret = mcp795_rtcc_write(dev, addr, &tmp, 1);
+	}
+
+	return ret;
+}
+
+static int mcp795_stop_oscillator(struct device *dev, bool *extosc)
+{
+	int retries = 5;
+	int ret;
+	u8 data;
+
+	ret = mcp795_rtcc_set_bits(dev, MCP795_REG_SECONDS, MCP795_ST_BIT, 0);
+	if (ret)
+		return ret;
+	ret = mcp795_rtcc_read(dev, MCP795_REG_CONTROL, &data, 1);
+	if (ret)
+		return ret;
+	*extosc = !!(data & MCP795_EXTOSC_BIT);
+	ret = mcp795_rtcc_set_bits(
+				dev, MCP795_REG_CONTROL, MCP795_EXTOSC_BIT, 0);
+	if (ret)
+		return ret;
+	/* wait for the OSCON bit to clear */
+	do {
+		usleep_range(700, 800);
+		ret = mcp795_rtcc_read(dev, MCP795_REG_DAY, &data, 1);
+		if (ret)
+			break;
+		if (!(data & MCP795_OSCON_BIT))
+			break;
+
+	} while (--retries);
+
+	return !retries ? -EIO : ret;
+}
+
+static int mcp795_start_oscillator(struct device *dev, bool *extosc)
+{
+	if (extosc) {
+		u8 data = *extosc ? MCP795_EXTOSC_BIT : 0;
+		int ret;
+
+		ret = mcp795_rtcc_set_bits(
+			dev, MCP795_REG_CONTROL, MCP795_EXTOSC_BIT, data);
+		if (ret)
+			return ret;
+	}
+	return mcp795_rtcc_set_bits(
+			dev, MCP795_REG_SECONDS, MCP795_ST_BIT, MCP795_ST_BIT);
+}
+
+/* Enable or disable Alarm 0 in RTC */
+static int mcp795_update_alarm(struct device *dev, bool enable)
+{
+	int ret;
+
+	dev_dbg(dev, "%s alarm\n", enable ? "Enable" : "Disable");
+
+	if (enable) {
+		/* clear ALM0IF (Alarm 0 Interrupt Flag) bit */
+		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_ALM0_DAY,
+					MCP795_ALM0IF_BIT, 0);
+		if (ret)
+			return ret;
+		/* enable alarm 0 */
+		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_CONTROL,
+					MCP795_ALM0_BIT, MCP795_ALM0_BIT);
+	} else {
+		/* disable alarm 0 and alarm 1 */
+		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_CONTROL,
+					MCP795_ALM0_BIT | MCP795_ALM1_BIT, 0);
+	}
+	return ret;
+}
+
+static int mcp795_set_time(struct device *dev, struct rtc_time *tim)
+{
+	int ret;
+	u8 data[7];
+	bool extosc;
+
+	/* Stop RTC and store current value of EXTOSC bit */
+	ret = mcp795_stop_oscillator(dev, &extosc);
+	if (ret)
+		return ret;
+
+	/* Read first, so we can leave config bits untouched */
+	ret = mcp795_rtcc_read(dev, MCP795_REG_SECONDS, data, sizeof(data));
+
+	if (ret)
+		return ret;
+
+	data[0] = (data[0] & 0x80) | bin2bcd(tim->tm_sec);
+	data[1] = (data[1] & 0x80) | bin2bcd(tim->tm_min);
+	data[2] = bin2bcd(tim->tm_hour);
+	data[3] = (data[3] & 0xF8) | bin2bcd(tim->tm_wday + 1);
+	data[4] = bin2bcd(tim->tm_mday);
+	data[5] = (data[5] & MCP795_LP_BIT) | bin2bcd(tim->tm_mon + 1);
+
+	if (tim->tm_year > 100)
+		tim->tm_year -= 100;
+
+	data[6] = bin2bcd(tim->tm_year);
+
+	/* Always write the date and month using a separate Write command.
+	 * This is a workaround for a know silicon issue that some combinations
+	 * of date and month values may result in the date being reset to 1.
+	 */
+	ret = mcp795_rtcc_write(dev, MCP795_REG_SECONDS, data, 5);
+	if (ret)
+		return ret;
+
+	ret = mcp795_rtcc_write(dev, MCP795_REG_MONTH, &data[5], 2);
+	if (ret)
+		return ret;
+
+	/* Start back RTC and restore previous value of EXTOSC bit.
+	 * There is no need to clear EXTOSC bit when the previous value was 0
+	 * because it was already cleared when stopping the RTC oscillator.
+	 */
+	ret = mcp795_start_oscillator(dev, extosc ? &extosc : NULL);
+	if (ret)
+		return ret;
+
+	dev_dbg(dev, "Set mcp795: %ptR\n", tim);
+
+	return 0;
+}
+
+static int mcp795_read_time(struct device *dev, struct rtc_time *tim)
+{
+	int ret;
+	u8 data[7];
+
+	ret = mcp795_rtcc_read(dev, MCP795_REG_SECONDS, data, sizeof(data));
+
+	if (ret)
+		return ret;
+
+	tim->tm_sec	= bcd2bin(data[0] & 0x7F);
+	tim->tm_min	= bcd2bin(data[1] & 0x7F);
+	tim->tm_hour	= bcd2bin(data[2] & 0x3F);
+	tim->tm_wday	= bcd2bin(data[3] & 0x07) - 1;
+	tim->tm_mday	= bcd2bin(data[4] & 0x3F);
+	tim->tm_mon	= bcd2bin(data[5] & 0x1F) - 1;
+	tim->tm_year	= bcd2bin(data[6]) + 100; /* Assume we are in 20xx */
+
+	dev_dbg(dev, "Read from mcp795: %ptR\n", tim);
+
+	return 0;
+}
+
+static int mcp795_set_alarm(struct device *dev, struct rtc_wkalrm *alm)
+{
+	struct rtc_time now_tm;
+	time64_t now;
+	time64_t later;
+	u8 tmp[6];
+	int ret;
+
+	/* Read current time from RTC hardware */
+	ret = mcp795_read_time(dev, &now_tm);
+	if (ret)
+		return ret;
+	/* Get the number of seconds since 1970 */
+	now = rtc_tm_to_time64(&now_tm);
+	later = rtc_tm_to_time64(&alm->time);
+	if (later <= now)
+		return -EINVAL;
+	/* make sure alarm fires within the next one year */
+	if ((later - now) >=
+		(SEC_PER_DAY * (365 + is_leap_year(alm->time.tm_year))))
+		return -EDOM;
+	/* disable alarm */
+	ret = mcp795_update_alarm(dev, false);
+	if (ret)
+		return ret;
+	/* Read registers, so we can leave configuration bits untouched */
+	ret = mcp795_rtcc_read(dev, MCP795_REG_ALM0_SECONDS, tmp, sizeof(tmp));
+	if (ret)
+		return ret;
+
+	alm->time.tm_year	= -1;
+	alm->time.tm_isdst	= -1;
+	alm->time.tm_yday	= -1;
+
+	tmp[0] = (tmp[0] & 0x80) | bin2bcd(alm->time.tm_sec);
+	tmp[1] = (tmp[1] & 0x80) | bin2bcd(alm->time.tm_min);
+	tmp[2] = (tmp[2] & 0xE0) | bin2bcd(alm->time.tm_hour);
+	tmp[3] = (tmp[3] & 0x80) | bin2bcd(alm->time.tm_wday + 1);
+	/* set alarm match: seconds, minutes, hour, day, date and month */
+	tmp[3] |= (MCP795_ALM0C2_BIT | MCP795_ALM0C1_BIT | MCP795_ALM0C0_BIT);
+	tmp[4] = (tmp[4] & 0xC0) | bin2bcd(alm->time.tm_mday);
+	tmp[5] = (tmp[5] & 0xE0) | bin2bcd(alm->time.tm_mon + 1);
+
+	ret = mcp795_rtcc_write(dev, MCP795_REG_ALM0_SECONDS, tmp, sizeof(tmp));
+	if (ret)
+		return ret;
+
+	/* enable alarm if requested */
+	if (alm->enabled) {
+		ret = mcp795_update_alarm(dev, true);
+		if (ret)
+			return ret;
+		dev_dbg(dev, "Alarm IRQ armed\n");
+	}
+	dev_dbg(dev, "Set alarm: %ptRdr(%d) %ptRt\n",
+		&alm->time, alm->time.tm_wday, &alm->time);
+	return 0;
+}
+
+static int mcp795_read_alarm(struct device *dev, struct rtc_wkalrm *alm)
+{
+	u8 data[6];
+	int ret;
+
+	ret = mcp795_rtcc_read(
+			dev, MCP795_REG_ALM0_SECONDS, data, sizeof(data));
+	if (ret)
+		return ret;
+
+	alm->time.tm_sec	= bcd2bin(data[0] & 0x7F);
+	alm->time.tm_min	= bcd2bin(data[1] & 0x7F);
+	alm->time.tm_hour	= bcd2bin(data[2] & 0x1F);
+	alm->time.tm_wday	= bcd2bin(data[3] & 0x07) - 1;
+	alm->time.tm_mday	= bcd2bin(data[4] & 0x3F);
+	alm->time.tm_mon	= bcd2bin(data[5] & 0x1F) - 1;
+	alm->time.tm_year	= -1;
+	alm->time.tm_isdst	= -1;
+	alm->time.tm_yday	= -1;
+
+	dev_dbg(dev, "Read alarm: %ptRdr(%d) %ptRt\n",
+		&alm->time, alm->time.tm_wday, &alm->time);
+	return 0;
+}
+
+static int mcp795_alarm_irq_enable(struct device *dev, unsigned int enabled)
+{
+	return mcp795_update_alarm(dev, !!enabled);
+}
+
+static irqreturn_t mcp795_irq(int irq, void *data)
+{
+	struct spi_device *spi = data;
+	struct rtc_device *rtc = spi_get_drvdata(spi);
+	int ret;
+
+	rtc_lock(rtc);
+
+	/* Disable alarm.
+	 * There is no need to clear ALM0IF (Alarm 0 Interrupt Flag) bit,
+	 * because it is done every time when alarm is enabled.
+	 */
+	ret = mcp795_update_alarm(&spi->dev, false);
+	if (ret)
+		dev_err(&spi->dev,
+			"Failed to disable alarm in IRQ (ret=%d)\n", ret);
+	rtc_update_irq(rtc, 1, RTC_AF | RTC_IRQF);
+
+	rtc_unlock(rtc);
+
+	return IRQ_HANDLED;
+}
+
+static const struct rtc_class_ops mcp795_rtc_ops = {
+		.read_time = mcp795_read_time,
+		.set_time = mcp795_set_time,
+		.read_alarm = mcp795_read_alarm,
+		.set_alarm = mcp795_set_alarm,
+		.alarm_irq_enable = mcp795_alarm_irq_enable
+};
+
+static int mcp795_probe(struct spi_device *spi)
+{
+	struct rtc_device *rtc;
+	int ret;
+
+	spi->mode = SPI_MODE_0;
+	spi->bits_per_word = 8;
+	ret = spi_setup(spi);
+	if (ret) {
+		dev_err(&spi->dev, "Unable to setup SPI\n");
+		return ret;
+	}
+
+	/* Start the oscillator but don't set the value of EXTOSC bit */
+	mcp795_start_oscillator(&spi->dev, NULL);
+	/* Clear the 12 hour mode flag*/
+	mcp795_rtcc_set_bits(&spi->dev, 0x03, MCP795_24_BIT, 0);
+
+	rtc = devm_rtc_device_register(&spi->dev, "rtc-mcp795",
+					&mcp795_rtc_ops, THIS_MODULE);
+	if (IS_ERR(rtc))
+		return PTR_ERR(rtc);
+
+	spi_set_drvdata(spi, rtc);
+
+	if (spi->irq > 0) {
+		dev_dbg(&spi->dev, "Alarm support enabled\n");
+
+		/* Clear any pending alarm (ALM0IF bit) before requesting
+		 * the interrupt.
+		 */
+		mcp795_rtcc_set_bits(&spi->dev, MCP795_REG_ALM0_DAY,
+					MCP795_ALM0IF_BIT, 0);
+		ret = devm_request_threaded_irq(&spi->dev, spi->irq, NULL,
+				mcp795_irq, IRQF_TRIGGER_FALLING | IRQF_ONESHOT,
+				dev_name(&rtc->dev), spi);
+		if (ret)
+			dev_err(&spi->dev, "Failed to request IRQ: %d: %d\n",
+						spi->irq, ret);
+		else
+			device_init_wakeup(&spi->dev, true);
+	}
+	return 0;
+}
+
+#ifdef CONFIG_OF
+static const struct of_device_id mcp795_of_match[] = {
+	{ .compatible = "maxim,mcp795" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, mcp795_of_match);
+#endif
+
+static const struct spi_device_id mcp795_spi_ids[] = {
+	{ .name = "mcp795" },
+	{ }
+};
+MODULE_DEVICE_TABLE(spi, mcp795_spi_ids);
+
+static struct spi_driver mcp795_driver = {
+		.driver = {
+				.name = "rtc-mcp795",
+				.of_match_table = of_match_ptr(mcp795_of_match),
+		},
+		.probe = mcp795_probe,
+		.id_table = mcp795_spi_ids,
+};
+
+module_spi_driver(mcp795_driver);
+
+MODULE_DESCRIPTION("MCP795 RTC SPI Driver");
+MODULE_AUTHOR("Josef Gajdusek <atx@atx.name>");
+MODULE_LICENSE("GPL");