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, 337 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-stk17ta8.c b/drivers/rtc/rtc-stk17ta8.c
new file mode 100644
index 000000000..7cb6be1b7
--- /dev/null
+++ b/drivers/rtc/rtc-stk17ta8.c
@@ -0,0 +1,337 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * A RTC driver for the Simtek STK17TA8
+ *
+ * By Thomas Hommel <thomas.hommel@ge.com>
+ *
+ * Based on the DS1553 driver from
+ * Atsushi Nemoto <anemo@mba.ocn.ne.jp>
+ */
+
+#include <linux/bcd.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/gfp.h>
+#include <linux/delay.h>
+#include <linux/jiffies.h>
+#include <linux/interrupt.h>
+#include <linux/rtc.h>
+#include <linux/platform_device.h>
+#include <linux/io.h>
+#include <linux/module.h>
+
+#define RTC_REG_SIZE		0x20000
+#define RTC_OFFSET		0x1fff0
+
+#define RTC_FLAGS		(RTC_OFFSET + 0)
+#define RTC_CENTURY		(RTC_OFFSET + 1)
+#define RTC_SECONDS_ALARM	(RTC_OFFSET + 2)
+#define RTC_MINUTES_ALARM	(RTC_OFFSET + 3)
+#define RTC_HOURS_ALARM		(RTC_OFFSET + 4)
+#define RTC_DATE_ALARM		(RTC_OFFSET + 5)
+#define RTC_INTERRUPTS		(RTC_OFFSET + 6)
+#define RTC_WATCHDOG		(RTC_OFFSET + 7)
+#define RTC_CALIBRATION		(RTC_OFFSET + 8)
+#define RTC_SECONDS		(RTC_OFFSET + 9)
+#define RTC_MINUTES		(RTC_OFFSET + 10)
+#define RTC_HOURS		(RTC_OFFSET + 11)
+#define RTC_DAY			(RTC_OFFSET + 12)
+#define RTC_DATE		(RTC_OFFSET + 13)
+#define RTC_MONTH		(RTC_OFFSET + 14)
+#define RTC_YEAR		(RTC_OFFSET + 15)
+
+#define RTC_SECONDS_MASK	0x7f
+#define RTC_DAY_MASK		0x07
+#define RTC_CAL_MASK		0x3f
+
+/* Bits in the Calibration register */
+#define RTC_STOP		0x80
+
+/* Bits in the Flags register */
+#define RTC_FLAGS_AF		0x40
+#define RTC_FLAGS_PF		0x20
+#define RTC_WRITE		0x02
+#define RTC_READ		0x01
+
+/* Bits in the Interrupts register */
+#define RTC_INTS_AIE		0x40
+
+struct rtc_plat_data {
+	struct rtc_device *rtc;
+	void __iomem *ioaddr;
+	unsigned long last_jiffies;
+	int irq;
+	unsigned int irqen;
+	int alrm_sec;
+	int alrm_min;
+	int alrm_hour;
+	int alrm_mday;
+	spinlock_t lock;
+};
+
+static int stk17ta8_rtc_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct rtc_plat_data *pdata = dev_get_drvdata(dev);
+	void __iomem *ioaddr = pdata->ioaddr;
+	u8 flags;
+
+	flags = readb(pdata->ioaddr + RTC_FLAGS);
+	writeb(flags | RTC_WRITE, pdata->ioaddr + RTC_FLAGS);
+
+	writeb(bin2bcd(tm->tm_year % 100), ioaddr + RTC_YEAR);
+	writeb(bin2bcd(tm->tm_mon + 1), ioaddr + RTC_MONTH);
+	writeb(bin2bcd(tm->tm_wday) & RTC_DAY_MASK, ioaddr + RTC_DAY);
+	writeb(bin2bcd(tm->tm_mday), ioaddr + RTC_DATE);
+	writeb(bin2bcd(tm->tm_hour), ioaddr + RTC_HOURS);
+	writeb(bin2bcd(tm->tm_min), ioaddr + RTC_MINUTES);
+	writeb(bin2bcd(tm->tm_sec) & RTC_SECONDS_MASK, ioaddr + RTC_SECONDS);
+	writeb(bin2bcd((tm->tm_year + 1900) / 100), ioaddr + RTC_CENTURY);
+
+	writeb(flags & ~RTC_WRITE, pdata->ioaddr + RTC_FLAGS);
+	return 0;
+}
+
+static int stk17ta8_rtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct rtc_plat_data *pdata = dev_get_drvdata(dev);
+	void __iomem *ioaddr = pdata->ioaddr;
+	unsigned int year, month, day, hour, minute, second, week;
+	unsigned int century;
+	u8 flags;
+
+	/* give enough time to update RTC in case of continuous read */
+	if (pdata->last_jiffies == jiffies)
+		msleep(1);
+	pdata->last_jiffies = jiffies;
+
+	flags = readb(pdata->ioaddr + RTC_FLAGS);
+	writeb(flags | RTC_READ, ioaddr + RTC_FLAGS);
+	second = readb(ioaddr + RTC_SECONDS) & RTC_SECONDS_MASK;
+	minute = readb(ioaddr + RTC_MINUTES);
+	hour = readb(ioaddr + RTC_HOURS);
+	day = readb(ioaddr + RTC_DATE);
+	week = readb(ioaddr + RTC_DAY) & RTC_DAY_MASK;
+	month = readb(ioaddr + RTC_MONTH);
+	year = readb(ioaddr + RTC_YEAR);
+	century = readb(ioaddr + RTC_CENTURY);
+	writeb(flags & ~RTC_READ, ioaddr + RTC_FLAGS);
+	tm->tm_sec = bcd2bin(second);
+	tm->tm_min = bcd2bin(minute);
+	tm->tm_hour = bcd2bin(hour);
+	tm->tm_mday = bcd2bin(day);
+	tm->tm_wday = bcd2bin(week);
+	tm->tm_mon = bcd2bin(month) - 1;
+	/* year is 1900 + tm->tm_year */
+	tm->tm_year = bcd2bin(year) + bcd2bin(century) * 100 - 1900;
+
+	return 0;
+}
+
+static void stk17ta8_rtc_update_alarm(struct rtc_plat_data *pdata)
+{
+	void __iomem *ioaddr = pdata->ioaddr;
+	unsigned long irqflags;
+	u8 flags;
+
+	spin_lock_irqsave(&pdata->lock, irqflags);
+
+	flags = readb(ioaddr + RTC_FLAGS);
+	writeb(flags | RTC_WRITE, ioaddr + RTC_FLAGS);
+
+	writeb(pdata->alrm_mday < 0 || (pdata->irqen & RTC_UF) ?
+	       0x80 : bin2bcd(pdata->alrm_mday),
+	       ioaddr + RTC_DATE_ALARM);
+	writeb(pdata->alrm_hour < 0 || (pdata->irqen & RTC_UF) ?
+	       0x80 : bin2bcd(pdata->alrm_hour),
+	       ioaddr + RTC_HOURS_ALARM);
+	writeb(pdata->alrm_min < 0 || (pdata->irqen & RTC_UF) ?
+	       0x80 : bin2bcd(pdata->alrm_min),
+	       ioaddr + RTC_MINUTES_ALARM);
+	writeb(pdata->alrm_sec < 0 || (pdata->irqen & RTC_UF) ?
+	       0x80 : bin2bcd(pdata->alrm_sec),
+	       ioaddr + RTC_SECONDS_ALARM);
+	writeb(pdata->irqen ? RTC_INTS_AIE : 0, ioaddr + RTC_INTERRUPTS);
+	readb(ioaddr + RTC_FLAGS);	/* clear interrupts */
+	writeb(flags & ~RTC_WRITE, ioaddr + RTC_FLAGS);
+	spin_unlock_irqrestore(&pdata->lock, irqflags);
+}
+
+static int stk17ta8_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
+{
+	struct rtc_plat_data *pdata = dev_get_drvdata(dev);
+
+	if (pdata->irq <= 0)
+		return -EINVAL;
+	pdata->alrm_mday = alrm->time.tm_mday;
+	pdata->alrm_hour = alrm->time.tm_hour;
+	pdata->alrm_min = alrm->time.tm_min;
+	pdata->alrm_sec = alrm->time.tm_sec;
+	if (alrm->enabled)
+		pdata->irqen |= RTC_AF;
+	stk17ta8_rtc_update_alarm(pdata);
+	return 0;
+}
+
+static int stk17ta8_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
+{
+	struct rtc_plat_data *pdata = dev_get_drvdata(dev);
+
+	if (pdata->irq <= 0)
+		return -EINVAL;
+	alrm->time.tm_mday = pdata->alrm_mday < 0 ? 0 : pdata->alrm_mday;
+	alrm->time.tm_hour = pdata->alrm_hour < 0 ? 0 : pdata->alrm_hour;
+	alrm->time.tm_min = pdata->alrm_min < 0 ? 0 : pdata->alrm_min;
+	alrm->time.tm_sec = pdata->alrm_sec < 0 ? 0 : pdata->alrm_sec;
+	alrm->enabled = (pdata->irqen & RTC_AF) ? 1 : 0;
+	return 0;
+}
+
+static irqreturn_t stk17ta8_rtc_interrupt(int irq, void *dev_id)
+{
+	struct platform_device *pdev = dev_id;
+	struct rtc_plat_data *pdata = platform_get_drvdata(pdev);
+	void __iomem *ioaddr = pdata->ioaddr;
+	unsigned long events = 0;
+
+	spin_lock(&pdata->lock);
+	/* read and clear interrupt */
+	if (readb(ioaddr + RTC_FLAGS) & RTC_FLAGS_AF) {
+		events = RTC_IRQF;
+		if (readb(ioaddr + RTC_SECONDS_ALARM) & 0x80)
+			events |= RTC_UF;
+		else
+			events |= RTC_AF;
+		rtc_update_irq(pdata->rtc, 1, events);
+	}
+	spin_unlock(&pdata->lock);
+	return events ? IRQ_HANDLED : IRQ_NONE;
+}
+
+static int stk17ta8_rtc_alarm_irq_enable(struct device *dev,
+	unsigned int enabled)
+{
+	struct rtc_plat_data *pdata = dev_get_drvdata(dev);
+
+	if (pdata->irq <= 0)
+		return -EINVAL;
+	if (enabled)
+		pdata->irqen |= RTC_AF;
+	else
+		pdata->irqen &= ~RTC_AF;
+	stk17ta8_rtc_update_alarm(pdata);
+	return 0;
+}
+
+static const struct rtc_class_ops stk17ta8_rtc_ops = {
+	.read_time		= stk17ta8_rtc_read_time,
+	.set_time		= stk17ta8_rtc_set_time,
+	.read_alarm		= stk17ta8_rtc_read_alarm,
+	.set_alarm		= stk17ta8_rtc_set_alarm,
+	.alarm_irq_enable	= stk17ta8_rtc_alarm_irq_enable,
+};
+
+static int stk17ta8_nvram_read(void *priv, unsigned int pos, void *val,
+			       size_t bytes)
+{
+	struct rtc_plat_data *pdata = priv;
+	void __iomem *ioaddr = pdata->ioaddr;
+	u8 *buf = val;
+
+	for (; bytes; bytes--)
+		*buf++ = readb(ioaddr + pos++);
+	return 0;
+}
+
+static int stk17ta8_nvram_write(void *priv, unsigned int pos, void *val,
+				size_t bytes)
+{
+	struct rtc_plat_data *pdata = priv;
+	void __iomem *ioaddr = pdata->ioaddr;
+	u8 *buf = val;
+
+	for (; bytes; bytes--)
+		writeb(*buf++, ioaddr + pos++);
+	return 0;
+}
+
+static int stk17ta8_rtc_probe(struct platform_device *pdev)
+{
+	unsigned int cal;
+	unsigned int flags;
+	struct rtc_plat_data *pdata;
+	void __iomem *ioaddr;
+	int ret = 0;
+	struct nvmem_config nvmem_cfg = {
+		.name = "stk17ta8_nvram",
+		.word_size = 1,
+		.stride = 1,
+		.size = RTC_OFFSET,
+		.reg_read = stk17ta8_nvram_read,
+		.reg_write = stk17ta8_nvram_write,
+	};
+
+	pdata = devm_kzalloc(&pdev->dev, sizeof(*pdata), GFP_KERNEL);
+	if (!pdata)
+		return -ENOMEM;
+
+	ioaddr = devm_platform_ioremap_resource(pdev, 0);
+	if (IS_ERR(ioaddr))
+		return PTR_ERR(ioaddr);
+	pdata->ioaddr = ioaddr;
+	pdata->irq = platform_get_irq(pdev, 0);
+
+	/* turn RTC on if it was not on */
+	cal = readb(ioaddr + RTC_CALIBRATION);
+	if (cal & RTC_STOP) {
+		cal &= RTC_CAL_MASK;
+		flags = readb(ioaddr + RTC_FLAGS);
+		writeb(flags | RTC_WRITE, ioaddr + RTC_FLAGS);
+		writeb(cal, ioaddr + RTC_CALIBRATION);
+		writeb(flags & ~RTC_WRITE, ioaddr + RTC_FLAGS);
+	}
+	if (readb(ioaddr + RTC_FLAGS) & RTC_FLAGS_PF)
+		dev_warn(&pdev->dev, "voltage-low detected.\n");
+
+	spin_lock_init(&pdata->lock);
+	pdata->last_jiffies = jiffies;
+	platform_set_drvdata(pdev, pdata);
+	if (pdata->irq > 0) {
+		writeb(0, ioaddr + RTC_INTERRUPTS);
+		if (devm_request_irq(&pdev->dev, pdata->irq,
+				stk17ta8_rtc_interrupt,
+				IRQF_SHARED,
+				pdev->name, pdev) < 0) {
+			dev_warn(&pdev->dev, "interrupt not available.\n");
+			pdata->irq = 0;
+		}
+	}
+
+	pdata->rtc = devm_rtc_allocate_device(&pdev->dev);
+	if (IS_ERR(pdata->rtc))
+		return PTR_ERR(pdata->rtc);
+
+	pdata->rtc->ops = &stk17ta8_rtc_ops;
+
+	nvmem_cfg.priv = pdata;
+	ret = devm_rtc_nvmem_register(pdata->rtc, &nvmem_cfg);
+	if (ret)
+		return ret;
+
+	return devm_rtc_register_device(pdata->rtc);
+}
+
+/* work with hotplug and coldplug */
+MODULE_ALIAS("platform:stk17ta8");
+
+static struct platform_driver stk17ta8_rtc_driver = {
+	.probe		= stk17ta8_rtc_probe,
+	.driver		= {
+		.name	= "stk17ta8",
+	},
+};
+
+module_platform_driver(stk17ta8_rtc_driver);
+
+MODULE_AUTHOR("Thomas Hommel <thomas.hommel@ge.com>");
+MODULE_DESCRIPTION("Simtek STK17TA8 RTC driver");
+MODULE_LICENSE("GPL");