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, 220 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-max8907.c b/drivers/rtc/rtc-max8907.c
new file mode 100644
index 000000000..af97140dd
--- /dev/null
+++ b/drivers/rtc/rtc-max8907.c
@@ -0,0 +1,220 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * RTC driver for Maxim MAX8907
+ *
+ * Copyright (c) 2011-2012, NVIDIA Corporation.
+ *
+ * Based on drivers/rtc/rtc-max8925.c,
+ * Copyright (C) 2009-2010 Marvell International Ltd.
+ */
+
+#include <linux/bcd.h>
+#include <linux/mfd/max8907.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/regmap.h>
+#include <linux/rtc.h>
+#include <linux/slab.h>
+
+enum {
+	RTC_SEC = 0,
+	RTC_MIN,
+	RTC_HOUR,
+	RTC_WEEKDAY,
+	RTC_DATE,
+	RTC_MONTH,
+	RTC_YEAR1,
+	RTC_YEAR2,
+};
+
+#define TIME_NUM			8
+#define ALARM_1SEC			(1 << 7)
+#define HOUR_12				(1 << 7)
+#define HOUR_AM_PM			(1 << 5)
+#define ALARM0_IRQ			(1 << 3)
+#define ALARM1_IRQ			(1 << 2)
+#define ALARM0_STATUS			(1 << 2)
+#define ALARM1_STATUS			(1 << 1)
+
+struct max8907_rtc {
+	struct max8907		*max8907;
+	struct regmap		*regmap;
+	struct rtc_device	*rtc_dev;
+	int			irq;
+};
+
+static irqreturn_t max8907_irq_handler(int irq, void *data)
+{
+	struct max8907_rtc *rtc = data;
+
+	regmap_write(rtc->regmap, MAX8907_REG_ALARM0_CNTL, 0);
+
+	rtc_update_irq(rtc->rtc_dev, 1, RTC_IRQF | RTC_AF);
+
+	return IRQ_HANDLED;
+}
+
+static void regs_to_tm(u8 *regs, struct rtc_time *tm)
+{
+	tm->tm_year = bcd2bin(regs[RTC_YEAR2]) * 100 +
+		bcd2bin(regs[RTC_YEAR1]) - 1900;
+	tm->tm_mon = bcd2bin(regs[RTC_MONTH] & 0x1f) - 1;
+	tm->tm_mday = bcd2bin(regs[RTC_DATE] & 0x3f);
+	tm->tm_wday = (regs[RTC_WEEKDAY] & 0x07);
+	if (regs[RTC_HOUR] & HOUR_12) {
+		tm->tm_hour = bcd2bin(regs[RTC_HOUR] & 0x01f);
+		if (tm->tm_hour == 12)
+			tm->tm_hour = 0;
+		if (regs[RTC_HOUR] & HOUR_AM_PM)
+			tm->tm_hour += 12;
+	} else {
+		tm->tm_hour = bcd2bin(regs[RTC_HOUR] & 0x03f);
+	}
+	tm->tm_min = bcd2bin(regs[RTC_MIN] & 0x7f);
+	tm->tm_sec = bcd2bin(regs[RTC_SEC] & 0x7f);
+}
+
+static void tm_to_regs(struct rtc_time *tm, u8 *regs)
+{
+	u8 high, low;
+
+	high = (tm->tm_year + 1900) / 100;
+	low = tm->tm_year % 100;
+	regs[RTC_YEAR2] = bin2bcd(high);
+	regs[RTC_YEAR1] = bin2bcd(low);
+	regs[RTC_MONTH] = bin2bcd(tm->tm_mon + 1);
+	regs[RTC_DATE] = bin2bcd(tm->tm_mday);
+	regs[RTC_WEEKDAY] = tm->tm_wday;
+	regs[RTC_HOUR] = bin2bcd(tm->tm_hour);
+	regs[RTC_MIN] = bin2bcd(tm->tm_min);
+	regs[RTC_SEC] = bin2bcd(tm->tm_sec);
+}
+
+static int max8907_rtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct max8907_rtc *rtc = dev_get_drvdata(dev);
+	u8 regs[TIME_NUM];
+	int ret;
+
+	ret = regmap_bulk_read(rtc->regmap, MAX8907_REG_RTC_SEC, regs,
+			       TIME_NUM);
+	if (ret < 0)
+		return ret;
+
+	regs_to_tm(regs, tm);
+
+	return 0;
+}
+
+static int max8907_rtc_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct max8907_rtc *rtc = dev_get_drvdata(dev);
+	u8 regs[TIME_NUM];
+
+	tm_to_regs(tm, regs);
+
+	return regmap_bulk_write(rtc->regmap, MAX8907_REG_RTC_SEC, regs,
+				 TIME_NUM);
+}
+
+static int max8907_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
+{
+	struct max8907_rtc *rtc = dev_get_drvdata(dev);
+	u8 regs[TIME_NUM];
+	unsigned int val;
+	int ret;
+
+	ret = regmap_bulk_read(rtc->regmap, MAX8907_REG_ALARM0_SEC, regs,
+			       TIME_NUM);
+	if (ret < 0)
+		return ret;
+
+	regs_to_tm(regs, &alrm->time);
+
+	ret = regmap_read(rtc->regmap, MAX8907_REG_ALARM0_CNTL, &val);
+	if (ret < 0)
+		return ret;
+
+	alrm->enabled = !!(val & 0x7f);
+
+	return 0;
+}
+
+static int max8907_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
+{
+	struct max8907_rtc *rtc = dev_get_drvdata(dev);
+	u8 regs[TIME_NUM];
+	int ret;
+
+	tm_to_regs(&alrm->time, regs);
+
+	/* Disable alarm while we update the target time */
+	ret = regmap_write(rtc->regmap, MAX8907_REG_ALARM0_CNTL, 0);
+	if (ret < 0)
+		return ret;
+
+	ret = regmap_bulk_write(rtc->regmap, MAX8907_REG_ALARM0_SEC, regs,
+				TIME_NUM);
+	if (ret < 0)
+		return ret;
+
+	if (alrm->enabled)
+		ret = regmap_write(rtc->regmap, MAX8907_REG_ALARM0_CNTL, 0x77);
+
+	return ret;
+}
+
+static const struct rtc_class_ops max8907_rtc_ops = {
+	.read_time	= max8907_rtc_read_time,
+	.set_time	= max8907_rtc_set_time,
+	.read_alarm	= max8907_rtc_read_alarm,
+	.set_alarm	= max8907_rtc_set_alarm,
+};
+
+static int max8907_rtc_probe(struct platform_device *pdev)
+{
+	struct max8907 *max8907 = dev_get_drvdata(pdev->dev.parent);
+	struct max8907_rtc *rtc;
+	int ret;
+
+	rtc = devm_kzalloc(&pdev->dev, sizeof(*rtc), GFP_KERNEL);
+	if (!rtc)
+		return -ENOMEM;
+	platform_set_drvdata(pdev, rtc);
+
+	rtc->max8907 = max8907;
+	rtc->regmap = max8907->regmap_rtc;
+
+	rtc->rtc_dev = devm_rtc_device_register(&pdev->dev, "max8907-rtc",
+					&max8907_rtc_ops, THIS_MODULE);
+	if (IS_ERR(rtc->rtc_dev)) {
+		ret = PTR_ERR(rtc->rtc_dev);
+		dev_err(&pdev->dev, "Failed to register RTC device: %d\n", ret);
+		return ret;
+	}
+
+	rtc->irq = regmap_irq_get_virq(max8907->irqc_rtc,
+				       MAX8907_IRQ_RTC_ALARM0);
+	if (rtc->irq < 0)
+		return rtc->irq;
+
+	ret = devm_request_threaded_irq(&pdev->dev, rtc->irq, NULL,
+				max8907_irq_handler,
+				IRQF_ONESHOT, "max8907-alarm0", rtc);
+	if (ret < 0)
+		dev_err(&pdev->dev, "Failed to request IRQ%d: %d\n",
+			rtc->irq, ret);
+
+	return ret;
+}
+
+static struct platform_driver max8907_rtc_driver = {
+	.driver = {
+		.name = "max8907-rtc",
+	},
+	.probe = max8907_rtc_probe,
+};
+module_platform_driver(max8907_rtc_driver);
+
+MODULE_DESCRIPTION("Maxim MAX8907 RTC driver");
+MODULE_LICENSE("GPL v2");