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, 167 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-spacemit-p1.c b/drivers/rtc/rtc-spacemit-p1.c
new file mode 100644
index 000000000..43ab62494
--- /dev/null
+++ b/drivers/rtc/rtc-spacemit-p1.c
@@ -0,0 +1,167 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Driver for the RTC found in the SpacemiT P1 PMIC
+ *
+ * Copyright (C) 2025 by RISCstar Solutions Corporation.  All rights reserved.
+ */
+
+#include <linux/bits.h>
+#include <linux/device.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/regmap.h>
+#include <linux/rtc.h>
+
+#define MOD_NAME	"spacemit-p1-rtc"
+
+/*
+ * Six consecutive 1-byte registers hold the seconds, minutes, hours,
+ * day-of-month, month, and year (respectively).
+ *
+ * The range of values in these registers is:
+ *    seconds	0-59
+ *    minutes	0-59
+ *    hours	0-59
+ *    day	0-30 (struct tm is 1-31)
+ *    month	0-11
+ *    year	years since 2000 (struct tm is since 1900)
+ *
+ * Note that the day and month must be converted after reading and
+ * before writing.
+ */
+#define RTC_TIME		0x0d	/* Offset of the seconds register */
+
+#define RTC_CTRL		0x1d
+#define RTC_EN		BIT(2)
+
+/* Number of attempts to read a consistent time stamp before giving up */
+#define RTC_READ_TRIES		20	/* At least 1 */
+
+struct p1_rtc {
+	struct regmap *regmap;
+	struct rtc_device *rtc;
+};
+
+/*
+ * The P1 hardware documentation states that the register values are
+ * latched to ensure a consistent time snapshot within the registers,
+ * but these are in fact unstable due to a bug in the hardware design.
+ * So we loop until we get two identical readings.
+ */
+static int p1_rtc_read_time(struct device *dev, struct rtc_time *t)
+{
+	struct p1_rtc *p1 = dev_get_drvdata(dev);
+	struct regmap *regmap = p1->regmap;
+	u32 count = RTC_READ_TRIES;
+	u8 seconds;
+	u8 time[6];
+	int ret;
+
+	if (!regmap_test_bits(regmap, RTC_CTRL, RTC_EN))
+		return -EINVAL;		/* RTC is disabled */
+
+	ret = regmap_bulk_read(regmap, RTC_TIME, time, sizeof(time));
+	if (ret)
+		return ret;
+
+	do {
+		seconds = time[0];
+		ret = regmap_bulk_read(regmap, RTC_TIME, time, sizeof(time));
+		if (ret)
+			return ret;
+	} while (time[0] != seconds && --count);
+
+	if (!count)
+		return -EIO;		/* Unable to get a consistent result */
+
+	t->tm_sec = time[0] & GENMASK(5, 0);
+	t->tm_min = time[1] & GENMASK(5, 0);
+	t->tm_hour = time[2] & GENMASK(4, 0);
+	t->tm_mday = (time[3] & GENMASK(4, 0)) + 1;
+	t->tm_mon = time[4] & GENMASK(3, 0);
+	t->tm_year = (time[5] & GENMASK(5, 0)) + 100;
+
+	return 0;
+}
+
+/*
+ * The P1 hardware documentation states that values in the registers are
+ * latched so when written they represent a consistent time snapshot.
+ * Nevertheless, this is not guaranteed by the implementation, so we must
+ * disable the RTC while updating it.
+ */
+static int p1_rtc_set_time(struct device *dev, struct rtc_time *t)
+{
+	struct p1_rtc *p1 = dev_get_drvdata(dev);
+	struct regmap *regmap = p1->regmap;
+	u8 time[6];
+	int ret;
+
+	time[0] = t->tm_sec;
+	time[1] = t->tm_min;
+	time[2] = t->tm_hour;
+	time[3] = t->tm_mday - 1;
+	time[4] = t->tm_mon;
+	time[5] = t->tm_year - 100;
+
+	/* Disable the RTC to update; re-enable again when done */
+	ret = regmap_clear_bits(regmap, RTC_CTRL, RTC_EN);
+	if (ret)
+		return ret;
+
+	/* If something goes wrong, leave the RTC disabled */
+	ret = regmap_bulk_write(regmap, RTC_TIME, time, sizeof(time));
+	if (ret)
+		return ret;
+
+	return regmap_set_bits(regmap, RTC_CTRL, RTC_EN);
+}
+
+static const struct rtc_class_ops p1_rtc_class_ops = {
+	.read_time = p1_rtc_read_time,
+	.set_time = p1_rtc_set_time,
+};
+
+static int p1_rtc_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct rtc_device *rtc;
+	struct p1_rtc *p1;
+
+	p1 = devm_kzalloc(dev, sizeof(*p1), GFP_KERNEL);
+	if (!p1)
+		return -ENOMEM;
+	dev_set_drvdata(dev, p1);
+
+	p1->regmap = dev_get_regmap(dev->parent, NULL);
+	if (!p1->regmap)
+		return dev_err_probe(dev, -ENODEV, "failed to get regmap\n");
+
+	rtc = devm_rtc_allocate_device(dev);
+	if (IS_ERR(rtc))
+		return dev_err_probe(dev, PTR_ERR(rtc),
+				     "error allocating device\n");
+	p1->rtc = rtc;
+
+	rtc->ops = &p1_rtc_class_ops;
+	rtc->range_min = RTC_TIMESTAMP_BEGIN_2000;
+	rtc->range_max = RTC_TIMESTAMP_END_2063;
+
+	clear_bit(RTC_FEATURE_ALARM, rtc->features);
+	clear_bit(RTC_FEATURE_UPDATE_INTERRUPT, rtc->features);
+
+	return devm_rtc_register_device(rtc);
+}
+
+static struct platform_driver p1_rtc_driver = {
+	.probe = p1_rtc_probe,
+	.driver = {
+		.name = MOD_NAME,
+	},
+};
+
+module_platform_driver(p1_rtc_driver);
+
+MODULE_DESCRIPTION("SpacemiT P1 RTC driver");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:" MOD_NAME);