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-ds1742.c b/drivers/rtc/rtc-ds1742.c
new file mode 100644
index 000000000..6ae8b9a29
--- /dev/null
+++ b/drivers/rtc/rtc-ds1742.c
@@ -0,0 +1,220 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * An rtc driver for the Dallas DS1742
+ *
+ * Copyright (C) 2006 Atsushi Nemoto <anemo@mba.ocn.ne.jp>
+ *
+ * Copyright (C) 2006 Torsten Ertbjerg Rasmussen <tr@newtec.dk>
+ *  - nvram size determined from resource
+ *  - this ds1742 driver now supports ds1743.
+ */
+
+#include <linux/bcd.h>
+#include <linux/kernel.h>
+#include <linux/gfp.h>
+#include <linux/delay.h>
+#include <linux/jiffies.h>
+#include <linux/rtc.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/io.h>
+#include <linux/module.h>
+
+#define RTC_SIZE		8
+
+#define RTC_CONTROL		0
+#define RTC_CENTURY		0
+#define RTC_SECONDS		1
+#define RTC_MINUTES		2
+#define RTC_HOURS		3
+#define RTC_DAY			4
+#define RTC_DATE		5
+#define RTC_MONTH		6
+#define RTC_YEAR		7
+
+#define RTC_CENTURY_MASK	0x3f
+#define RTC_SECONDS_MASK	0x7f
+#define RTC_DAY_MASK		0x07
+
+/* Bits in the Control/Century register */
+#define RTC_WRITE		0x80
+#define RTC_READ		0x40
+
+/* Bits in the Seconds register */
+#define RTC_STOP		0x80
+
+/* Bits in the Day register */
+#define RTC_BATT_FLAG		0x80
+
+struct rtc_plat_data {
+	void __iomem *ioaddr_nvram;
+	void __iomem *ioaddr_rtc;
+	unsigned long last_jiffies;
+};
+
+static int ds1742_rtc_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct rtc_plat_data *pdata = dev_get_drvdata(dev);
+	void __iomem *ioaddr = pdata->ioaddr_rtc;
+	u8 century;
+
+	century = bin2bcd((tm->tm_year + 1900) / 100);
+
+	writeb(RTC_WRITE, ioaddr + RTC_CONTROL);
+
+	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);
+
+	/* RTC_CENTURY and RTC_CONTROL share same register */
+	writeb(RTC_WRITE | (century & RTC_CENTURY_MASK), ioaddr + RTC_CENTURY);
+	writeb(century & RTC_CENTURY_MASK, ioaddr + RTC_CONTROL);
+	return 0;
+}
+
+static int ds1742_rtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct rtc_plat_data *pdata = dev_get_drvdata(dev);
+	void __iomem *ioaddr = pdata->ioaddr_rtc;
+	unsigned int year, month, day, hour, minute, second, week;
+	unsigned int century;
+
+	/* give enough time to update RTC in case of continuous read */
+	if (pdata->last_jiffies == jiffies)
+		msleep(1);
+	pdata->last_jiffies = jiffies;
+	writeb(RTC_READ, ioaddr + RTC_CONTROL);
+	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) & RTC_CENTURY_MASK;
+	writeb(0, ioaddr + RTC_CONTROL);
+	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 const struct rtc_class_ops ds1742_rtc_ops = {
+	.read_time	= ds1742_rtc_read_time,
+	.set_time	= ds1742_rtc_set_time,
+};
+
+static int ds1742_nvram_read(void *priv, unsigned int pos, void *val,
+			     size_t bytes)
+{
+	struct rtc_plat_data *pdata = priv;
+	void __iomem *ioaddr = pdata->ioaddr_nvram;
+	u8 *buf = val;
+
+	for (; bytes; bytes--)
+		*buf++ = readb(ioaddr + pos++);
+	return 0;
+}
+
+static int ds1742_nvram_write(void *priv, unsigned int pos, void *val,
+			      size_t bytes)
+{
+	struct rtc_plat_data *pdata = priv;
+	void __iomem *ioaddr = pdata->ioaddr_nvram;
+	u8 *buf = val;
+
+	for (; bytes; bytes--)
+		writeb(*buf++, ioaddr + pos++);
+	return 0;
+}
+
+static int ds1742_rtc_probe(struct platform_device *pdev)
+{
+	struct rtc_device *rtc;
+	struct resource *res;
+	unsigned int cen, sec;
+	struct rtc_plat_data *pdata;
+	void __iomem *ioaddr;
+	int ret = 0;
+	struct nvmem_config nvmem_cfg = {
+		.name = "ds1742_nvram",
+		.reg_read = ds1742_nvram_read,
+		.reg_write = ds1742_nvram_write,
+	};
+
+
+	pdata = devm_kzalloc(&pdev->dev, sizeof(*pdata), GFP_KERNEL);
+	if (!pdata)
+		return -ENOMEM;
+
+	ioaddr = devm_platform_get_and_ioremap_resource(pdev, 0, &res);
+	if (IS_ERR(ioaddr))
+		return PTR_ERR(ioaddr);
+
+	pdata->ioaddr_nvram = ioaddr;
+	pdata->ioaddr_rtc = ioaddr + resource_size(res) - RTC_SIZE;
+
+	nvmem_cfg.size = resource_size(res) - RTC_SIZE;
+	nvmem_cfg.priv = pdata;
+
+	/* turn RTC on if it was not on */
+	ioaddr = pdata->ioaddr_rtc;
+	sec = readb(ioaddr + RTC_SECONDS);
+	if (sec & RTC_STOP) {
+		sec &= RTC_SECONDS_MASK;
+		cen = readb(ioaddr + RTC_CENTURY) & RTC_CENTURY_MASK;
+		writeb(RTC_WRITE, ioaddr + RTC_CONTROL);
+		writeb(sec, ioaddr + RTC_SECONDS);
+		writeb(cen & RTC_CENTURY_MASK, ioaddr + RTC_CONTROL);
+	}
+	if (!(readb(ioaddr + RTC_DAY) & RTC_BATT_FLAG))
+		dev_warn(&pdev->dev, "voltage-low detected.\n");
+
+	pdata->last_jiffies = jiffies;
+	platform_set_drvdata(pdev, pdata);
+
+	rtc = devm_rtc_allocate_device(&pdev->dev);
+	if (IS_ERR(rtc))
+		return PTR_ERR(rtc);
+
+	rtc->ops = &ds1742_rtc_ops;
+
+	ret = devm_rtc_register_device(rtc);
+	if (ret)
+		return ret;
+
+	devm_rtc_nvmem_register(rtc, &nvmem_cfg);
+
+	return 0;
+}
+
+static const struct of_device_id __maybe_unused ds1742_rtc_of_match[] = {
+	{ .compatible = "maxim,ds1742", },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, ds1742_rtc_of_match);
+
+static struct platform_driver ds1742_rtc_driver = {
+	.probe		= ds1742_rtc_probe,
+	.driver		= {
+		.name	= "rtc-ds1742",
+		.of_match_table = of_match_ptr(ds1742_rtc_of_match),
+	},
+};
+
+module_platform_driver(ds1742_rtc_driver);
+
+MODULE_AUTHOR("Atsushi Nemoto <anemo@mba.ocn.ne.jp>");
+MODULE_DESCRIPTION("Dallas DS1742 RTC driver");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:rtc-ds1742");