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, 202 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-bq4802.c b/drivers/rtc/rtc-bq4802.c
new file mode 100644
index 000000000..472e75668
--- /dev/null
+++ b/drivers/rtc/rtc-bq4802.c
@@ -0,0 +1,202 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* rtc-bq4802.c: TI BQ4802 RTC driver.
+ *
+ * Copyright (C) 2008 David S. Miller <davem@davemloft.net>
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/io.h>
+#include <linux/platform_device.h>
+#include <linux/rtc.h>
+#include <linux/bcd.h>
+#include <linux/slab.h>
+
+MODULE_AUTHOR("David S. Miller <davem@davemloft.net>");
+MODULE_DESCRIPTION("TI BQ4802 RTC driver");
+MODULE_LICENSE("GPL");
+
+struct bq4802 {
+	void __iomem		*regs;
+	unsigned long		ioport;
+	struct rtc_device	*rtc;
+	spinlock_t		lock;
+	struct resource		*r;
+	u8 (*read)(struct bq4802 *, int);
+	void (*write)(struct bq4802 *, int, u8);
+};
+
+static u8 bq4802_read_io(struct bq4802 *p, int off)
+{
+	return inb(p->ioport + off);
+}
+
+static void bq4802_write_io(struct bq4802 *p, int off, u8 val)
+{
+	outb(val, p->ioport + off);
+}
+
+static u8 bq4802_read_mem(struct bq4802 *p, int off)
+{
+	return readb(p->regs + off);
+}
+
+static void bq4802_write_mem(struct bq4802 *p, int off, u8 val)
+{
+	writeb(val, p->regs + off);
+}
+
+static int bq4802_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct bq4802 *p = dev_get_drvdata(dev);
+	unsigned long flags;
+	unsigned int century;
+	u8 val;
+
+	spin_lock_irqsave(&p->lock, flags);
+
+	val = p->read(p, 0x0e);
+	p->write(p, 0xe, val | 0x08);
+
+	tm->tm_sec = p->read(p, 0x00);
+	tm->tm_min = p->read(p, 0x02);
+	tm->tm_hour = p->read(p, 0x04);
+	tm->tm_mday = p->read(p, 0x06);
+	tm->tm_mon = p->read(p, 0x09);
+	tm->tm_year = p->read(p, 0x0a);
+	tm->tm_wday = p->read(p, 0x08);
+	century = p->read(p, 0x0f);
+
+	p->write(p, 0x0e, val);
+
+	spin_unlock_irqrestore(&p->lock, flags);
+
+	tm->tm_sec = bcd2bin(tm->tm_sec);
+	tm->tm_min = bcd2bin(tm->tm_min);
+	tm->tm_hour = bcd2bin(tm->tm_hour);
+	tm->tm_mday = bcd2bin(tm->tm_mday);
+	tm->tm_mon = bcd2bin(tm->tm_mon);
+	tm->tm_year = bcd2bin(tm->tm_year);
+	tm->tm_wday = bcd2bin(tm->tm_wday);
+	century = bcd2bin(century);
+
+	tm->tm_year += (century * 100);
+	tm->tm_year -= 1900;
+
+	tm->tm_mon--;
+
+	return 0;
+}
+
+static int bq4802_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct bq4802 *p = dev_get_drvdata(dev);
+	u8 sec, min, hrs, day, mon, yrs, century, val;
+	unsigned long flags;
+	unsigned int year;
+
+	year = tm->tm_year + 1900;
+	century = year / 100;
+	yrs = year % 100;
+
+	mon = tm->tm_mon + 1;   /* tm_mon starts at zero */
+	day = tm->tm_mday;
+	hrs = tm->tm_hour;
+	min = tm->tm_min;
+	sec = tm->tm_sec;
+
+	sec = bin2bcd(sec);
+	min = bin2bcd(min);
+	hrs = bin2bcd(hrs);
+	day = bin2bcd(day);
+	mon = bin2bcd(mon);
+	yrs = bin2bcd(yrs);
+	century = bin2bcd(century);
+
+	spin_lock_irqsave(&p->lock, flags);
+
+	val = p->read(p, 0x0e);
+	p->write(p, 0x0e, val | 0x08);
+
+	p->write(p, 0x00, sec);
+	p->write(p, 0x02, min);
+	p->write(p, 0x04, hrs);
+	p->write(p, 0x06, day);
+	p->write(p, 0x09, mon);
+	p->write(p, 0x0a, yrs);
+	p->write(p, 0x0f, century);
+
+	p->write(p, 0x0e, val);
+
+	spin_unlock_irqrestore(&p->lock, flags);
+
+	return 0;
+}
+
+static const struct rtc_class_ops bq4802_ops = {
+	.read_time	= bq4802_read_time,
+	.set_time	= bq4802_set_time,
+};
+
+static int bq4802_probe(struct platform_device *pdev)
+{
+	struct bq4802 *p = devm_kzalloc(&pdev->dev, sizeof(*p), GFP_KERNEL);
+	int err = -ENOMEM;
+
+	if (!p)
+		goto out;
+
+	spin_lock_init(&p->lock);
+
+	p->r = platform_get_resource(pdev, IORESOURCE_MEM, 0);
+	if (!p->r) {
+		p->r = platform_get_resource(pdev, IORESOURCE_IO, 0);
+		err = -EINVAL;
+		if (!p->r)
+			goto out;
+	}
+	if (p->r->flags & IORESOURCE_IO) {
+		p->ioport = p->r->start;
+		p->read = bq4802_read_io;
+		p->write = bq4802_write_io;
+	} else if (p->r->flags & IORESOURCE_MEM) {
+		p->regs = devm_ioremap(&pdev->dev, p->r->start,
+					resource_size(p->r));
+		if (!p->regs){
+			err = -ENOMEM;
+			goto out;
+		}
+		p->read = bq4802_read_mem;
+		p->write = bq4802_write_mem;
+	} else {
+		err = -EINVAL;
+		goto out;
+	}
+
+	platform_set_drvdata(pdev, p);
+
+	p->rtc = devm_rtc_device_register(&pdev->dev, "bq4802",
+					&bq4802_ops, THIS_MODULE);
+	if (IS_ERR(p->rtc)) {
+		err = PTR_ERR(p->rtc);
+		goto out;
+	}
+
+	err = 0;
+out:
+	return err;
+
+}
+
+/* work with hotplug and coldplug */
+MODULE_ALIAS("platform:rtc-bq4802");
+
+static struct platform_driver bq4802_driver = {
+	.driver		= {
+		.name	= "rtc-bq4802",
+	},
+	.probe		= bq4802_probe,
+};
+
+module_platform_driver(bq4802_driver);