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, 215 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-rs5c348.c b/drivers/rtc/rtc-rs5c348.c
new file mode 100644
index 000000000..fec633f80
--- /dev/null
+++ b/drivers/rtc/rtc-rs5c348.c
@@ -0,0 +1,215 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * A SPI driver for the Ricoh RS5C348 RTC
+ *
+ * Copyright (C) 2006 Atsushi Nemoto <anemo@mba.ocn.ne.jp>
+ *
+ * The board specific init code should provide characteristics of this
+ * device:
+ *     Mode 1 (High-Active, Shift-Then-Sample), High Avtive CS
+ */
+
+#include <linux/bcd.h>
+#include <linux/delay.h>
+#include <linux/device.h>
+#include <linux/errno.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/string.h>
+#include <linux/slab.h>
+#include <linux/rtc.h>
+#include <linux/workqueue.h>
+#include <linux/spi/spi.h>
+#include <linux/module.h>
+
+#define RS5C348_REG_SECS	0
+#define RS5C348_REG_MINS	1
+#define RS5C348_REG_HOURS	2
+#define RS5C348_REG_WDAY	3
+#define RS5C348_REG_DAY	4
+#define RS5C348_REG_MONTH	5
+#define RS5C348_REG_YEAR	6
+#define RS5C348_REG_CTL1	14
+#define RS5C348_REG_CTL2	15
+
+#define RS5C348_SECS_MASK	0x7f
+#define RS5C348_MINS_MASK	0x7f
+#define RS5C348_HOURS_MASK	0x3f
+#define RS5C348_WDAY_MASK	0x03
+#define RS5C348_DAY_MASK	0x3f
+#define RS5C348_MONTH_MASK	0x1f
+
+#define RS5C348_BIT_PM	0x20	/* REG_HOURS */
+#define RS5C348_BIT_Y2K	0x80	/* REG_MONTH */
+#define RS5C348_BIT_24H	0x20	/* REG_CTL1 */
+#define RS5C348_BIT_XSTP	0x10	/* REG_CTL2 */
+#define RS5C348_BIT_VDET	0x40	/* REG_CTL2 */
+
+#define RS5C348_CMD_W(addr)	(((addr) << 4) | 0x08)	/* single write */
+#define RS5C348_CMD_R(addr)	(((addr) << 4) | 0x0c)	/* single read */
+#define RS5C348_CMD_MW(addr)	(((addr) << 4) | 0x00)	/* burst write */
+#define RS5C348_CMD_MR(addr)	(((addr) << 4) | 0x04)	/* burst read */
+
+struct rs5c348_plat_data {
+	struct rtc_device *rtc;
+	int rtc_24h;
+};
+
+static int
+rs5c348_rtc_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct spi_device *spi = to_spi_device(dev);
+	struct rs5c348_plat_data *pdata = dev_get_platdata(&spi->dev);
+	u8 txbuf[5+7], *txp;
+	int ret;
+
+	ret = spi_w8r8(spi, RS5C348_CMD_R(RS5C348_REG_CTL2));
+	if (ret < 0)
+		return ret;
+	if (ret & RS5C348_BIT_XSTP) {
+		txbuf[0] = RS5C348_CMD_W(RS5C348_REG_CTL2);
+		txbuf[1] = 0;
+		ret = spi_write_then_read(spi, txbuf, 2, NULL, 0);
+		if (ret < 0)
+			return ret;
+	}
+
+	/* Transfer 5 bytes before writing SEC.  This gives 31us for carry. */
+	txp = txbuf;
+	txbuf[0] = RS5C348_CMD_R(RS5C348_REG_CTL2); /* cmd, ctl2 */
+	txbuf[1] = 0;	/* dummy */
+	txbuf[2] = RS5C348_CMD_R(RS5C348_REG_CTL2); /* cmd, ctl2 */
+	txbuf[3] = 0;	/* dummy */
+	txbuf[4] = RS5C348_CMD_MW(RS5C348_REG_SECS); /* cmd, sec, ... */
+	txp = &txbuf[5];
+	txp[RS5C348_REG_SECS] = bin2bcd(tm->tm_sec);
+	txp[RS5C348_REG_MINS] = bin2bcd(tm->tm_min);
+	if (pdata->rtc_24h) {
+		txp[RS5C348_REG_HOURS] = bin2bcd(tm->tm_hour);
+	} else {
+		/* hour 0 is AM12, noon is PM12 */
+		txp[RS5C348_REG_HOURS] = bin2bcd((tm->tm_hour + 11) % 12 + 1) |
+			(tm->tm_hour >= 12 ? RS5C348_BIT_PM : 0);
+	}
+	txp[RS5C348_REG_WDAY] = bin2bcd(tm->tm_wday);
+	txp[RS5C348_REG_DAY] = bin2bcd(tm->tm_mday);
+	txp[RS5C348_REG_MONTH] = bin2bcd(tm->tm_mon + 1) |
+		(tm->tm_year >= 100 ? RS5C348_BIT_Y2K : 0);
+	txp[RS5C348_REG_YEAR] = bin2bcd(tm->tm_year % 100);
+	/* write in one transfer to avoid data inconsistency */
+	ret = spi_write_then_read(spi, txbuf, sizeof(txbuf), NULL, 0);
+	udelay(62);	/* Tcsr 62us */
+	return ret;
+}
+
+static int
+rs5c348_rtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct spi_device *spi = to_spi_device(dev);
+	struct rs5c348_plat_data *pdata = dev_get_platdata(&spi->dev);
+	u8 txbuf[5], rxbuf[7];
+	int ret;
+
+	ret = spi_w8r8(spi, RS5C348_CMD_R(RS5C348_REG_CTL2));
+	if (ret < 0)
+		return ret;
+	if (ret & RS5C348_BIT_VDET)
+		dev_warn(&spi->dev, "voltage-low detected.\n");
+	if (ret & RS5C348_BIT_XSTP) {
+		dev_warn(&spi->dev, "oscillator-stop detected.\n");
+		return -EINVAL;
+	}
+
+	/* Transfer 5 byte befores reading SEC.  This gives 31us for carry. */
+	txbuf[0] = RS5C348_CMD_R(RS5C348_REG_CTL2); /* cmd, ctl2 */
+	txbuf[1] = 0;	/* dummy */
+	txbuf[2] = RS5C348_CMD_R(RS5C348_REG_CTL2); /* cmd, ctl2 */
+	txbuf[3] = 0;	/* dummy */
+	txbuf[4] = RS5C348_CMD_MR(RS5C348_REG_SECS); /* cmd, sec, ... */
+
+	/* read in one transfer to avoid data inconsistency */
+	ret = spi_write_then_read(spi, txbuf, sizeof(txbuf),
+				  rxbuf, sizeof(rxbuf));
+	udelay(62);	/* Tcsr 62us */
+	if (ret < 0)
+		return ret;
+
+	tm->tm_sec = bcd2bin(rxbuf[RS5C348_REG_SECS] & RS5C348_SECS_MASK);
+	tm->tm_min = bcd2bin(rxbuf[RS5C348_REG_MINS] & RS5C348_MINS_MASK);
+	tm->tm_hour = bcd2bin(rxbuf[RS5C348_REG_HOURS] & RS5C348_HOURS_MASK);
+	if (!pdata->rtc_24h) {
+		if (rxbuf[RS5C348_REG_HOURS] & RS5C348_BIT_PM) {
+			tm->tm_hour -= 20;
+			tm->tm_hour %= 12;
+			tm->tm_hour += 12;
+		} else
+			tm->tm_hour %= 12;
+	}
+	tm->tm_wday = bcd2bin(rxbuf[RS5C348_REG_WDAY] & RS5C348_WDAY_MASK);
+	tm->tm_mday = bcd2bin(rxbuf[RS5C348_REG_DAY] & RS5C348_DAY_MASK);
+	tm->tm_mon =
+		bcd2bin(rxbuf[RS5C348_REG_MONTH] & RS5C348_MONTH_MASK) - 1;
+	/* year is 1900 + tm->tm_year */
+	tm->tm_year = bcd2bin(rxbuf[RS5C348_REG_YEAR]) +
+		((rxbuf[RS5C348_REG_MONTH] & RS5C348_BIT_Y2K) ? 100 : 0);
+
+	return 0;
+}
+
+static const struct rtc_class_ops rs5c348_rtc_ops = {
+	.read_time	= rs5c348_rtc_read_time,
+	.set_time	= rs5c348_rtc_set_time,
+};
+
+static int rs5c348_probe(struct spi_device *spi)
+{
+	int ret;
+	struct rtc_device *rtc;
+	struct rs5c348_plat_data *pdata;
+
+	pdata = devm_kzalloc(&spi->dev, sizeof(struct rs5c348_plat_data),
+				GFP_KERNEL);
+	if (!pdata)
+		return -ENOMEM;
+	spi->dev.platform_data = pdata;
+
+	/* Check D7 of SECOND register */
+	ret = spi_w8r8(spi, RS5C348_CMD_R(RS5C348_REG_SECS));
+	if (ret < 0 || (ret & 0x80)) {
+		dev_err(&spi->dev, "not found.\n");
+		return ret;
+	}
+
+	dev_info(&spi->dev, "spiclk %u KHz.\n",
+		 (spi->max_speed_hz + 500) / 1000);
+
+	ret = spi_w8r8(spi, RS5C348_CMD_R(RS5C348_REG_CTL1));
+	if (ret < 0)
+		return ret;
+	if (ret & RS5C348_BIT_24H)
+		pdata->rtc_24h = 1;
+
+	rtc = devm_rtc_allocate_device(&spi->dev);
+	if (IS_ERR(rtc))
+		return PTR_ERR(rtc);
+
+	pdata->rtc = rtc;
+
+	rtc->ops = &rs5c348_rtc_ops;
+
+	return devm_rtc_register_device(rtc);
+}
+
+static struct spi_driver rs5c348_driver = {
+	.driver = {
+		.name	= "rtc-rs5c348",
+	},
+	.probe	= rs5c348_probe,
+};
+
+module_spi_driver(rs5c348_driver);
+
+MODULE_AUTHOR("Atsushi Nemoto <anemo@mba.ocn.ne.jp>");
+MODULE_DESCRIPTION("Ricoh RS5C348 RTC driver");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("spi:rtc-rs5c348");