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, 229 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-sd2405al.c b/drivers/rtc/rtc-sd2405al.c
new file mode 100644
index 000000000..7982a910a
--- /dev/null
+++ b/drivers/rtc/rtc-sd2405al.c
@@ -0,0 +1,229 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * RTC driver for the SD2405AL Real-Time Clock
+ *
+ * Datasheet:
+ * https://image.dfrobot.com/image/data/TOY0021/SD2405AL%20datasheet%20(Angelo%20v0.1).pdf
+ *
+ * I2C slave address: 0x32
+ *
+ * Copyright (C) 2024-2025 Tóth János <gomba007@gmail.com>
+ */
+
+#include <linux/bcd.h>
+#include <linux/i2c.h>
+#include <linux/regmap.h>
+#include <linux/rtc.h>
+
+/* Real time clock registers */
+#define SD2405AL_REG_T_SEC	0x00
+#define SD2405AL_REG_T_MIN	0x01
+#define SD2405AL_REG_T_HOUR	0x02
+#	define SD2405AL_BIT_12H_PM	BIT(5)
+#	define SD2405AL_BIT_24H		BIT(7)
+#define SD2405AL_REG_T_WEEK	0x03
+#define SD2405AL_REG_T_DAY	0x04
+#define SD2405AL_REG_T_MON	0x05
+#define SD2405AL_REG_T_YEAR	0x06
+
+#define SD2405AL_NUM_T_REGS	(SD2405AL_REG_T_YEAR - SD2405AL_REG_T_SEC + 1)
+
+/* Control registers */
+#define SD2405AL_REG_CTR1	0x0F
+#	define SD2405AL_BIT_WRTC2	BIT(2)
+#	define SD2405AL_BIT_WRTC3	BIT(7)
+#define SD2405AL_REG_CTR2	0x10
+#	define SD2405AL_BIT_WRTC1	BIT(7)
+#define SD2405AL_REG_CTR3	0x11
+#define SD2405AL_REG_TTF	0x12
+#define SD2405AL_REG_CNTDWN	0x13
+
+/* General RAM */
+#define SD2405AL_REG_M_START	0x14
+#define SD2405AL_REG_M_END	0x1F
+
+struct sd2405al {
+	struct device		*dev;
+	struct regmap		*regmap;
+};
+
+static int sd2405al_enable_reg_write(struct sd2405al *sd2405al)
+{
+	int ret;
+
+	/* order of writes is important */
+	ret = regmap_update_bits(sd2405al->regmap, SD2405AL_REG_CTR2,
+				 SD2405AL_BIT_WRTC1, SD2405AL_BIT_WRTC1);
+	if (ret < 0)
+		return ret;
+
+	ret = regmap_update_bits(sd2405al->regmap, SD2405AL_REG_CTR1,
+				 SD2405AL_BIT_WRTC2 | SD2405AL_BIT_WRTC3,
+				 SD2405AL_BIT_WRTC2 | SD2405AL_BIT_WRTC3);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+}
+
+static int sd2405al_disable_reg_write(struct sd2405al *sd2405al)
+{
+	int ret;
+
+	/* order of writes is important */
+	ret = regmap_update_bits(sd2405al->regmap, SD2405AL_REG_CTR1,
+				 SD2405AL_BIT_WRTC2 | SD2405AL_BIT_WRTC3, 0x00);
+	if (ret < 0)
+		return ret;
+
+	ret = regmap_update_bits(sd2405al->regmap, SD2405AL_REG_CTR2,
+				 SD2405AL_BIT_WRTC1, 0x00);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+}
+
+static int sd2405al_read_time(struct device *dev, struct rtc_time *time)
+{
+	u8 data[SD2405AL_NUM_T_REGS] = { 0 };
+	struct sd2405al *sd2405al = dev_get_drvdata(dev);
+	int ret;
+
+	ret = regmap_bulk_read(sd2405al->regmap, SD2405AL_REG_T_SEC, data,
+			       SD2405AL_NUM_T_REGS);
+	if (ret < 0)
+		return ret;
+
+	time->tm_sec = bcd2bin(data[SD2405AL_REG_T_SEC] & 0x7F);
+	time->tm_min = bcd2bin(data[SD2405AL_REG_T_MIN] & 0x7F);
+
+	if (data[SD2405AL_REG_T_HOUR] & SD2405AL_BIT_24H)
+		time->tm_hour = bcd2bin(data[SD2405AL_REG_T_HOUR] & 0x3F);
+	else
+		if (data[SD2405AL_REG_T_HOUR] & SD2405AL_BIT_12H_PM)
+			time->tm_hour = bcd2bin(data[SD2405AL_REG_T_HOUR]
+						& 0x1F) + 12;
+		else /* 12 hour mode, AM */
+			time->tm_hour = bcd2bin(data[SD2405AL_REG_T_HOUR]
+						& 0x1F);
+
+	time->tm_wday = bcd2bin(data[SD2405AL_REG_T_WEEK] & 0x07);
+	time->tm_mday = bcd2bin(data[SD2405AL_REG_T_DAY] & 0x3F);
+	time->tm_mon = bcd2bin(data[SD2405AL_REG_T_MON] & 0x1F) - 1;
+	time->tm_year = bcd2bin(data[SD2405AL_REG_T_YEAR]) + 100;
+
+	dev_dbg(sd2405al->dev, "read time: %ptR (%d)\n", time, time->tm_wday);
+
+	return 0;
+}
+
+static int sd2405al_set_time(struct device *dev, struct rtc_time *time)
+{
+	u8 data[SD2405AL_NUM_T_REGS];
+	struct sd2405al *sd2405al = dev_get_drvdata(dev);
+	int ret;
+
+	data[SD2405AL_REG_T_SEC] = bin2bcd(time->tm_sec);
+	data[SD2405AL_REG_T_MIN] = bin2bcd(time->tm_min);
+	data[SD2405AL_REG_T_HOUR] = bin2bcd(time->tm_hour) | SD2405AL_BIT_24H;
+	data[SD2405AL_REG_T_DAY] = bin2bcd(time->tm_mday);
+	data[SD2405AL_REG_T_WEEK] = bin2bcd(time->tm_wday);
+	data[SD2405AL_REG_T_MON] = bin2bcd(time->tm_mon) + 1;
+	data[SD2405AL_REG_T_YEAR] = bin2bcd(time->tm_year - 100);
+
+	ret = sd2405al_enable_reg_write(sd2405al);
+	if (ret < 0)
+		return ret;
+
+	ret = regmap_bulk_write(sd2405al->regmap, SD2405AL_REG_T_SEC, data,
+				SD2405AL_NUM_T_REGS);
+	if (ret < 0)
+		return ret;
+
+	ret = regmap_write(sd2405al->regmap, SD2405AL_REG_TTF, 0x00);
+	if (ret < 0)
+		return ret;
+
+	ret = sd2405al_disable_reg_write(sd2405al);
+	if (ret < 0)
+		return ret;
+
+	dev_dbg(sd2405al->dev, "set time: %ptR (%d)\n", time, time->tm_wday);
+
+	return 0;
+}
+
+static const struct rtc_class_ops sd2405al_rtc_ops = {
+	.read_time = sd2405al_read_time,
+	.set_time = sd2405al_set_time,
+};
+
+static const struct regmap_config sd2405al_regmap_conf = {
+	.reg_bits = 8,
+	.val_bits = 8,
+	.max_register = SD2405AL_REG_M_END,
+};
+
+static int sd2405al_probe(struct i2c_client *client)
+{
+	struct sd2405al *sd2405al;
+	struct rtc_device *rtc;
+	int ret;
+
+	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
+		return -ENODEV;
+
+	sd2405al = devm_kzalloc(&client->dev, sizeof(*sd2405al), GFP_KERNEL);
+	if (!sd2405al)
+		return -ENOMEM;
+
+	sd2405al->dev = &client->dev;
+
+	sd2405al->regmap = devm_regmap_init_i2c(client, &sd2405al_regmap_conf);
+	if (IS_ERR(sd2405al->regmap))
+		return PTR_ERR(sd2405al->regmap);
+
+	rtc = devm_rtc_allocate_device(&client->dev);
+	if (IS_ERR(rtc))
+		return PTR_ERR(rtc);
+
+	rtc->ops = &sd2405al_rtc_ops;
+	rtc->range_min = RTC_TIMESTAMP_BEGIN_2000;
+	rtc->range_max = RTC_TIMESTAMP_END_2099;
+
+	dev_set_drvdata(&client->dev, sd2405al);
+
+	ret = devm_rtc_register_device(rtc);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+}
+
+static const struct i2c_device_id sd2405al_id[] = {
+	{ .name = "sd2405al" },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(i2c, sd2405al_id);
+
+static const __maybe_unused struct of_device_id sd2405al_of_match[] = {
+	{ .compatible = "dfrobot,sd2405al" },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, sd2405al_of_match);
+
+static struct i2c_driver sd2405al_driver = {
+	.driver = {
+		.name = "sd2405al",
+		.of_match_table = of_match_ptr(sd2405al_of_match),
+	},
+	.probe = sd2405al_probe,
+	.id_table = sd2405al_id,
+};
+
+module_i2c_driver(sd2405al_driver);
+
+MODULE_AUTHOR("Tóth János <gomba007@gmail.com>");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("SD2405AL RTC driver");