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, 213 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-sd3078.c b/drivers/rtc/rtc-sd3078.c
new file mode 100644
index 000000000..2c61f0e20
--- /dev/null
+++ b/drivers/rtc/rtc-sd3078.c
@@ -0,0 +1,213 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Real Time Clock (RTC) Driver for sd3078
+ * Copyright (C) 2018 Zoro Li
+ */
+
+#include <linux/bcd.h>
+#include <linux/i2c.h>
+#include <linux/module.h>
+#include <linux/regmap.h>
+#include <linux/rtc.h>
+#include <linux/slab.h>
+
+#define SD3078_REG_SC			0x00
+#define SD3078_REG_MN			0x01
+#define SD3078_REG_HR			0x02
+#define SD3078_REG_DW			0x03
+#define SD3078_REG_DM			0x04
+#define SD3078_REG_MO			0x05
+#define SD3078_REG_YR			0x06
+
+#define SD3078_REG_CTRL1		0x0f
+#define SD3078_REG_CTRL2		0x10
+#define SD3078_REG_CTRL3		0x11
+
+#define KEY_WRITE1		0x80
+#define KEY_WRITE2		0x04
+#define KEY_WRITE3		0x80
+
+#define NUM_TIME_REGS   (SD3078_REG_YR - SD3078_REG_SC + 1)
+
+/*
+ * The sd3078 has write protection
+ * and we can choose whether or not to use it.
+ * Write protection is turned off by default.
+ */
+#define WRITE_PROTECT_EN	0
+
+/*
+ * In order to prevent arbitrary modification of the time register,
+ * when modification of the register,
+ * the "write" bit needs to be written in a certain order.
+ * 1. set WRITE1 bit
+ * 2. set WRITE2 bit
+ * 3. set WRITE3 bit
+ */
+static void sd3078_enable_reg_write(struct regmap *regmap)
+{
+	regmap_update_bits(regmap, SD3078_REG_CTRL2, KEY_WRITE1, KEY_WRITE1);
+	regmap_update_bits(regmap, SD3078_REG_CTRL1, KEY_WRITE2, KEY_WRITE2);
+	regmap_update_bits(regmap, SD3078_REG_CTRL1, KEY_WRITE3, KEY_WRITE3);
+}
+
+#if WRITE_PROTECT_EN
+/*
+ * In order to prevent arbitrary modification of the time register,
+ * we should disable the write function.
+ * when disable write,
+ * the "write" bit needs to be clear in a certain order.
+ * 1. clear WRITE2 bit
+ * 2. clear WRITE3 bit
+ * 3. clear WRITE1 bit
+ */
+static void sd3078_disable_reg_write(struct regmap *regmap)
+{
+	regmap_update_bits(regmap, SD3078_REG_CTRL1, KEY_WRITE2, 0);
+	regmap_update_bits(regmap, SD3078_REG_CTRL1, KEY_WRITE3, 0);
+	regmap_update_bits(regmap, SD3078_REG_CTRL2, KEY_WRITE1, 0);
+}
+#endif
+
+static int sd3078_rtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	unsigned char hour;
+	unsigned char rtc_data[NUM_TIME_REGS] = {0};
+	struct i2c_client *client = to_i2c_client(dev);
+	struct regmap *regmap = i2c_get_clientdata(client);
+	int ret;
+
+	ret = regmap_bulk_read(regmap, SD3078_REG_SC, rtc_data, NUM_TIME_REGS);
+	if (ret < 0) {
+		dev_err(dev, "reading from RTC failed with err:%d\n", ret);
+		return ret;
+	}
+
+	tm->tm_sec	= bcd2bin(rtc_data[SD3078_REG_SC] & 0x7F);
+	tm->tm_min	= bcd2bin(rtc_data[SD3078_REG_MN] & 0x7F);
+
+	/*
+	 * The sd3078 supports 12/24 hour mode.
+	 * When getting time,
+	 * we need to convert the 12 hour mode to the 24 hour mode.
+	 */
+	hour = rtc_data[SD3078_REG_HR];
+	if (hour & 0x80) /* 24H MODE */
+		tm->tm_hour = bcd2bin(rtc_data[SD3078_REG_HR] & 0x3F);
+	else if (hour & 0x20) /* 12H MODE PM */
+		tm->tm_hour = bcd2bin(rtc_data[SD3078_REG_HR] & 0x1F) + 12;
+	else /* 12H MODE AM */
+		tm->tm_hour = bcd2bin(rtc_data[SD3078_REG_HR] & 0x1F);
+
+	tm->tm_mday = bcd2bin(rtc_data[SD3078_REG_DM] & 0x3F);
+	tm->tm_wday = rtc_data[SD3078_REG_DW] & 0x07;
+	tm->tm_mon	= bcd2bin(rtc_data[SD3078_REG_MO] & 0x1F) - 1;
+	tm->tm_year = bcd2bin(rtc_data[SD3078_REG_YR]) + 100;
+
+	return 0;
+}
+
+static int sd3078_rtc_set_time(struct device *dev, struct rtc_time *tm)
+{
+	unsigned char rtc_data[NUM_TIME_REGS];
+	struct i2c_client *client = to_i2c_client(dev);
+	struct regmap *regmap = i2c_get_clientdata(client);
+	int ret;
+
+	rtc_data[SD3078_REG_SC] = bin2bcd(tm->tm_sec);
+	rtc_data[SD3078_REG_MN] = bin2bcd(tm->tm_min);
+	rtc_data[SD3078_REG_HR] = bin2bcd(tm->tm_hour) | 0x80;
+	rtc_data[SD3078_REG_DM] = bin2bcd(tm->tm_mday);
+	rtc_data[SD3078_REG_DW] = tm->tm_wday & 0x07;
+	rtc_data[SD3078_REG_MO] = bin2bcd(tm->tm_mon) + 1;
+	rtc_data[SD3078_REG_YR] = bin2bcd(tm->tm_year - 100);
+
+#if WRITE_PROTECT_EN
+	sd3078_enable_reg_write(regmap);
+#endif
+
+	ret = regmap_bulk_write(regmap, SD3078_REG_SC, rtc_data,
+				NUM_TIME_REGS);
+	if (ret < 0) {
+		dev_err(dev, "writing to RTC failed with err:%d\n", ret);
+		return ret;
+	}
+
+#if WRITE_PROTECT_EN
+	sd3078_disable_reg_write(regmap);
+#endif
+
+	return 0;
+}
+
+static const struct rtc_class_ops sd3078_rtc_ops = {
+	.read_time	= sd3078_rtc_read_time,
+	.set_time	= sd3078_rtc_set_time,
+};
+
+static const struct regmap_config regmap_config = {
+	.reg_bits = 8,
+	.val_bits = 8,
+	.max_register = 0x11,
+};
+
+static int sd3078_probe(struct i2c_client *client)
+{
+	int ret;
+	struct regmap *regmap;
+	struct rtc_device *rtc;
+
+	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
+		return -ENODEV;
+
+	regmap = devm_regmap_init_i2c(client, &regmap_config);
+	if (IS_ERR(regmap)) {
+		dev_err(&client->dev, "regmap allocation failed\n");
+		return PTR_ERR(regmap);
+	}
+
+	i2c_set_clientdata(client, regmap);
+
+	rtc = devm_rtc_allocate_device(&client->dev);
+	if (IS_ERR(rtc))
+		return PTR_ERR(rtc);
+
+	rtc->ops = &sd3078_rtc_ops;
+	rtc->range_min = RTC_TIMESTAMP_BEGIN_2000;
+	rtc->range_max = RTC_TIMESTAMP_END_2099;
+
+	ret = devm_rtc_register_device(rtc);
+	if (ret)
+		return ret;
+
+	sd3078_enable_reg_write(regmap);
+
+	return 0;
+}
+
+static const struct i2c_device_id sd3078_id[] = {
+	{ .name = "sd3078" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, sd3078_id);
+
+static const __maybe_unused struct of_device_id rtc_dt_match[] = {
+	{ .compatible = "whwave,sd3078" },
+	{},
+};
+MODULE_DEVICE_TABLE(of, rtc_dt_match);
+
+static struct i2c_driver sd3078_driver = {
+	.driver     = {
+		.name   = "sd3078",
+		.of_match_table = of_match_ptr(rtc_dt_match),
+	},
+	.probe      = sd3078_probe,
+	.id_table   = sd3078_id,
+};
+
+module_i2c_driver(sd3078_driver);
+
+MODULE_AUTHOR("Dianlong Li <long17.cool@163.com>");
+MODULE_DESCRIPTION("SD3078 RTC driver");
+MODULE_LICENSE("GPL v2");