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, 317 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-pcf8583.c b/drivers/rtc/rtc-pcf8583.c
new file mode 100644
index 000000000..df5e20cbc
--- /dev/null
+++ b/drivers/rtc/rtc-pcf8583.c
@@ -0,0 +1,317 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  drivers/rtc/rtc-pcf8583.c
+ *
+ *  Copyright (C) 2000 Russell King
+ *  Copyright (C) 2008 Wolfram Sang & Juergen Beisert, Pengutronix
+ *
+ *  Driver for PCF8583 RTC & RAM chip
+ *
+ *  Converted to the generic RTC susbsystem by G. Liakhovetski (2006)
+ */
+#include <linux/module.h>
+#include <linux/i2c.h>
+#include <linux/slab.h>
+#include <linux/rtc.h>
+#include <linux/init.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/bcd.h>
+
+struct rtc_mem {
+	unsigned int	loc;
+	unsigned int	nr;
+	unsigned char	*data;
+};
+
+struct pcf8583 {
+	struct rtc_device *rtc;
+	unsigned char ctrl;
+};
+
+#define CTRL_STOP	0x80
+#define CTRL_HOLD	0x40
+#define CTRL_32KHZ	0x00
+#define CTRL_MASK	0x08
+#define CTRL_ALARMEN	0x04
+#define CTRL_ALARM	0x02
+#define CTRL_TIMER	0x01
+
+
+static struct i2c_driver pcf8583_driver;
+
+#define get_ctrl(x)    ((struct pcf8583 *)i2c_get_clientdata(x))->ctrl
+#define set_ctrl(x, v) get_ctrl(x) = v
+
+#define CMOS_YEAR	(64 + 128)
+#define CMOS_CHECKSUM	(63)
+
+static int pcf8583_get_datetime(struct i2c_client *client, struct rtc_time *dt)
+{
+	unsigned char buf[8], addr[1] = { 1 };
+	struct i2c_msg msgs[2] = {
+		{
+			.addr = client->addr,
+			.flags = 0,
+			.len = 1,
+			.buf = addr,
+		}, {
+			.addr = client->addr,
+			.flags = I2C_M_RD,
+			.len = 6,
+			.buf = buf,
+		}
+	};
+	int ret;
+
+	memset(buf, 0, sizeof(buf));
+
+	ret = i2c_transfer(client->adapter, msgs, 2);
+	if (ret == 2) {
+		dt->tm_year = buf[4] >> 6;
+		dt->tm_wday = buf[5] >> 5;
+
+		buf[4] &= 0x3f;
+		buf[5] &= 0x1f;
+
+		dt->tm_sec = bcd2bin(buf[1]);
+		dt->tm_min = bcd2bin(buf[2]);
+		dt->tm_hour = bcd2bin(buf[3]);
+		dt->tm_mday = bcd2bin(buf[4]);
+		dt->tm_mon = bcd2bin(buf[5]) - 1;
+	}
+
+	return ret == 2 ? 0 : -EIO;
+}
+
+static int pcf8583_set_datetime(struct i2c_client *client, struct rtc_time *dt, int datetoo)
+{
+	unsigned char buf[8];
+	int ret, len = 6;
+
+	buf[0] = 0;
+	buf[1] = get_ctrl(client) | 0x80;
+	buf[2] = 0;
+	buf[3] = bin2bcd(dt->tm_sec);
+	buf[4] = bin2bcd(dt->tm_min);
+	buf[5] = bin2bcd(dt->tm_hour);
+
+	if (datetoo) {
+		len = 8;
+		buf[6] = bin2bcd(dt->tm_mday) | (dt->tm_year << 6);
+		buf[7] = bin2bcd(dt->tm_mon + 1)  | (dt->tm_wday << 5);
+	}
+
+	ret = i2c_master_send(client, (char *)buf, len);
+	if (ret != len)
+		return -EIO;
+
+	buf[1] = get_ctrl(client);
+	ret = i2c_master_send(client, (char *)buf, 2);
+
+	return ret == 2 ? 0 : -EIO;
+}
+
+static int pcf8583_get_ctrl(struct i2c_client *client, unsigned char *ctrl)
+{
+	*ctrl = get_ctrl(client);
+	return 0;
+}
+
+static int pcf8583_set_ctrl(struct i2c_client *client, unsigned char *ctrl)
+{
+	unsigned char buf[2];
+
+	buf[0] = 0;
+	buf[1] = *ctrl;
+	set_ctrl(client, *ctrl);
+
+	return i2c_master_send(client, (char *)buf, 2);
+}
+
+static int pcf8583_read_mem(struct i2c_client *client, struct rtc_mem *mem)
+{
+	unsigned char addr[1];
+	struct i2c_msg msgs[2] = {
+		{
+			.addr = client->addr,
+			.flags = 0,
+			.len = 1,
+			.buf = addr,
+		}, {
+			.addr = client->addr,
+			.flags = I2C_M_RD,
+			.len = mem->nr,
+			.buf = mem->data,
+		}
+	};
+
+	if (mem->loc < 8)
+		return -EINVAL;
+
+	addr[0] = mem->loc;
+
+	return i2c_transfer(client->adapter, msgs, 2) == 2 ? 0 : -EIO;
+}
+
+static int pcf8583_write_mem(struct i2c_client *client, struct rtc_mem *mem)
+{
+	unsigned char buf[9];
+	int ret;
+
+	if (mem->loc < 8 || mem->nr > 8)
+		return -EINVAL;
+
+	buf[0] = mem->loc;
+	memcpy(buf + 1, mem->data, mem->nr);
+
+	ret = i2c_master_send(client, buf, mem->nr + 1);
+	return ret == mem->nr + 1 ? 0 : -EIO;
+}
+
+static int pcf8583_rtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	unsigned char ctrl, year[2];
+	struct rtc_mem mem = {
+		.loc = CMOS_YEAR,
+		.nr = sizeof(year),
+		.data = year
+	};
+	int real_year, year_offset, err;
+
+	/*
+	 * Ensure that the RTC is running.
+	 */
+	pcf8583_get_ctrl(client, &ctrl);
+	if (ctrl & (CTRL_STOP | CTRL_HOLD)) {
+		unsigned char new_ctrl = ctrl & ~(CTRL_STOP | CTRL_HOLD);
+
+		dev_warn(dev, "resetting control %02x -> %02x\n",
+			ctrl, new_ctrl);
+
+		err = pcf8583_set_ctrl(client, &new_ctrl);
+		if (err < 0)
+			return err;
+	}
+
+	if (pcf8583_get_datetime(client, tm) ||
+	    pcf8583_read_mem(client, &mem))
+		return -EIO;
+
+	real_year = year[0];
+
+	/*
+	 * The RTC year holds the LSB two bits of the current
+	 * year, which should reflect the LSB two bits of the
+	 * CMOS copy of the year.  Any difference indicates
+	 * that we have to correct the CMOS version.
+	 */
+	year_offset = tm->tm_year - (real_year & 3);
+	if (year_offset < 0)
+		/*
+		 * RTC year wrapped.  Adjust it appropriately.
+		 */
+		year_offset += 4;
+
+	tm->tm_year = (real_year + year_offset + year[1] * 100) - 1900;
+
+	return 0;
+}
+
+static int pcf8583_rtc_set_time(struct device *dev, struct rtc_time *tm)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	unsigned char year[2], chk;
+	struct rtc_mem cmos_year  = {
+		.loc = CMOS_YEAR,
+		.nr = sizeof(year),
+		.data = year
+	};
+	struct rtc_mem cmos_check = {
+		.loc = CMOS_CHECKSUM,
+		.nr = 1,
+		.data = &chk
+	};
+	unsigned int proper_year = tm->tm_year + 1900;
+	int ret;
+
+	/*
+	 * The RTC's own 2-bit year must reflect the least
+	 * significant two bits of the CMOS year.
+	 */
+
+	ret = pcf8583_set_datetime(client, tm, 1);
+	if (ret)
+		return ret;
+
+	ret = pcf8583_read_mem(client, &cmos_check);
+	if (ret)
+		return ret;
+
+	ret = pcf8583_read_mem(client, &cmos_year);
+	if (ret)
+		return ret;
+
+	chk -= year[1] + year[0];
+
+	year[1] = proper_year / 100;
+	year[0] = proper_year % 100;
+
+	chk += year[1] + year[0];
+
+	ret = pcf8583_write_mem(client, &cmos_year);
+
+	if (ret)
+		return ret;
+
+	ret = pcf8583_write_mem(client, &cmos_check);
+
+	return ret;
+}
+
+static const struct rtc_class_ops pcf8583_rtc_ops = {
+	.read_time	= pcf8583_rtc_read_time,
+	.set_time	= pcf8583_rtc_set_time,
+};
+
+static int pcf8583_probe(struct i2c_client *client)
+{
+	struct pcf8583 *pcf8583;
+
+	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
+		return -ENODEV;
+
+	pcf8583 = devm_kzalloc(&client->dev, sizeof(struct pcf8583),
+				GFP_KERNEL);
+	if (!pcf8583)
+		return -ENOMEM;
+
+	i2c_set_clientdata(client, pcf8583);
+
+	pcf8583->rtc = devm_rtc_device_register(&client->dev,
+				pcf8583_driver.driver.name,
+				&pcf8583_rtc_ops, THIS_MODULE);
+
+	return PTR_ERR_OR_ZERO(pcf8583->rtc);
+}
+
+static const struct i2c_device_id pcf8583_id[] = {
+	{ .name = "pcf8583" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, pcf8583_id);
+
+static struct i2c_driver pcf8583_driver = {
+	.driver = {
+		.name	= "pcf8583",
+	},
+	.probe		= pcf8583_probe,
+	.id_table	= pcf8583_id,
+};
+
+module_i2c_driver(pcf8583_driver);
+
+MODULE_AUTHOR("Russell King");
+MODULE_DESCRIPTION("PCF8583 I2C RTC driver");
+MODULE_LICENSE("GPL");