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, 270 insertions, 0 deletions

diff --git a/drivers/mfd/ntxec.c b/drivers/mfd/ntxec.c
new file mode 100644
index 000000000..08c68de0f
--- /dev/null
+++ b/drivers/mfd/ntxec.c
@@ -0,0 +1,270 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * The Netronix embedded controller is a microcontroller found in some
+ * e-book readers designed by the original design manufacturer Netronix, Inc.
+ * It contains RTC, battery monitoring, system power management, and PWM
+ * functionality.
+ *
+ * This driver implements register access, version detection, and system
+ * power-off/reset.
+ *
+ * Copyright 2020 Jonathan Neuschäfer <j.neuschaefer@gmx.net>
+ */
+
+#include <linux/delay.h>
+#include <linux/errno.h>
+#include <linux/i2c.h>
+#include <linux/mfd/core.h>
+#include <linux/mfd/ntxec.h>
+#include <linux/module.h>
+#include <linux/pm.h>
+#include <linux/reboot.h>
+#include <linux/regmap.h>
+#include <linux/types.h>
+#include <linux/unaligned.h>
+
+#define NTXEC_REG_VERSION	0x00
+#define NTXEC_REG_POWEROFF	0x50
+#define NTXEC_REG_POWERKEEP	0x70
+#define NTXEC_REG_RESET		0x90
+
+#define NTXEC_POWEROFF_VALUE	0x0100
+#define NTXEC_POWERKEEP_VALUE	0x0800
+#define NTXEC_RESET_VALUE	0xff00
+
+static struct i2c_client *poweroff_restart_client;
+
+static void ntxec_poweroff(void)
+{
+	int res;
+	u8 buf[3] = { NTXEC_REG_POWEROFF };
+	struct i2c_msg msgs[] = {
+		{
+			.addr = poweroff_restart_client->addr,
+			.flags = 0,
+			.len = sizeof(buf),
+			.buf = buf,
+		},
+	};
+
+	put_unaligned_be16(NTXEC_POWEROFF_VALUE, buf + 1);
+
+	res = i2c_transfer(poweroff_restart_client->adapter, msgs, ARRAY_SIZE(msgs));
+	if (res < 0)
+		dev_warn(&poweroff_restart_client->dev,
+			 "Failed to power off (err = %d)\n", res);
+
+	/*
+	 * The time from the register write until the host CPU is powered off
+	 * has been observed to be about 2.5 to 3 seconds. Sleep long enough to
+	 * safely avoid returning from the poweroff handler.
+	 */
+	msleep(5000);
+}
+
+static int ntxec_restart(struct notifier_block *nb,
+			 unsigned long action, void *data)
+{
+	int res;
+	u8 buf[3] = { NTXEC_REG_RESET };
+	/*
+	 * NOTE: The lower half of the reset value is not sent, because sending
+	 * it causes an I2C error. (The reset handler in the downstream driver
+	 * does send the full two-byte value, but doesn't check the result).
+	 */
+	struct i2c_msg msgs[] = {
+		{
+			.addr = poweroff_restart_client->addr,
+			.flags = 0,
+			.len = sizeof(buf) - 1,
+			.buf = buf,
+		},
+	};
+
+	put_unaligned_be16(NTXEC_RESET_VALUE, buf + 1);
+
+	res = i2c_transfer(poweroff_restart_client->adapter, msgs, ARRAY_SIZE(msgs));
+	if (res < 0)
+		dev_warn(&poweroff_restart_client->dev,
+			 "Failed to restart (err = %d)\n", res);
+
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block ntxec_restart_handler = {
+	.notifier_call = ntxec_restart,
+	.priority = 128,
+};
+
+static int regmap_ignore_write(void *context,
+			       unsigned int reg, unsigned int val)
+
+{
+	struct regmap *regmap = context;
+
+	regmap_write(regmap, reg, val);
+
+	return 0;
+}
+
+static int regmap_wrap_read(void *context, unsigned int reg,
+			    unsigned int *val)
+{
+	struct regmap *regmap = context;
+
+	return regmap_read(regmap, reg, val);
+}
+
+/*
+ * Some firmware versions do not ack written data, add a wrapper. It
+ * is used to stack another regmap on top.
+ */
+static const struct regmap_config regmap_config_noack = {
+	.name = "ntxec_noack",
+	.reg_bits = 8,
+	.val_bits = 16,
+	.cache_type = REGCACHE_NONE,
+	.reg_write = regmap_ignore_write,
+	.reg_read = regmap_wrap_read
+};
+
+static const struct regmap_config regmap_config = {
+	.name = "ntxec",
+	.reg_bits = 8,
+	.val_bits = 16,
+	.cache_type = REGCACHE_NONE,
+	.val_format_endian = REGMAP_ENDIAN_BIG,
+};
+
+static const struct mfd_cell ntxec_subdev[] = {
+	{ .name = "ntxec-rtc" },
+	{ .name = "ntxec-pwm" },
+};
+
+static const struct mfd_cell ntxec_subdev_pwm[] = {
+	{ .name = "ntxec-pwm" },
+};
+
+static int ntxec_probe(struct i2c_client *client)
+{
+	struct ntxec *ec;
+	unsigned int version;
+	int res;
+	const struct mfd_cell *subdevs;
+	size_t n_subdevs;
+
+	ec = devm_kmalloc(&client->dev, sizeof(*ec), GFP_KERNEL);
+	if (!ec)
+		return -ENOMEM;
+
+	ec->dev = &client->dev;
+
+	ec->regmap = devm_regmap_init_i2c(client, &regmap_config);
+	if (IS_ERR(ec->regmap)) {
+		dev_err(ec->dev, "Failed to set up regmap for device\n");
+		return PTR_ERR(ec->regmap);
+	}
+
+	/* Determine the firmware version */
+	res = regmap_read(ec->regmap, NTXEC_REG_VERSION, &version);
+	if (res < 0) {
+		dev_err(ec->dev, "Failed to read firmware version number\n");
+		return res;
+	}
+
+	/* Bail out if we encounter an unknown firmware version */
+	switch (version) {
+	case NTXEC_VERSION_KOBO_AURA:
+	case NTXEC_VERSION_TOLINO_VISION:
+		subdevs = ntxec_subdev;
+		n_subdevs = ARRAY_SIZE(ntxec_subdev);
+		break;
+	case NTXEC_VERSION_TOLINO_SHINE2:
+		subdevs = ntxec_subdev_pwm;
+		n_subdevs = ARRAY_SIZE(ntxec_subdev_pwm);
+		/* Another regmap stacked on top of the other */
+		ec->regmap = devm_regmap_init(ec->dev, NULL,
+					      ec->regmap,
+					      &regmap_config_noack);
+		if (IS_ERR(ec->regmap))
+			return PTR_ERR(ec->regmap);
+		break;
+	default:
+		dev_err(ec->dev,
+			"Netronix embedded controller version %04x is not supported.\n",
+			version);
+		return -ENODEV;
+	}
+
+	dev_info(ec->dev,
+		 "Netronix embedded controller version %04x detected.\n", version);
+
+	if (of_device_is_system_power_controller(ec->dev->of_node)) {
+		/*
+		 * Set the 'powerkeep' bit. This is necessary on some boards
+		 * in order to keep the system running.
+		 */
+		res = regmap_write(ec->regmap, NTXEC_REG_POWERKEEP,
+				   NTXEC_POWERKEEP_VALUE);
+		if (res < 0)
+			return res;
+
+		if (poweroff_restart_client)
+			/*
+			 * Another instance of the driver already took
+			 * poweroff/restart duties.
+			 */
+			dev_err(ec->dev, "poweroff_restart_client already assigned\n");
+		else
+			poweroff_restart_client = client;
+
+		if (pm_power_off)
+			/* Another driver already registered a poweroff handler. */
+			dev_err(ec->dev, "pm_power_off already assigned\n");
+		else
+			pm_power_off = ntxec_poweroff;
+
+		res = register_restart_handler(&ntxec_restart_handler);
+		if (res)
+			dev_err(ec->dev,
+				"Failed to register restart handler: %d\n", res);
+	}
+
+	i2c_set_clientdata(client, ec);
+
+	res = devm_mfd_add_devices(ec->dev, PLATFORM_DEVID_NONE,
+				   subdevs, n_subdevs, NULL, 0, NULL);
+	if (res)
+		dev_err(ec->dev, "Failed to add subdevices: %d\n", res);
+
+	return res;
+}
+
+static void ntxec_remove(struct i2c_client *client)
+{
+	if (client == poweroff_restart_client) {
+		poweroff_restart_client = NULL;
+		pm_power_off = NULL;
+		unregister_restart_handler(&ntxec_restart_handler);
+	}
+}
+
+static const struct of_device_id of_ntxec_match_table[] = {
+	{ .compatible = "netronix,ntxec", },
+	{}
+};
+MODULE_DEVICE_TABLE(of, of_ntxec_match_table);
+
+static struct i2c_driver ntxec_driver = {
+	.driver = {
+		.name = "ntxec",
+		.of_match_table = of_ntxec_match_table,
+	},
+	.probe = ntxec_probe,
+	.remove = ntxec_remove,
+};
+module_i2c_driver(ntxec_driver);
+
+MODULE_AUTHOR("Jonathan Neuschäfer <j.neuschaefer@gmx.net>");
+MODULE_DESCRIPTION("Core driver for Netronix EC");
+MODULE_LICENSE("GPL");