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, 276 insertions, 0 deletions

diff --git a/drivers/gnss/serial.c b/drivers/gnss/serial.c
new file mode 100644
index 000000000..0e43bf629
--- /dev/null
+++ b/drivers/gnss/serial.c
@@ -0,0 +1,276 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Generic serial GNSS receiver driver
+ *
+ * Copyright (C) 2018 Johan Hovold <johan@kernel.org>
+ */
+
+#include <linux/errno.h>
+#include <linux/gnss.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/pm.h>
+#include <linux/pm_runtime.h>
+#include <linux/sched.h>
+#include <linux/serdev.h>
+#include <linux/slab.h>
+
+#include "serial.h"
+
+static int gnss_serial_open(struct gnss_device *gdev)
+{
+	struct gnss_serial *gserial = gnss_get_drvdata(gdev);
+	struct serdev_device *serdev = gserial->serdev;
+	int ret;
+
+	ret = serdev_device_open(serdev);
+	if (ret)
+		return ret;
+
+	serdev_device_set_baudrate(serdev, gserial->speed);
+	serdev_device_set_flow_control(serdev, false);
+
+	ret = pm_runtime_get_sync(&serdev->dev);
+	if (ret < 0) {
+		pm_runtime_put_noidle(&serdev->dev);
+		goto err_close;
+	}
+
+	return 0;
+
+err_close:
+	serdev_device_close(serdev);
+
+	return ret;
+}
+
+static void gnss_serial_close(struct gnss_device *gdev)
+{
+	struct gnss_serial *gserial = gnss_get_drvdata(gdev);
+	struct serdev_device *serdev = gserial->serdev;
+
+	serdev_device_close(serdev);
+
+	pm_runtime_put(&serdev->dev);
+}
+
+static int gnss_serial_write_raw(struct gnss_device *gdev,
+		const unsigned char *buf, size_t count)
+{
+	struct gnss_serial *gserial = gnss_get_drvdata(gdev);
+	struct serdev_device *serdev = gserial->serdev;
+	int ret;
+
+	/* write is only buffered synchronously */
+	ret = serdev_device_write(serdev, buf, count, MAX_SCHEDULE_TIMEOUT);
+	if (ret < 0 || ret < count)
+		return ret;
+
+	/* FIXME: determine if interrupted? */
+	serdev_device_wait_until_sent(serdev, 0);
+
+	return count;
+}
+
+static const struct gnss_operations gnss_serial_gnss_ops = {
+	.open		= gnss_serial_open,
+	.close		= gnss_serial_close,
+	.write_raw	= gnss_serial_write_raw,
+};
+
+static size_t gnss_serial_receive_buf(struct serdev_device *serdev,
+				       const u8 *buf, size_t count)
+{
+	struct gnss_serial *gserial = serdev_device_get_drvdata(serdev);
+	struct gnss_device *gdev = gserial->gdev;
+
+	return gnss_insert_raw(gdev, buf, count);
+}
+
+static const struct serdev_device_ops gnss_serial_serdev_ops = {
+	.receive_buf	= gnss_serial_receive_buf,
+	.write_wakeup	= serdev_device_write_wakeup,
+};
+
+static int gnss_serial_set_power(struct gnss_serial *gserial,
+					enum gnss_serial_pm_state state)
+{
+	if (!gserial->ops || !gserial->ops->set_power)
+		return 0;
+
+	return gserial->ops->set_power(gserial, state);
+}
+
+/*
+ * FIXME: need to provide subdriver defaults or separate dt parsing from
+ * allocation.
+ */
+static int gnss_serial_parse_dt(struct serdev_device *serdev)
+{
+	struct gnss_serial *gserial = serdev_device_get_drvdata(serdev);
+	struct device_node *node = serdev->dev.of_node;
+	u32 speed = 4800;
+
+	of_property_read_u32(node, "current-speed", &speed);
+
+	gserial->speed = speed;
+
+	return 0;
+}
+
+struct gnss_serial *gnss_serial_allocate(struct serdev_device *serdev,
+						size_t data_size)
+{
+	struct gnss_serial *gserial;
+	struct gnss_device *gdev;
+	int ret;
+
+	gserial = kzalloc(sizeof(*gserial) + data_size, GFP_KERNEL);
+	if (!gserial)
+		return ERR_PTR(-ENOMEM);
+
+	gdev = gnss_allocate_device(&serdev->dev);
+	if (!gdev) {
+		ret = -ENOMEM;
+		goto err_free_gserial;
+	}
+
+	gdev->ops = &gnss_serial_gnss_ops;
+	gnss_set_drvdata(gdev, gserial);
+
+	gserial->serdev = serdev;
+	gserial->gdev = gdev;
+
+	serdev_device_set_drvdata(serdev, gserial);
+	serdev_device_set_client_ops(serdev, &gnss_serial_serdev_ops);
+
+	ret = gnss_serial_parse_dt(serdev);
+	if (ret)
+		goto err_put_device;
+
+	return gserial;
+
+err_put_device:
+	gnss_put_device(gserial->gdev);
+err_free_gserial:
+	kfree(gserial);
+
+	return ERR_PTR(ret);
+}
+EXPORT_SYMBOL_GPL(gnss_serial_allocate);
+
+void gnss_serial_free(struct gnss_serial *gserial)
+{
+	gnss_put_device(gserial->gdev);
+	kfree(gserial);
+}
+EXPORT_SYMBOL_GPL(gnss_serial_free);
+
+int gnss_serial_register(struct gnss_serial *gserial)
+{
+	struct serdev_device *serdev = gserial->serdev;
+	int ret;
+
+	if (IS_ENABLED(CONFIG_PM)) {
+		pm_runtime_enable(&serdev->dev);
+	} else {
+		ret = gnss_serial_set_power(gserial, GNSS_SERIAL_ACTIVE);
+		if (ret < 0)
+			return ret;
+	}
+
+	ret = gnss_register_device(gserial->gdev);
+	if (ret)
+		goto err_disable_rpm;
+
+	return 0;
+
+err_disable_rpm:
+	if (IS_ENABLED(CONFIG_PM))
+		pm_runtime_disable(&serdev->dev);
+	else
+		gnss_serial_set_power(gserial, GNSS_SERIAL_OFF);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(gnss_serial_register);
+
+void gnss_serial_deregister(struct gnss_serial *gserial)
+{
+	struct serdev_device *serdev = gserial->serdev;
+
+	gnss_deregister_device(gserial->gdev);
+
+	if (IS_ENABLED(CONFIG_PM))
+		pm_runtime_disable(&serdev->dev);
+	else
+		gnss_serial_set_power(gserial, GNSS_SERIAL_OFF);
+}
+EXPORT_SYMBOL_GPL(gnss_serial_deregister);
+
+#ifdef CONFIG_PM
+static int gnss_serial_runtime_suspend(struct device *dev)
+{
+	struct gnss_serial *gserial = dev_get_drvdata(dev);
+
+	return gnss_serial_set_power(gserial, GNSS_SERIAL_STANDBY);
+}
+
+static int gnss_serial_runtime_resume(struct device *dev)
+{
+	struct gnss_serial *gserial = dev_get_drvdata(dev);
+
+	return gnss_serial_set_power(gserial, GNSS_SERIAL_ACTIVE);
+}
+#endif /* CONFIG_PM */
+
+static int gnss_serial_prepare(struct device *dev)
+{
+	if (pm_runtime_suspended(dev))
+		return 1;
+
+	return 0;
+}
+
+#ifdef CONFIG_PM_SLEEP
+static int gnss_serial_suspend(struct device *dev)
+{
+	struct gnss_serial *gserial = dev_get_drvdata(dev);
+	int ret = 0;
+
+	/*
+	 * FIXME: serdev currently lacks support for managing the underlying
+	 * device's wakeup settings. A workaround would be to close the serdev
+	 * device here if it is open.
+	 */
+
+	if (!pm_runtime_suspended(dev))
+		ret = gnss_serial_set_power(gserial, GNSS_SERIAL_STANDBY);
+
+	return ret;
+}
+
+static int gnss_serial_resume(struct device *dev)
+{
+	struct gnss_serial *gserial = dev_get_drvdata(dev);
+	int ret = 0;
+
+	if (!pm_runtime_suspended(dev))
+		ret = gnss_serial_set_power(gserial, GNSS_SERIAL_ACTIVE);
+
+	return ret;
+}
+#endif /* CONFIG_PM_SLEEP */
+
+const struct dev_pm_ops gnss_serial_pm_ops = {
+	.prepare	= gnss_serial_prepare,
+	SET_SYSTEM_SLEEP_PM_OPS(gnss_serial_suspend, gnss_serial_resume)
+	SET_RUNTIME_PM_OPS(gnss_serial_runtime_suspend, gnss_serial_runtime_resume, NULL)
+};
+EXPORT_SYMBOL_GPL(gnss_serial_pm_ops);
+
+MODULE_AUTHOR("Johan Hovold <johan@kernel.org>");
+MODULE_DESCRIPTION("Generic serial GNSS receiver driver");
+MODULE_LICENSE("GPL v2");