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, 150 insertions, 0 deletions

diff --git a/drivers/rtc/rtc-meson-vrtc.c b/drivers/rtc/rtc-meson-vrtc.c
new file mode 100644
index 000000000..7d38258cb
--- /dev/null
+++ b/drivers/rtc/rtc-meson-vrtc.c
@@ -0,0 +1,150 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 BayLibre, SAS
+ * Author: Neil Armstrong <narmstrong@baylibre.com>
+ * Copyright (C) 2015 Amlogic, Inc. All rights reserved.
+ */
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/rtc.h>
+#include <linux/io.h>
+#include <linux/of.h>
+#include <linux/time64.h>
+
+struct meson_vrtc_data {
+	void __iomem *io_alarm;
+	unsigned long alarm_time;
+	bool enabled;
+};
+
+static int meson_vrtc_read_time(struct device *dev, struct rtc_time *tm)
+{
+	struct timespec64 time;
+
+	dev_dbg(dev, "%s\n", __func__);
+	ktime_get_real_ts64(&time);
+	rtc_time64_to_tm(time.tv_sec, tm);
+
+	return 0;
+}
+
+static void meson_vrtc_set_wakeup_time(struct meson_vrtc_data *vrtc,
+				       unsigned long time)
+{
+	writel_relaxed(time, vrtc->io_alarm);
+}
+
+static int meson_vrtc_set_alarm(struct device *dev, struct rtc_wkalrm *alarm)
+{
+	struct meson_vrtc_data *vrtc = dev_get_drvdata(dev);
+
+	dev_dbg(dev, "%s: alarm->enabled=%d\n", __func__, alarm->enabled);
+	if (alarm->enabled)
+		vrtc->alarm_time = rtc_tm_to_time64(&alarm->time);
+	else
+		vrtc->alarm_time = 0;
+
+	return 0;
+}
+
+static int meson_vrtc_alarm_irq_enable(struct device *dev, unsigned int enabled)
+{
+	struct meson_vrtc_data *vrtc = dev_get_drvdata(dev);
+
+	vrtc->enabled = enabled;
+	return 0;
+}
+
+static const struct rtc_class_ops meson_vrtc_ops = {
+	.read_time = meson_vrtc_read_time,
+	.set_alarm = meson_vrtc_set_alarm,
+	.alarm_irq_enable = meson_vrtc_alarm_irq_enable,
+};
+
+static int meson_vrtc_probe(struct platform_device *pdev)
+{
+	struct meson_vrtc_data *vrtc;
+	struct rtc_device *rtc;
+
+	vrtc = devm_kzalloc(&pdev->dev, sizeof(*vrtc), GFP_KERNEL);
+	if (!vrtc)
+		return -ENOMEM;
+
+	vrtc->io_alarm = devm_platform_ioremap_resource(pdev, 0);
+	if (IS_ERR(vrtc->io_alarm))
+		return PTR_ERR(vrtc->io_alarm);
+
+	device_init_wakeup(&pdev->dev, true);
+
+	platform_set_drvdata(pdev, vrtc);
+
+	rtc = devm_rtc_allocate_device(&pdev->dev);
+	if (IS_ERR(rtc))
+		return PTR_ERR(rtc);
+
+	rtc->ops = &meson_vrtc_ops;
+	return devm_rtc_register_device(rtc);
+}
+
+static int __maybe_unused meson_vrtc_suspend(struct device *dev)
+{
+	struct meson_vrtc_data *vrtc = dev_get_drvdata(dev);
+
+	dev_dbg(dev, "%s\n", __func__);
+	if (vrtc->alarm_time) {
+		unsigned long local_time;
+		long alarm_secs;
+		struct timespec64 time;
+
+		ktime_get_real_ts64(&time);
+		local_time = time.tv_sec;
+
+		dev_dbg(dev, "alarm_time = %lus, local_time=%lus\n",
+			vrtc->alarm_time, local_time);
+		alarm_secs = vrtc->alarm_time - local_time;
+		if (alarm_secs > 0) {
+			meson_vrtc_set_wakeup_time(vrtc, alarm_secs);
+			dev_dbg(dev, "system will wakeup in %lds.\n",
+				alarm_secs);
+		} else {
+			dev_err(dev, "alarm time already passed: %lds.\n",
+				alarm_secs);
+		}
+	}
+
+	return 0;
+}
+
+static int __maybe_unused meson_vrtc_resume(struct device *dev)
+{
+	struct meson_vrtc_data *vrtc = dev_get_drvdata(dev);
+
+	dev_dbg(dev, "%s\n", __func__);
+
+	vrtc->alarm_time = 0;
+	meson_vrtc_set_wakeup_time(vrtc, 0);
+	return 0;
+}
+
+static SIMPLE_DEV_PM_OPS(meson_vrtc_pm_ops,
+			 meson_vrtc_suspend, meson_vrtc_resume);
+
+static const struct of_device_id meson_vrtc_dt_match[] = {
+	{ .compatible = "amlogic,meson-vrtc"},
+	{},
+};
+MODULE_DEVICE_TABLE(of, meson_vrtc_dt_match);
+
+static struct platform_driver meson_vrtc_driver = {
+	.probe = meson_vrtc_probe,
+	.driver = {
+		.name = "meson-vrtc",
+		.of_match_table = meson_vrtc_dt_match,
+		.pm = &meson_vrtc_pm_ops,
+	},
+};
+
+module_platform_driver(meson_vrtc_driver);
+
+MODULE_DESCRIPTION("Amlogic Virtual Wakeup RTC Timer driver");
+MODULE_LICENSE("GPL");