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, 233 insertions, 0 deletions

diff --git a/drivers/video/backlight/rt4831-backlight.c b/drivers/video/backlight/rt4831-backlight.c
new file mode 100644
index 000000000..7ead75929
--- /dev/null
+++ b/drivers/video/backlight/rt4831-backlight.c
@@ -0,0 +1,233 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <dt-bindings/leds/rt4831-backlight.h>
+#include <linux/backlight.h>
+#include <linux/bitops.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/property.h>
+#include <linux/regmap.h>
+
+#define RT4831_REG_BLCFG	0x02
+#define RT4831_REG_BLDIML	0x04
+#define RT4831_REG_ENABLE	0x08
+#define RT4831_REG_BLOPT2	0x11
+
+#define RT4831_BLMAX_BRIGHTNESS	2048
+
+#define RT4831_BLOVP_MASK	GENMASK(7, 5)
+#define RT4831_BLOVP_SHIFT	5
+#define RT4831_BLPWMEN_MASK	BIT(0)
+#define RT4831_BLEN_MASK	BIT(4)
+#define RT4831_BLCH_MASK	GENMASK(3, 0)
+#define RT4831_BLDIML_MASK	GENMASK(2, 0)
+#define RT4831_BLDIMH_MASK	GENMASK(10, 3)
+#define RT4831_BLDIMH_SHIFT	3
+#define RT4831_BLOCP_MASK	GENMASK(1, 0)
+
+#define RT4831_BLOCP_MINUA	900000
+#define RT4831_BLOCP_MAXUA	1800000
+#define RT4831_BLOCP_STEPUA	300000
+
+struct rt4831_priv {
+	struct device *dev;
+	struct regmap *regmap;
+	struct backlight_device *bl;
+};
+
+static int rt4831_bl_update_status(struct backlight_device *bl_dev)
+{
+	struct rt4831_priv *priv = bl_get_data(bl_dev);
+	int brightness = backlight_get_brightness(bl_dev);
+	unsigned int enable = brightness ? RT4831_BLEN_MASK : 0;
+	u8 v[2];
+	int ret;
+
+	if (brightness) {
+		v[0] = (brightness - 1) & RT4831_BLDIML_MASK;
+		v[1] = ((brightness - 1) & RT4831_BLDIMH_MASK) >> RT4831_BLDIMH_SHIFT;
+
+		ret = regmap_raw_write(priv->regmap, RT4831_REG_BLDIML, v, sizeof(v));
+		if (ret)
+			return ret;
+	}
+
+	return regmap_update_bits(priv->regmap, RT4831_REG_ENABLE, RT4831_BLEN_MASK, enable);
+
+}
+
+static int rt4831_bl_get_brightness(struct backlight_device *bl_dev)
+{
+	struct rt4831_priv *priv = bl_get_data(bl_dev);
+	unsigned int val;
+	u8 v[2];
+	int ret;
+
+	ret = regmap_read(priv->regmap, RT4831_REG_ENABLE, &val);
+	if (ret)
+		return ret;
+
+	if (!(val & RT4831_BLEN_MASK))
+		return 0;
+
+	ret = regmap_raw_read(priv->regmap, RT4831_REG_BLDIML, v, sizeof(v));
+	if (ret)
+		return ret;
+
+	ret = (v[1] << RT4831_BLDIMH_SHIFT) + (v[0] & RT4831_BLDIML_MASK) + 1;
+
+	return ret;
+}
+
+static const struct backlight_ops rt4831_bl_ops = {
+	.options = BL_CORE_SUSPENDRESUME,
+	.update_status = rt4831_bl_update_status,
+	.get_brightness = rt4831_bl_get_brightness,
+};
+
+static int rt4831_parse_backlight_properties(struct rt4831_priv *priv,
+					     struct backlight_properties *bl_props)
+{
+	struct device *dev = priv->dev;
+	u8 propval;
+	u32 brightness, ocp_uA;
+	unsigned int val = 0;
+	int ret;
+
+	/* common properties */
+	ret = device_property_read_u32(dev, "max-brightness", &brightness);
+	if (ret)
+		brightness = RT4831_BLMAX_BRIGHTNESS;
+
+	bl_props->max_brightness = min_t(u32, brightness, RT4831_BLMAX_BRIGHTNESS);
+
+	ret = device_property_read_u32(dev, "default-brightness", &brightness);
+	if (ret)
+		brightness = bl_props->max_brightness;
+
+	bl_props->brightness = min_t(u32, brightness, bl_props->max_brightness);
+
+	/* vendor properties */
+	if (device_property_read_bool(dev, "richtek,pwm-enable"))
+		val = RT4831_BLPWMEN_MASK;
+
+	ret = regmap_update_bits(priv->regmap, RT4831_REG_BLCFG, RT4831_BLPWMEN_MASK, val);
+	if (ret)
+		return ret;
+
+	ret = device_property_read_u8(dev, "richtek,bled-ovp-sel", &propval);
+	if (ret)
+		propval = RT4831_BLOVPLVL_21V;
+
+	propval = min_t(u8, propval, RT4831_BLOVPLVL_29V);
+	ret = regmap_update_bits(priv->regmap, RT4831_REG_BLCFG, RT4831_BLOVP_MASK,
+				 propval << RT4831_BLOVP_SHIFT);
+	if (ret)
+		return ret;
+
+	/*
+	 * This OCP level is used to protect and limit the inductor current.
+	 * If inductor peak current reach the level, low-side MOSFET will be
+	 * turned off. Meanwhile, the output channel current may be limited.
+	 * To match the configured channel current, the inductor chosen must
+	 * be higher than the OCP level.
+	 *
+	 * Not like the OVP level, the default 21V can be used in the most
+	 * application. But if the chosen OCP level is smaller than needed,
+	 * it will also affect the backlight channel output current to be
+	 * smaller than the register setting.
+	 */
+	ret = device_property_read_u32(dev, "richtek,bled-ocp-microamp",
+				       &ocp_uA);
+	if (!ret) {
+		ocp_uA = clamp_val(ocp_uA, RT4831_BLOCP_MINUA,
+				   RT4831_BLOCP_MAXUA);
+		val = DIV_ROUND_UP(ocp_uA - RT4831_BLOCP_MINUA,
+				   RT4831_BLOCP_STEPUA);
+		ret = regmap_update_bits(priv->regmap, RT4831_REG_BLOPT2,
+					 RT4831_BLOCP_MASK, val);
+		if (ret)
+			return ret;
+	}
+
+	ret = device_property_read_u8(dev, "richtek,channel-use", &propval);
+	if (ret) {
+		dev_err(dev, "richtek,channel-use DT property missing\n");
+		return ret;
+	}
+
+	if (!(propval & RT4831_BLCH_MASK)) {
+		dev_err(dev, "No channel specified\n");
+		return -EINVAL;
+	}
+
+	return regmap_update_bits(priv->regmap, RT4831_REG_ENABLE, RT4831_BLCH_MASK, propval);
+}
+
+static int rt4831_bl_probe(struct platform_device *pdev)
+{
+	struct rt4831_priv *priv;
+	struct backlight_properties bl_props = { .type = BACKLIGHT_RAW,
+						 .scale = BACKLIGHT_SCALE_LINEAR };
+	int ret;
+
+	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->dev = &pdev->dev;
+
+	priv->regmap = dev_get_regmap(pdev->dev.parent, NULL);
+	if (!priv->regmap) {
+		dev_err(&pdev->dev, "Failed to init regmap\n");
+		return -ENODEV;
+	}
+
+	ret = rt4831_parse_backlight_properties(priv, &bl_props);
+	if (ret) {
+		dev_err(&pdev->dev, "Failed to parse backlight properties\n");
+		return ret;
+	}
+
+	priv->bl = devm_backlight_device_register(&pdev->dev, pdev->name, &pdev->dev, priv,
+						  &rt4831_bl_ops, &bl_props);
+	if (IS_ERR(priv->bl)) {
+		dev_err(&pdev->dev, "Failed to register backlight\n");
+		return PTR_ERR(priv->bl);
+	}
+
+	backlight_update_status(priv->bl);
+	platform_set_drvdata(pdev, priv);
+
+	return 0;
+}
+
+static void rt4831_bl_remove(struct platform_device *pdev)
+{
+	struct rt4831_priv *priv = platform_get_drvdata(pdev);
+	struct backlight_device *bl_dev = priv->bl;
+
+	bl_dev->props.brightness = 0;
+	backlight_update_status(priv->bl);
+}
+
+static const struct of_device_id __maybe_unused rt4831_bl_of_match[] = {
+	{ .compatible = "richtek,rt4831-backlight", },
+	{}
+};
+MODULE_DEVICE_TABLE(of, rt4831_bl_of_match);
+
+static struct platform_driver rt4831_bl_driver = {
+	.driver = {
+		.name = "rt4831-backlight",
+		.of_match_table = rt4831_bl_of_match,
+	},
+	.probe = rt4831_bl_probe,
+	.remove = rt4831_bl_remove,
+};
+module_platform_driver(rt4831_bl_driver);
+
+MODULE_AUTHOR("ChiYuan Huang <cy_huang@richtek.com>");
+MODULE_DESCRIPTION("Richtek RT4831 Backlight Driver");
+MODULE_LICENSE("GPL v2");