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, 144 insertions, 0 deletions

diff --git a/drivers/misc/qcom-coincell.c b/drivers/misc/qcom-coincell.c
new file mode 100644
index 000000000..3c57f7429
--- /dev/null
+++ b/drivers/misc/qcom-coincell.c
@@ -0,0 +1,144 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2013, The Linux Foundation. All rights reserved.
+ * Copyright (c) 2015, Sony Mobile Communications Inc.
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/of.h>
+#include <linux/regmap.h>
+#include <linux/platform_device.h>
+
+struct qcom_coincell {
+	struct device	*dev;
+	struct regmap	*regmap;
+	u32		base_addr;
+};
+
+#define QCOM_COINCELL_REG_RSET		0x44
+#define QCOM_COINCELL_REG_VSET		0x45
+#define QCOM_COINCELL_REG_ENABLE	0x46
+
+#define QCOM_COINCELL_ENABLE		BIT(7)
+
+static const int qcom_rset_map[] = { 2100, 1700, 1200, 800 };
+static const int qcom_vset_map[] = { 2500, 3200, 3100, 3000 };
+/* NOTE: for pm8921 and others, voltage of 2500 is 16 (10000b), not 0 */
+
+/* if enable==0, rset and vset are ignored */
+static int qcom_coincell_chgr_config(struct qcom_coincell *chgr, int rset,
+				     int vset, bool enable)
+{
+	int i, j, rc;
+
+	/* if disabling, just do that and skip other operations */
+	if (!enable)
+		return regmap_write(chgr->regmap,
+			  chgr->base_addr + QCOM_COINCELL_REG_ENABLE, 0);
+
+	/* find index for current-limiting resistor */
+	for (i = 0; i < ARRAY_SIZE(qcom_rset_map); i++)
+		if (rset == qcom_rset_map[i])
+			break;
+
+	if (i >= ARRAY_SIZE(qcom_rset_map)) {
+		dev_err(chgr->dev, "invalid rset-ohms value %d\n", rset);
+		return -EINVAL;
+	}
+
+	/* find index for charge voltage */
+	for (j = 0; j < ARRAY_SIZE(qcom_vset_map); j++)
+		if (vset == qcom_vset_map[j])
+			break;
+
+	if (j >= ARRAY_SIZE(qcom_vset_map)) {
+		dev_err(chgr->dev, "invalid vset-millivolts value %d\n", vset);
+		return -EINVAL;
+	}
+
+	rc = regmap_write(chgr->regmap,
+			  chgr->base_addr + QCOM_COINCELL_REG_RSET, i);
+	if (rc) {
+		/*
+		 * This is mainly to flag a bad base_addr (reg) from dts.
+		 * Other failures writing to the registers should be
+		 * extremely rare, or indicative of problems that
+		 * should be reported elsewhere (eg. spmi failure).
+		 */
+		dev_err(chgr->dev, "could not write to RSET register\n");
+		return rc;
+	}
+
+	rc = regmap_write(chgr->regmap,
+		chgr->base_addr + QCOM_COINCELL_REG_VSET, j);
+	if (rc)
+		return rc;
+
+	/* set 'enable' register */
+	return regmap_write(chgr->regmap,
+			    chgr->base_addr + QCOM_COINCELL_REG_ENABLE,
+			    QCOM_COINCELL_ENABLE);
+}
+
+static int qcom_coincell_probe(struct platform_device *pdev)
+{
+	struct device_node *node = pdev->dev.of_node;
+	struct qcom_coincell chgr;
+	u32 rset = 0;
+	u32 vset = 0;
+	bool enable;
+	int rc;
+
+	chgr.dev = &pdev->dev;
+
+	chgr.regmap = dev_get_regmap(pdev->dev.parent, NULL);
+	if (!chgr.regmap) {
+		dev_err(chgr.dev, "Unable to get regmap\n");
+		return -EINVAL;
+	}
+
+	rc = of_property_read_u32(node, "reg", &chgr.base_addr);
+	if (rc)
+		return rc;
+
+	enable = !of_property_read_bool(node, "qcom,charger-disable");
+
+	if (enable) {
+		rc = of_property_read_u32(node, "qcom,rset-ohms", &rset);
+		if (rc) {
+			dev_err(chgr.dev,
+				"can't find 'qcom,rset-ohms' in DT block");
+			return rc;
+		}
+
+		rc = of_property_read_u32(node, "qcom,vset-millivolts", &vset);
+		if (rc) {
+			dev_err(chgr.dev,
+			    "can't find 'qcom,vset-millivolts' in DT block");
+			return rc;
+		}
+	}
+
+	return qcom_coincell_chgr_config(&chgr, rset, vset, enable);
+}
+
+static const struct of_device_id qcom_coincell_match_table[] = {
+	{ .compatible = "qcom,pm8941-coincell", },
+	{}
+};
+
+MODULE_DEVICE_TABLE(of, qcom_coincell_match_table);
+
+static struct platform_driver qcom_coincell_driver = {
+	.driver	= {
+		.name		= "qcom-spmi-coincell",
+		.of_match_table	= qcom_coincell_match_table,
+	},
+	.probe		= qcom_coincell_probe,
+};
+
+module_platform_driver(qcom_coincell_driver);
+
+MODULE_DESCRIPTION("Qualcomm PMIC coincell charger driver");
+MODULE_LICENSE("GPL v2");