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, 157 insertions, 0 deletions

diff --git a/drivers/nvmem/snvs_lpgpr.c b/drivers/nvmem/snvs_lpgpr.c
new file mode 100644
index 000000000..89c271123
--- /dev/null
+++ b/drivers/nvmem/snvs_lpgpr.c
@@ -0,0 +1,157 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2015 Pengutronix, Steffen Trumtrar <kernel@pengutronix.de>
+ * Copyright (c) 2017 Pengutronix, Oleksij Rempel <kernel@pengutronix.de>
+ */
+
+#include <linux/mfd/syscon.h>
+#include <linux/module.h>
+#include <linux/nvmem-provider.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/regmap.h>
+
+#define IMX6Q_SNVS_HPLR		0x00
+#define IMX6Q_SNVS_LPLR		0x34
+#define IMX6Q_SNVS_LPGPR	0x68
+
+#define IMX7D_SNVS_HPLR		0x00
+#define IMX7D_SNVS_LPLR		0x34
+#define IMX7D_SNVS_LPGPR	0x90
+
+#define IMX_GPR_SL		BIT(5)
+#define IMX_GPR_HL		BIT(5)
+
+struct snvs_lpgpr_cfg {
+	int offset;
+	int offset_hplr;
+	int offset_lplr;
+	int size;
+};
+
+struct snvs_lpgpr_priv {
+	struct device_d			*dev;
+	struct regmap			*regmap;
+	struct nvmem_config		cfg;
+	const struct snvs_lpgpr_cfg	*dcfg;
+};
+
+static const struct snvs_lpgpr_cfg snvs_lpgpr_cfg_imx6q = {
+	.offset		= IMX6Q_SNVS_LPGPR,
+	.offset_hplr	= IMX6Q_SNVS_HPLR,
+	.offset_lplr	= IMX6Q_SNVS_LPLR,
+	.size		= 4,
+};
+
+static const struct snvs_lpgpr_cfg snvs_lpgpr_cfg_imx7d = {
+	.offset		= IMX7D_SNVS_LPGPR,
+	.offset_hplr	= IMX7D_SNVS_HPLR,
+	.offset_lplr	= IMX7D_SNVS_LPLR,
+	.size		= 16,
+};
+
+static int snvs_lpgpr_write(void *context, unsigned int offset, void *val,
+			    size_t bytes)
+{
+	struct snvs_lpgpr_priv *priv = context;
+	const struct snvs_lpgpr_cfg *dcfg = priv->dcfg;
+	unsigned int lock_reg;
+	int ret;
+
+	ret = regmap_read(priv->regmap, dcfg->offset_hplr, &lock_reg);
+	if (ret < 0)
+		return ret;
+
+	if (lock_reg & IMX_GPR_SL)
+		return -EPERM;
+
+	ret = regmap_read(priv->regmap, dcfg->offset_lplr, &lock_reg);
+	if (ret < 0)
+		return ret;
+
+	if (lock_reg & IMX_GPR_HL)
+		return -EPERM;
+
+	return regmap_bulk_write(priv->regmap, dcfg->offset + offset, val,
+				bytes / 4);
+}
+
+static int snvs_lpgpr_read(void *context, unsigned int offset, void *val,
+			   size_t bytes)
+{
+	struct snvs_lpgpr_priv *priv = context;
+	const struct snvs_lpgpr_cfg *dcfg = priv->dcfg;
+
+	return regmap_bulk_read(priv->regmap, dcfg->offset + offset,
+			       val, bytes / 4);
+}
+
+static int snvs_lpgpr_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct device_node *node = dev->of_node;
+	struct device_node *syscon_node;
+	struct snvs_lpgpr_priv *priv;
+	struct nvmem_config *cfg;
+	struct nvmem_device *nvmem;
+	const struct snvs_lpgpr_cfg *dcfg;
+
+	if (!node)
+		return -ENOENT;
+
+	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	dcfg = of_device_get_match_data(dev);
+	if (!dcfg)
+		return -EINVAL;
+
+	syscon_node = of_get_parent(node);
+	if (!syscon_node)
+		return -ENODEV;
+
+	priv->regmap = syscon_node_to_regmap(syscon_node);
+	of_node_put(syscon_node);
+	if (IS_ERR(priv->regmap))
+		return PTR_ERR(priv->regmap);
+
+	priv->dcfg = dcfg;
+
+	cfg = &priv->cfg;
+	cfg->priv = priv;
+	cfg->name = dev_name(dev);
+	cfg->dev = dev;
+	cfg->stride = 4;
+	cfg->word_size = 4;
+	cfg->size = dcfg->size;
+	cfg->owner = THIS_MODULE;
+	cfg->reg_read  = snvs_lpgpr_read;
+	cfg->reg_write = snvs_lpgpr_write;
+
+	nvmem = devm_nvmem_register(dev, cfg);
+
+	return PTR_ERR_OR_ZERO(nvmem);
+}
+
+static const struct of_device_id snvs_lpgpr_dt_ids[] = {
+	{ .compatible = "fsl,imx6q-snvs-lpgpr", .data = &snvs_lpgpr_cfg_imx6q },
+	{ .compatible = "fsl,imx6ul-snvs-lpgpr",
+	  .data = &snvs_lpgpr_cfg_imx6q },
+	{ .compatible = "fsl,imx7d-snvs-lpgpr",	.data = &snvs_lpgpr_cfg_imx7d },
+	{ },
+};
+MODULE_DEVICE_TABLE(of, snvs_lpgpr_dt_ids);
+
+static struct platform_driver snvs_lpgpr_driver = {
+	.probe	= snvs_lpgpr_probe,
+	.driver = {
+		.name	= "snvs_lpgpr",
+		.of_match_table = snvs_lpgpr_dt_ids,
+	},
+};
+module_platform_driver(snvs_lpgpr_driver);
+
+MODULE_AUTHOR("Oleksij Rempel <o.rempel@pengutronix.de>");
+MODULE_DESCRIPTION("Low Power General Purpose Register in i.MX6 and i.MX7 Secure Non-Volatile Storage");
+MODULE_LICENSE("GPL v2");