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, 143 insertions, 0 deletions

diff --git a/drivers/mfd/khadas-mcu.c b/drivers/mfd/khadas-mcu.c
new file mode 100644
index 000000000..ba981a788
--- /dev/null
+++ b/drivers/mfd/khadas-mcu.c
@@ -0,0 +1,143 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Driver for Khadas System control Microcontroller
+ *
+ * Copyright (C) 2020 BayLibre SAS
+ *
+ * Author(s): Neil Armstrong <narmstrong@baylibre.com>
+ */
+#include <linux/bitfield.h>
+#include <linux/i2c.h>
+#include <linux/mfd/core.h>
+#include <linux/mfd/khadas-mcu.h>
+#include <linux/module.h>
+#include <linux/regmap.h>
+
+static bool khadas_mcu_reg_volatile(struct device *dev, unsigned int reg)
+{
+	if (reg >= KHADAS_MCU_USER_DATA_0_REG &&
+	    reg < KHADAS_MCU_PWR_OFF_CMD_REG)
+		return true;
+
+	switch (reg) {
+	case KHADAS_MCU_PWR_OFF_CMD_REG:
+	case KHADAS_MCU_PASSWD_START_REG:
+	case KHADAS_MCU_CHECK_VEN_PASSWD_REG:
+	case KHADAS_MCU_CHECK_USER_PASSWD_REG:
+	case KHADAS_MCU_WOL_INIT_START_REG:
+	case KHADAS_MCU_CMD_FAN_STATUS_CTRL_REG:
+		return true;
+	default:
+		return false;
+	}
+}
+
+static bool khadas_mcu_reg_writeable(struct device *dev, unsigned int reg)
+{
+	switch (reg) {
+	case KHADAS_MCU_PASSWD_VEN_0_REG:
+	case KHADAS_MCU_PASSWD_VEN_1_REG:
+	case KHADAS_MCU_PASSWD_VEN_2_REG:
+	case KHADAS_MCU_PASSWD_VEN_3_REG:
+	case KHADAS_MCU_PASSWD_VEN_4_REG:
+	case KHADAS_MCU_PASSWD_VEN_5_REG:
+	case KHADAS_MCU_MAC_0_REG:
+	case KHADAS_MCU_MAC_1_REG:
+	case KHADAS_MCU_MAC_2_REG:
+	case KHADAS_MCU_MAC_3_REG:
+	case KHADAS_MCU_MAC_4_REG:
+	case KHADAS_MCU_MAC_5_REG:
+	case KHADAS_MCU_USID_0_REG:
+	case KHADAS_MCU_USID_1_REG:
+	case KHADAS_MCU_USID_2_REG:
+	case KHADAS_MCU_USID_3_REG:
+	case KHADAS_MCU_USID_4_REG:
+	case KHADAS_MCU_USID_5_REG:
+	case KHADAS_MCU_VERSION_0_REG:
+	case KHADAS_MCU_VERSION_1_REG:
+	case KHADAS_MCU_DEVICE_NO_0_REG:
+	case KHADAS_MCU_DEVICE_NO_1_REG:
+	case KHADAS_MCU_FACTORY_TEST_REG:
+	case KHADAS_MCU_SHUTDOWN_NORMAL_STATUS_REG:
+		return false;
+	default:
+		return true;
+	}
+}
+
+static const struct regmap_config khadas_mcu_regmap_config = {
+	.reg_bits	= 8,
+	.reg_stride	= 1,
+	.val_bits	= 8,
+	.max_register	= KHADAS_MCU_CMD_FAN_STATUS_CTRL_REG,
+	.volatile_reg	= khadas_mcu_reg_volatile,
+	.writeable_reg	= khadas_mcu_reg_writeable,
+	.cache_type	= REGCACHE_MAPLE,
+};
+
+static struct mfd_cell khadas_mcu_fan_cells[] = {
+	/* VIM1/2 Rev13+ and VIM3 only */
+	{ .name = "khadas-mcu-fan-ctrl", },
+};
+
+static struct mfd_cell khadas_mcu_cells[] = {
+	{ .name = "khadas-mcu-user-mem", },
+};
+
+static int khadas_mcu_probe(struct i2c_client *client)
+{
+	struct device *dev = &client->dev;
+	struct khadas_mcu *ddata;
+	int ret;
+
+	ddata = devm_kzalloc(dev, sizeof(*ddata), GFP_KERNEL);
+	if (!ddata)
+		return -ENOMEM;
+
+	i2c_set_clientdata(client, ddata);
+
+	ddata->dev = dev;
+
+	ddata->regmap = devm_regmap_init_i2c(client, &khadas_mcu_regmap_config);
+	if (IS_ERR(ddata->regmap)) {
+		ret = PTR_ERR(ddata->regmap);
+		dev_err(dev, "Failed to allocate register map: %d\n", ret);
+		return ret;
+	}
+
+	ret = devm_mfd_add_devices(dev, PLATFORM_DEVID_NONE,
+				   khadas_mcu_cells,
+				   ARRAY_SIZE(khadas_mcu_cells),
+				   NULL, 0, NULL);
+	if (ret)
+		return ret;
+
+	if (of_property_present(dev->of_node, "#cooling-cells"))
+		return devm_mfd_add_devices(dev, PLATFORM_DEVID_NONE,
+					    khadas_mcu_fan_cells,
+					    ARRAY_SIZE(khadas_mcu_fan_cells),
+					    NULL, 0, NULL);
+
+	return 0;
+}
+
+#ifdef CONFIG_OF
+static const struct of_device_id khadas_mcu_of_match[] = {
+	{ .compatible = "khadas,mcu", },
+	{},
+};
+MODULE_DEVICE_TABLE(of, khadas_mcu_of_match);
+#endif
+
+static struct i2c_driver khadas_mcu_driver = {
+	.driver = {
+		.name = "khadas-mcu-core",
+		.of_match_table = of_match_ptr(khadas_mcu_of_match),
+	},
+	.probe = khadas_mcu_probe,
+};
+module_i2c_driver(khadas_mcu_driver);
+
+MODULE_DESCRIPTION("Khadas MCU core driver");
+MODULE_AUTHOR("Neil Armstrong <narmstrong@baylibre.com>");
+MODULE_LICENSE("GPL v2");