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, 165 insertions, 0 deletions

diff --git a/drivers/memory/da8xx-ddrctl.c b/drivers/memory/da8xx-ddrctl.c
new file mode 100644
index 000000000..2bf34da85
--- /dev/null
+++ b/drivers/memory/da8xx-ddrctl.c
@@ -0,0 +1,165 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * TI da8xx DDR2/mDDR controller driver
+ *
+ * Copyright (C) 2016 BayLibre SAS
+ *
+ * Author:
+ *   Bartosz Golaszewski <bgolaszewski@baylibre.com>
+ */
+
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/io.h>
+
+/*
+ * REVISIT: Linux doesn't have a good framework for the kind of performance
+ * knobs this driver controls. We can't use device tree properties as it deals
+ * with hardware configuration rather than description. We also don't want to
+ * commit to maintaining some random sysfs attributes.
+ *
+ * For now we just hardcode the register values for the boards that need
+ * some changes (as is the case for the LCD controller on da850-lcdk - the
+ * first board we support here). When linux gets an appropriate framework,
+ * we'll easily convert the driver to it.
+ */
+
+struct da8xx_ddrctl_config_knob {
+	const char *name;
+	u32 reg;
+	u32 mask;
+	u32 shift;
+};
+
+static const struct da8xx_ddrctl_config_knob da8xx_ddrctl_knobs[] = {
+	{
+		.name = "da850-pbbpr",
+		.reg = 0x20,
+		.mask = 0xffffff00,
+		.shift = 0,
+	},
+};
+
+struct da8xx_ddrctl_setting {
+	const char *name;
+	u32 val;
+};
+
+struct da8xx_ddrctl_board_settings {
+	const char *board;
+	const struct da8xx_ddrctl_setting *settings;
+};
+
+static const struct da8xx_ddrctl_setting da850_lcdk_ddrctl_settings[] = {
+	{
+		.name = "da850-pbbpr",
+		.val = 0x20,
+	},
+	{ }
+};
+
+static const struct da8xx_ddrctl_board_settings da8xx_ddrctl_board_confs[] = {
+	{
+		.board = "ti,da850-lcdk",
+		.settings = da850_lcdk_ddrctl_settings,
+	},
+};
+
+static const struct da8xx_ddrctl_config_knob *
+da8xx_ddrctl_match_knob(const struct da8xx_ddrctl_setting *setting)
+{
+	const struct da8xx_ddrctl_config_knob *knob;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(da8xx_ddrctl_knobs); i++) {
+		knob = &da8xx_ddrctl_knobs[i];
+
+		if (strcmp(knob->name, setting->name) == 0)
+			return knob;
+	}
+
+	return NULL;
+}
+
+static const struct da8xx_ddrctl_setting *da8xx_ddrctl_get_board_settings(void)
+{
+	const struct da8xx_ddrctl_board_settings *board_settings;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(da8xx_ddrctl_board_confs); i++) {
+		board_settings = &da8xx_ddrctl_board_confs[i];
+
+		if (of_machine_is_compatible(board_settings->board))
+			return board_settings->settings;
+	}
+
+	return NULL;
+}
+
+static int da8xx_ddrctl_probe(struct platform_device *pdev)
+{
+	const struct da8xx_ddrctl_config_knob *knob;
+	const struct da8xx_ddrctl_setting *setting;
+	struct resource *res;
+	void __iomem *ddrctl;
+	struct device *dev;
+	u32 reg;
+
+	dev = &pdev->dev;
+
+	setting = da8xx_ddrctl_get_board_settings();
+	if (!setting) {
+		dev_err(dev, "no settings defined for this board\n");
+		return -EINVAL;
+	}
+
+	ddrctl = devm_platform_get_and_ioremap_resource(pdev, 0, &res);
+	if (IS_ERR(ddrctl)) {
+		dev_err(dev, "unable to map memory controller registers\n");
+		return PTR_ERR(ddrctl);
+	}
+
+	for (; setting->name; setting++) {
+		knob = da8xx_ddrctl_match_knob(setting);
+		if (!knob) {
+			dev_warn(dev,
+				 "no such config option: %s\n", setting->name);
+			continue;
+		}
+
+		if (knob->reg + sizeof(u32) > resource_size(res)) {
+			dev_warn(dev,
+				 "register offset of '%s' exceeds mapped memory size\n",
+				 knob->name);
+			continue;
+		}
+
+		reg = readl(ddrctl + knob->reg);
+		reg &= knob->mask;
+		reg |= setting->val << knob->shift;
+
+		dev_dbg(dev, "writing 0x%08x to %s\n", reg, setting->name);
+
+		writel(reg, ddrctl + knob->reg);
+	}
+
+	return 0;
+}
+
+static const struct of_device_id da8xx_ddrctl_of_match[] = {
+	{ .compatible = "ti,da850-ddr-controller", },
+	{ },
+};
+
+static struct platform_driver da8xx_ddrctl_driver = {
+	.probe = da8xx_ddrctl_probe,
+	.driver = {
+		.name = "da850-ddr-controller",
+		.of_match_table = da8xx_ddrctl_of_match,
+	},
+};
+module_platform_driver(da8xx_ddrctl_driver);
+
+MODULE_AUTHOR("Bartosz Golaszewski <bgolaszewski@baylibre.com>");
+MODULE_DESCRIPTION("TI da8xx DDR2/mDDR controller driver");