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, 196 insertions, 0 deletions

diff --git a/drivers/spi/spi-sh-sci.c b/drivers/spi/spi-sh-sci.c
new file mode 100644
index 000000000..f66efaabc
--- /dev/null
+++ b/drivers/spi/spi-sh-sci.c
@@ -0,0 +1,196 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * SH SCI SPI interface
+ *
+ * Copyright (c) 2008 Magnus Damm
+ *
+ * Based on S3C24XX GPIO based SPI driver, which is:
+ *   Copyright (c) 2006 Ben Dooks
+ *   Copyright (c) 2006 Simtec Electronics
+ */
+
+#include <linux/kernel.h>
+#include <linux/delay.h>
+#include <linux/spinlock.h>
+#include <linux/platform_device.h>
+
+#include <linux/spi/spi.h>
+#include <linux/spi/spi_bitbang.h>
+#include <linux/module.h>
+
+#include <asm/spi.h>
+#include <asm/io.h>
+
+struct sh_sci_spi {
+	struct spi_bitbang bitbang;
+
+	void __iomem *membase;
+	unsigned char val;
+	struct sh_spi_info *info;
+	struct platform_device *dev;
+};
+
+#define SCSPTR(sp)	(sp->membase + 0x1c)
+#define PIN_SCK		(1 << 2)
+#define PIN_TXD		(1 << 0)
+#define PIN_RXD		PIN_TXD
+#define PIN_INIT	((1 << 1) | (1 << 3) | PIN_SCK | PIN_TXD)
+
+static inline void setbits(struct sh_sci_spi *sp, int bits, int on)
+{
+	/*
+	 * We are the only user of SCSPTR so no locking is required.
+	 * Reading bit 2 and 0 in SCSPTR gives pin state as input.
+	 * Writing the same bits sets the output value.
+	 * This makes regular read-modify-write difficult so we
+	 * use sp->val to keep track of the latest register value.
+	 */
+
+	if (on)
+		sp->val |= bits;
+	else
+		sp->val &= ~bits;
+
+	iowrite8(sp->val, SCSPTR(sp));
+}
+
+static inline void setsck(struct spi_device *dev, int on)
+{
+	setbits(spi_controller_get_devdata(dev->controller), PIN_SCK, on);
+}
+
+static inline void setmosi(struct spi_device *dev, int on)
+{
+	setbits(spi_controller_get_devdata(dev->controller), PIN_TXD, on);
+}
+
+static inline u32 getmiso(struct spi_device *dev)
+{
+	struct sh_sci_spi *sp = spi_controller_get_devdata(dev->controller);
+
+	return (ioread8(SCSPTR(sp)) & PIN_RXD) ? 1 : 0;
+}
+
+#define spidelay(x) ndelay(x)
+
+#include "spi-bitbang-txrx.h"
+
+static u32 sh_sci_spi_txrx_mode0(struct spi_device *spi,
+				 unsigned nsecs, u32 word, u8 bits,
+				 unsigned flags)
+{
+	return bitbang_txrx_be_cpha0(spi, nsecs, 0, flags, word, bits);
+}
+
+static u32 sh_sci_spi_txrx_mode1(struct spi_device *spi,
+				 unsigned nsecs, u32 word, u8 bits,
+				 unsigned flags)
+{
+	return bitbang_txrx_be_cpha1(spi, nsecs, 0, flags, word, bits);
+}
+
+static u32 sh_sci_spi_txrx_mode2(struct spi_device *spi,
+				 unsigned nsecs, u32 word, u8 bits,
+				 unsigned flags)
+{
+	return bitbang_txrx_be_cpha0(spi, nsecs, 1, flags, word, bits);
+}
+
+static u32 sh_sci_spi_txrx_mode3(struct spi_device *spi,
+				 unsigned nsecs, u32 word, u8 bits,
+				 unsigned flags)
+{
+	return bitbang_txrx_be_cpha1(spi, nsecs, 1, flags, word, bits);
+}
+
+static void sh_sci_spi_chipselect(struct spi_device *dev, int value)
+{
+	struct sh_sci_spi *sp = spi_controller_get_devdata(dev->controller);
+
+	if (sp->info->chip_select)
+		(sp->info->chip_select)(sp->info, spi_get_chipselect(dev, 0), value);
+}
+
+static int sh_sci_spi_probe(struct platform_device *dev)
+{
+	struct resource	*r;
+	struct spi_controller *host;
+	struct sh_sci_spi *sp;
+	int ret;
+
+	host = spi_alloc_host(&dev->dev, sizeof(struct sh_sci_spi));
+	if (host == NULL) {
+		dev_err(&dev->dev, "failed to allocate spi host\n");
+		ret = -ENOMEM;
+		goto err0;
+	}
+
+	sp = spi_controller_get_devdata(host);
+
+	platform_set_drvdata(dev, sp);
+	sp->info = dev_get_platdata(&dev->dev);
+	if (!sp->info) {
+		dev_err(&dev->dev, "platform data is missing\n");
+		ret = -ENOENT;
+		goto err1;
+	}
+
+	/* setup spi bitbang adaptor */
+	sp->bitbang.ctlr = host;
+	sp->bitbang.ctlr->bus_num = sp->info->bus_num;
+	sp->bitbang.ctlr->num_chipselect = sp->info->num_chipselect;
+	sp->bitbang.chipselect = sh_sci_spi_chipselect;
+
+	sp->bitbang.txrx_word[SPI_MODE_0] = sh_sci_spi_txrx_mode0;
+	sp->bitbang.txrx_word[SPI_MODE_1] = sh_sci_spi_txrx_mode1;
+	sp->bitbang.txrx_word[SPI_MODE_2] = sh_sci_spi_txrx_mode2;
+	sp->bitbang.txrx_word[SPI_MODE_3] = sh_sci_spi_txrx_mode3;
+
+	r = platform_get_resource(dev, IORESOURCE_MEM, 0);
+	if (r == NULL) {
+		ret = -ENOENT;
+		goto err1;
+	}
+	sp->membase = ioremap(r->start, resource_size(r));
+	if (!sp->membase) {
+		ret = -ENXIO;
+		goto err1;
+	}
+	sp->val = ioread8(SCSPTR(sp));
+	setbits(sp, PIN_INIT, 1);
+
+	ret = spi_bitbang_start(&sp->bitbang);
+	if (!ret)
+		return 0;
+
+	setbits(sp, PIN_INIT, 0);
+	iounmap(sp->membase);
+ err1:
+	spi_controller_put(sp->bitbang.ctlr);
+ err0:
+	return ret;
+}
+
+static void sh_sci_spi_remove(struct platform_device *dev)
+{
+	struct sh_sci_spi *sp = platform_get_drvdata(dev);
+
+	spi_bitbang_stop(&sp->bitbang);
+	setbits(sp, PIN_INIT, 0);
+	iounmap(sp->membase);
+	spi_controller_put(sp->bitbang.ctlr);
+}
+
+static struct platform_driver sh_sci_spi_drv = {
+	.probe		= sh_sci_spi_probe,
+	.remove		= sh_sci_spi_remove,
+	.driver		= {
+		.name	= "spi_sh_sci",
+	},
+};
+module_platform_driver(sh_sci_spi_drv);
+
+MODULE_DESCRIPTION("SH SCI SPI Driver");
+MODULE_AUTHOR("Magnus Damm <damm@opensource.se>");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:spi_sh_sci");