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, 145 insertions, 0 deletions

diff --git a/arch/mips/bcm63xx/cs.c b/arch/mips/bcm63xx/cs.c
new file mode 100644
index 000000000..29205badc
--- /dev/null
+++ b/arch/mips/bcm63xx/cs.c
@@ -0,0 +1,145 @@
+/*
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * Copyright (C) 2008 Maxime Bizon <mbizon@freebox.fr>
+ */
+
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/export.h>
+#include <linux/spinlock.h>
+#include <linux/log2.h>
+#include <bcm63xx_cpu.h>
+#include <bcm63xx_io.h>
+#include <bcm63xx_regs.h>
+#include <bcm63xx_cs.h>
+
+static DEFINE_SPINLOCK(bcm63xx_cs_lock);
+
+/*
+ * check if given chip select exists
+ */
+static int is_valid_cs(unsigned int cs)
+{
+	if (cs > 6)
+		return 0;
+	return 1;
+}
+
+/*
+ * Configure chipselect base address and size (bytes).
+ * Size must be a power of two between 8k and 256M.
+ */
+int bcm63xx_set_cs_base(unsigned int cs, u32 base, unsigned int size)
+{
+	unsigned long flags;
+	u32 val;
+
+	if (!is_valid_cs(cs))
+		return -EINVAL;
+
+	/* sanity check on size */
+	if (size != roundup_pow_of_two(size))
+		return -EINVAL;
+
+	if (size < 8 * 1024 || size > 256 * 1024 * 1024)
+		return -EINVAL;
+
+	val = (base & MPI_CSBASE_BASE_MASK);
+	/* 8k => 0 - 256M => 15 */
+	val |= (ilog2(size) - ilog2(8 * 1024)) << MPI_CSBASE_SIZE_SHIFT;
+
+	spin_lock_irqsave(&bcm63xx_cs_lock, flags);
+	bcm_mpi_writel(val, MPI_CSBASE_REG(cs));
+	spin_unlock_irqrestore(&bcm63xx_cs_lock, flags);
+
+	return 0;
+}
+
+EXPORT_SYMBOL(bcm63xx_set_cs_base);
+
+/*
+ * configure chipselect timing (ns)
+ */
+int bcm63xx_set_cs_timing(unsigned int cs, unsigned int wait,
+			   unsigned int setup, unsigned int hold)
+{
+	unsigned long flags;
+	u32 val;
+
+	if (!is_valid_cs(cs))
+		return -EINVAL;
+
+	spin_lock_irqsave(&bcm63xx_cs_lock, flags);
+	val = bcm_mpi_readl(MPI_CSCTL_REG(cs));
+	val &= ~(MPI_CSCTL_WAIT_MASK);
+	val &= ~(MPI_CSCTL_SETUP_MASK);
+	val &= ~(MPI_CSCTL_HOLD_MASK);
+	val |= wait << MPI_CSCTL_WAIT_SHIFT;
+	val |= setup << MPI_CSCTL_SETUP_SHIFT;
+	val |= hold << MPI_CSCTL_HOLD_SHIFT;
+	bcm_mpi_writel(val, MPI_CSCTL_REG(cs));
+	spin_unlock_irqrestore(&bcm63xx_cs_lock, flags);
+
+	return 0;
+}
+
+EXPORT_SYMBOL(bcm63xx_set_cs_timing);
+
+/*
+ * configure other chipselect parameter (data bus size, ...)
+ */
+int bcm63xx_set_cs_param(unsigned int cs, u32 params)
+{
+	unsigned long flags;
+	u32 val;
+
+	if (!is_valid_cs(cs))
+		return -EINVAL;
+
+	/* none of this fields apply to pcmcia */
+	if (cs == MPI_CS_PCMCIA_COMMON ||
+	    cs == MPI_CS_PCMCIA_ATTR ||
+	    cs == MPI_CS_PCMCIA_IO)
+		return -EINVAL;
+
+	spin_lock_irqsave(&bcm63xx_cs_lock, flags);
+	val = bcm_mpi_readl(MPI_CSCTL_REG(cs));
+	val &= ~(MPI_CSCTL_DATA16_MASK);
+	val &= ~(MPI_CSCTL_SYNCMODE_MASK);
+	val &= ~(MPI_CSCTL_TSIZE_MASK);
+	val &= ~(MPI_CSCTL_ENDIANSWAP_MASK);
+	val |= params;
+	bcm_mpi_writel(val, MPI_CSCTL_REG(cs));
+	spin_unlock_irqrestore(&bcm63xx_cs_lock, flags);
+
+	return 0;
+}
+
+EXPORT_SYMBOL(bcm63xx_set_cs_param);
+
+/*
+ * set cs status (enable/disable)
+ */
+int bcm63xx_set_cs_status(unsigned int cs, int enable)
+{
+	unsigned long flags;
+	u32 val;
+
+	if (!is_valid_cs(cs))
+		return -EINVAL;
+
+	spin_lock_irqsave(&bcm63xx_cs_lock, flags);
+	val = bcm_mpi_readl(MPI_CSCTL_REG(cs));
+	if (enable)
+		val |= MPI_CSCTL_ENABLE_MASK;
+	else
+		val &= ~MPI_CSCTL_ENABLE_MASK;
+	bcm_mpi_writel(val, MPI_CSCTL_REG(cs));
+	spin_unlock_irqrestore(&bcm63xx_cs_lock, flags);
+	return 0;
+}
+
+EXPORT_SYMBOL(bcm63xx_set_cs_status);