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, 231 insertions, 0 deletions

diff --git a/drivers/mtd/maps/dc21285.c b/drivers/mtd/maps/dc21285.c
new file mode 100644
index 000000000..70a3db3ab
--- /dev/null
+++ b/drivers/mtd/maps/dc21285.c
@@ -0,0 +1,231 @@
+/*
+ * MTD map driver for flash on the DC21285 (the StrongARM-110 companion chip)
+ *
+ * (C) 2000  Nicolas Pitre <nico@fluxnic.net>
+ *
+ * This code is GPL
+ */
+#include <linux/module.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/delay.h>
+#include <linux/slab.h>
+
+#include <linux/mtd/mtd.h>
+#include <linux/mtd/map.h>
+#include <linux/mtd/partitions.h>
+
+#include <asm/io.h>
+#include <asm/hardware/dec21285.h>
+#include <asm/mach-types.h>
+
+
+static struct mtd_info *dc21285_mtd;
+
+#ifdef CONFIG_ARCH_NETWINDER
+/*
+ * This is really ugly, but it seams to be the only
+ * realiable way to do it, as the cpld state machine
+ * is unpredictible. So we have a 25us penalty per
+ * write access.
+ */
+static void nw_en_write(void)
+{
+	unsigned long flags;
+
+	/*
+	 * we want to write a bit pattern XXX1 to Xilinx to enable
+	 * the write gate, which will be open for about the next 2ms.
+	 */
+	raw_spin_lock_irqsave(&nw_gpio_lock, flags);
+	nw_cpld_modify(CPLD_FLASH_WR_ENABLE, CPLD_FLASH_WR_ENABLE);
+	raw_spin_unlock_irqrestore(&nw_gpio_lock, flags);
+
+	/*
+	 * let the ISA bus to catch on...
+	 */
+	udelay(25);
+}
+#else
+#define nw_en_write() do { } while (0)
+#endif
+
+static map_word dc21285_read8(struct map_info *map, unsigned long ofs)
+{
+	map_word val;
+	val.x[0] = *(uint8_t*)(map->virt + ofs);
+	return val;
+}
+
+static map_word dc21285_read16(struct map_info *map, unsigned long ofs)
+{
+	map_word val;
+	val.x[0] = *(uint16_t*)(map->virt + ofs);
+	return val;
+}
+
+static map_word dc21285_read32(struct map_info *map, unsigned long ofs)
+{
+	map_word val;
+	val.x[0] = *(uint32_t*)(map->virt + ofs);
+	return val;
+}
+
+static void dc21285_copy_from(struct map_info *map, void *to, unsigned long from, ssize_t len)
+{
+	memcpy(to, (void*)(map->virt + from), len);
+}
+
+static void dc21285_write8(struct map_info *map, const map_word d, unsigned long adr)
+{
+	if (machine_is_netwinder())
+		nw_en_write();
+	*CSR_ROMWRITEREG = adr & 3;
+	adr &= ~3;
+	*(uint8_t*)(map->virt + adr) = d.x[0];
+}
+
+static void dc21285_write16(struct map_info *map, const map_word d, unsigned long adr)
+{
+	if (machine_is_netwinder())
+		nw_en_write();
+	*CSR_ROMWRITEREG = adr & 3;
+	adr &= ~3;
+	*(uint16_t*)(map->virt + adr) = d.x[0];
+}
+
+static void dc21285_write32(struct map_info *map, const map_word d, unsigned long adr)
+{
+	if (machine_is_netwinder())
+		nw_en_write();
+	*(uint32_t*)(map->virt + adr) = d.x[0];
+}
+
+static void dc21285_copy_to_32(struct map_info *map, unsigned long to, const void *from, ssize_t len)
+{
+	while (len > 0) {
+		map_word d;
+		d.x[0] = *((uint32_t*)from);
+		dc21285_write32(map, d, to);
+		from += 4;
+		to += 4;
+		len -= 4;
+	}
+}
+
+static void dc21285_copy_to_16(struct map_info *map, unsigned long to, const void *from, ssize_t len)
+{
+	while (len > 0) {
+		map_word d;
+		d.x[0] = *((uint16_t*)from);
+		dc21285_write16(map, d, to);
+		from += 2;
+		to += 2;
+		len -= 2;
+	}
+}
+
+static void dc21285_copy_to_8(struct map_info *map, unsigned long to, const void *from, ssize_t len)
+{
+	map_word d;
+	d.x[0] = *((uint8_t*)from);
+	dc21285_write8(map, d, to);
+	from++;
+	to++;
+	len--;
+}
+
+static struct map_info dc21285_map = {
+	.name = "DC21285 flash",
+	.phys = NO_XIP,
+	.size = 16*1024*1024,
+	.copy_from = dc21285_copy_from,
+};
+
+/* Partition stuff */
+static const char * const probes[] = { "RedBoot", "cmdlinepart", NULL };
+
+static int __init init_dc21285(void)
+{
+	/* Determine bankwidth */
+	switch (*CSR_SA110_CNTL & (3<<14)) {
+		case SA110_CNTL_ROMWIDTH_8:
+			dc21285_map.bankwidth = 1;
+			dc21285_map.read = dc21285_read8;
+			dc21285_map.write = dc21285_write8;
+			dc21285_map.copy_to = dc21285_copy_to_8;
+			break;
+		case SA110_CNTL_ROMWIDTH_16:
+			dc21285_map.bankwidth = 2;
+			dc21285_map.read = dc21285_read16;
+			dc21285_map.write = dc21285_write16;
+			dc21285_map.copy_to = dc21285_copy_to_16;
+			break;
+		case SA110_CNTL_ROMWIDTH_32:
+			dc21285_map.bankwidth = 4;
+			dc21285_map.read = dc21285_read32;
+			dc21285_map.write = dc21285_write32;
+			dc21285_map.copy_to = dc21285_copy_to_32;
+			break;
+		default:
+			printk (KERN_ERR "DC21285 flash: undefined bankwidth\n");
+			return -ENXIO;
+	}
+	printk (KERN_NOTICE "DC21285 flash support (%d-bit bankwidth)\n",
+		dc21285_map.bankwidth*8);
+
+	/* Let's map the flash area */
+	dc21285_map.virt = ioremap(DC21285_FLASH, 16*1024*1024);
+	if (!dc21285_map.virt) {
+		printk("Failed to ioremap\n");
+		return -EIO;
+	}
+
+	if (machine_is_ebsa285()) {
+		dc21285_mtd = do_map_probe("cfi_probe", &dc21285_map);
+	} else {
+		dc21285_mtd = do_map_probe("jedec_probe", &dc21285_map);
+	}
+
+	if (!dc21285_mtd) {
+		iounmap(dc21285_map.virt);
+		return -ENXIO;
+	}
+
+	dc21285_mtd->owner = THIS_MODULE;
+
+	mtd_device_parse_register(dc21285_mtd, probes, NULL, NULL, 0);
+
+	if(machine_is_ebsa285()) {
+		/*
+		 * Flash timing is determined with bits 19-16 of the
+		 * CSR_SA110_CNTL.  The value is the number of wait cycles, or
+		 * 0 for 16 cycles (the default).  Cycles are 20 ns.
+		 * Here we use 7 for 140 ns flash chips.
+		 */
+		/* access time */
+		*CSR_SA110_CNTL = ((*CSR_SA110_CNTL & ~0x000f0000) | (7 << 16));
+		/* burst time */
+		*CSR_SA110_CNTL = ((*CSR_SA110_CNTL & ~0x00f00000) | (7 << 20));
+		/* tristate time */
+		*CSR_SA110_CNTL = ((*CSR_SA110_CNTL & ~0x0f000000) | (7 << 24));
+	}
+
+	return 0;
+}
+
+static void __exit cleanup_dc21285(void)
+{
+	mtd_device_unregister(dc21285_mtd);
+	map_destroy(dc21285_mtd);
+	iounmap(dc21285_map.virt);
+}
+
+module_init(init_dc21285);
+module_exit(cleanup_dc21285);
+
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Nicolas Pitre <nico@fluxnic.net>");
+MODULE_DESCRIPTION("MTD map driver for DC21285 boards");