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, 174 insertions, 0 deletions

diff --git a/drivers/video/fbdev/maxinefb.c b/drivers/video/fbdev/maxinefb.c
new file mode 100644
index 000000000..52528eb4d
--- /dev/null
+++ b/drivers/video/fbdev/maxinefb.c
@@ -0,0 +1,174 @@
+/*
+ *      linux/drivers/video/maxinefb.c
+ *
+ *	DECstation 5000/xx onboard framebuffer support ... derived from:
+ *	"HP300 Topcat framebuffer support (derived from macfb of all things)
+ *	Phil Blundell <philb@gnu.org> 1998", the original code can be
+ *      found in the file hpfb.c in the same directory.
+ *
+ *      DECstation related code Copyright (C) 1999,2000,2001 by
+ *      Michael Engel <engel@unix-ag.org> and
+ *      Karsten Merker <merker@linuxtag.org>.
+ *      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.
+ *
+ */
+
+/*
+ * Changes:
+ * 2001/01/27 removed debugging and testing code, fixed fb_ops
+ *            initialization which had caused a crash before,
+ *            general cleanup, first official release (KM)
+ *
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/mm.h>
+#include <linux/delay.h>
+#include <linux/init.h>
+#include <linux/fb.h>
+#include <video/maxinefb.h>
+
+/* bootinfo.h defines the machine type values, needed when checking */
+/* whether are really running on a maxine, KM                       */
+#include <asm/bootinfo.h>
+
+static struct fb_info fb_info;
+
+static const struct fb_var_screeninfo maxinefb_defined = {
+	.xres =		1024,
+	.yres =		768,
+	.xres_virtual =	1024,
+	.yres_virtual =	768,
+	.bits_per_pixel =8,
+	.activate =	FB_ACTIVATE_NOW,
+	.height =	-1,
+	.width =	-1,
+	.vmode =	FB_VMODE_NONINTERLACED,
+};
+
+static struct fb_fix_screeninfo maxinefb_fix __initdata = {
+	.id =		"Maxine",
+	.smem_len =	(1024*768),
+	.type =		FB_TYPE_PACKED_PIXELS,
+	.visual =	FB_VISUAL_PSEUDOCOLOR,
+	.line_length =	1024,
+};
+
+/* Handle the funny Inmos RamDAC/video controller ... */
+
+void maxinefb_ims332_write_register(int regno, register unsigned int val)
+{
+	register unsigned char *regs = (char *) MAXINEFB_IMS332_ADDRESS;
+	unsigned char *wptr;
+
+	wptr = regs + 0xa0000 + (regno << 4);
+	*((volatile unsigned int *) (regs)) = (val >> 8) & 0xff00;
+	*((volatile unsigned short *) (wptr)) = val;
+}
+
+unsigned int maxinefb_ims332_read_register(int regno)
+{
+	register unsigned char *regs = (char *) MAXINEFB_IMS332_ADDRESS;
+	unsigned char *rptr;
+	register unsigned int j, k;
+
+	rptr = regs + 0x80000 + (regno << 4);
+	j = *((volatile unsigned short *) rptr);
+	k = *((volatile unsigned short *) regs);
+
+	return (j & 0xffff) | ((k & 0xff00) << 8);
+}
+
+/* Set the palette */
+static int maxinefb_setcolreg(unsigned regno, unsigned red, unsigned green,
+			      unsigned blue, unsigned transp, struct fb_info *info)
+{
+	/* value to be written into the palette reg. */
+	unsigned long hw_colorvalue = 0;
+
+	if (regno > 255)
+		return 1;
+
+	red   >>= 8;    /* The cmap fields are 16 bits    */
+	green >>= 8;    /* wide, but the harware colormap */
+	blue  >>= 8;    /* registers are only 8 bits wide */
+
+	hw_colorvalue = (blue << 16) + (green << 8) + (red);
+
+	maxinefb_ims332_write_register(IMS332_REG_COLOR_PALETTE + regno,
+				       hw_colorvalue);
+	return 0;
+}
+
+static const struct fb_ops maxinefb_ops = {
+	.owner		= THIS_MODULE,
+	FB_DEFAULT_IOMEM_OPS,
+	.fb_setcolreg	= maxinefb_setcolreg,
+};
+
+int __init maxinefb_init(void)
+{
+	unsigned long fboff;
+	unsigned long fb_start;
+	int i;
+
+	if (fb_get_options("maxinefb", NULL))
+		return -ENODEV;
+
+	/* Validate we're on the proper machine type */
+	if (mips_machtype != MACH_DS5000_XX) {
+		return -EINVAL;
+	}
+
+	printk(KERN_INFO "Maxinefb: Personal DECstation detected\n");
+	printk(KERN_INFO "Maxinefb: initializing onboard framebuffer\n");
+
+	/* Framebuffer display memory base address */
+	fb_start = DS5000_xx_ONBOARD_FBMEM_START;
+
+	/* Clear screen */
+	for (fboff = fb_start; fboff < fb_start + 0x1ffff; fboff++)
+		*(volatile unsigned char *)fboff = 0x0;
+
+	maxinefb_fix.smem_start = fb_start;
+
+	/* erase hardware cursor */
+	for (i = 0; i < 512; i++) {
+		maxinefb_ims332_write_register(IMS332_REG_CURSOR_RAM + i,
+					       0);
+		/*
+		   if (i&0x8 == 0)
+		   maxinefb_ims332_write_register (IMS332_REG_CURSOR_RAM + i, 0x0f);
+		   else
+		   maxinefb_ims332_write_register (IMS332_REG_CURSOR_RAM + i, 0xf0);
+		 */
+	}
+
+	fb_info.fbops = &maxinefb_ops;
+	fb_info.screen_base = (char *)maxinefb_fix.smem_start;
+	fb_info.var = maxinefb_defined;
+	fb_info.fix = maxinefb_fix;
+
+	fb_alloc_cmap(&fb_info.cmap, 256, 0);
+
+	if (register_framebuffer(&fb_info) < 0)
+		return 1;
+	return 0;
+}
+
+static void __exit maxinefb_exit(void)
+{
+	unregister_framebuffer(&fb_info);
+}
+
+#ifdef MODULE
+MODULE_LICENSE("GPL");
+#endif
+module_init(maxinefb_init);
+module_exit(maxinefb_exit);
+