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, 290 insertions, 0 deletions

diff --git a/drivers/video/fbdev/pmag-ba-fb.c b/drivers/video/fbdev/pmag-ba-fb.c
new file mode 100644
index 000000000..1e010520b
--- /dev/null
+++ b/drivers/video/fbdev/pmag-ba-fb.c
@@ -0,0 +1,290 @@
+/*
+ *	linux/drivers/video/pmag-ba-fb.c
+ *
+ *	PMAG-BA TURBOchannel Color Frame Buffer (CFB) card 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.
+ *
+ *	Based on digital document:
+ * 	"PMAG-BA TURBOchannel Color Frame Buffer
+ *	 Functional Specification", Revision 1.2, August 27, 1990
+ *
+ *	DECstation related code Copyright (C) 1999, 2000, 2001 by
+ *	Michael Engel <engel@unix-ag.org>,
+ *	Karsten Merker <merker@linuxtag.org> and
+ *	Harald Koerfgen.
+ *	Copyright (c) 2005, 2006  Maciej W. Rozycki
+ *	Copyright (c) 2005  James Simmons
+ *
+ *	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.
+ */
+
+#include <linux/compiler.h>
+#include <linux/errno.h>
+#include <linux/fb.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/tc.h>
+#include <linux/types.h>
+
+#include <asm/io.h>
+
+#include <video/pmag-ba-fb.h>
+
+
+struct pmagbafb_par {
+	volatile void __iomem *mmio;
+	volatile u32 __iomem *dac;
+};
+
+
+static const struct fb_var_screeninfo pmagbafb_defined = {
+	.xres		= 1024,
+	.yres		= 864,
+	.xres_virtual	= 1024,
+	.yres_virtual	= 864,
+	.bits_per_pixel	= 8,
+	.red.length	= 8,
+	.green.length	= 8,
+	.blue.length	= 8,
+	.activate	= FB_ACTIVATE_NOW,
+	.height		= -1,
+	.width		= -1,
+	.accel_flags	= FB_ACCEL_NONE,
+	.pixclock	= 14452,
+	.left_margin	= 116,
+	.right_margin	= 12,
+	.upper_margin	= 34,
+	.lower_margin	= 0,
+	.hsync_len	= 128,
+	.vsync_len	= 3,
+	.sync		= FB_SYNC_ON_GREEN,
+	.vmode		= FB_VMODE_NONINTERLACED,
+};
+
+static const struct fb_fix_screeninfo pmagbafb_fix = {
+	.id		= "PMAG-BA",
+	.smem_len	= (1024 * 1024),
+	.type		= FB_TYPE_PACKED_PIXELS,
+	.visual		= FB_VISUAL_PSEUDOCOLOR,
+	.line_length	= 1024,
+	.mmio_len	= PMAG_BA_SIZE - PMAG_BA_BT459,
+};
+
+
+static inline void dac_write(struct pmagbafb_par *par, unsigned int reg, u8 v)
+{
+	writeb(v, par->dac + reg / 4);
+}
+
+static inline u8 dac_read(struct pmagbafb_par *par, unsigned int reg)
+{
+	return readb(par->dac + reg / 4);
+}
+
+
+/*
+ * Set the palette.
+ */
+static int pmagbafb_setcolreg(unsigned int regno, unsigned int red,
+			      unsigned int green, unsigned int blue,
+			      unsigned int transp, struct fb_info *info)
+{
+	struct pmagbafb_par *par = info->par;
+
+	if (regno >= info->cmap.len)
+		return 1;
+
+	red   >>= 8;	/* The cmap fields are 16 bits    */
+	green >>= 8;	/* wide, but the hardware colormap */
+	blue  >>= 8;	/* registers are only 8 bits wide */
+
+	mb();
+	dac_write(par, BT459_ADDR_LO, regno);
+	dac_write(par, BT459_ADDR_HI, 0x00);
+	wmb();
+	dac_write(par, BT459_CMAP, red);
+	wmb();
+	dac_write(par, BT459_CMAP, green);
+	wmb();
+	dac_write(par, BT459_CMAP, blue);
+
+	return 0;
+}
+
+static const struct fb_ops pmagbafb_ops = {
+	.owner		= THIS_MODULE,
+	FB_DEFAULT_IOMEM_OPS,
+	.fb_setcolreg	= pmagbafb_setcolreg,
+};
+
+
+/*
+ * Turn the hardware cursor off.
+ */
+static void pmagbafb_erase_cursor(struct fb_info *info)
+{
+	struct pmagbafb_par *par = info->par;
+
+	mb();
+	dac_write(par, BT459_ADDR_LO, 0x00);
+	dac_write(par, BT459_ADDR_HI, 0x03);
+	wmb();
+	dac_write(par, BT459_DATA, 0x00);
+}
+
+
+static int pmagbafb_probe(struct device *dev)
+{
+	struct tc_dev *tdev = to_tc_dev(dev);
+	resource_size_t start, len;
+	struct fb_info *info;
+	struct pmagbafb_par *par;
+	int err;
+
+	info = framebuffer_alloc(sizeof(struct pmagbafb_par), dev);
+	if (!info)
+		return -ENOMEM;
+
+	par = info->par;
+	dev_set_drvdata(dev, info);
+
+	if (fb_alloc_cmap(&info->cmap, 256, 0) < 0) {
+		printk(KERN_ERR "%s: Cannot allocate color map\n",
+		       dev_name(dev));
+		err = -ENOMEM;
+		goto err_alloc;
+	}
+
+	info->fbops = &pmagbafb_ops;
+	info->fix = pmagbafb_fix;
+	info->var = pmagbafb_defined;
+
+	/* Request the I/O MEM resource.  */
+	start = tdev->resource.start;
+	len = tdev->resource.end - start + 1;
+	if (!request_mem_region(start, len, dev_name(dev))) {
+		printk(KERN_ERR "%s: Cannot reserve FB region\n",
+		       dev_name(dev));
+		err = -EBUSY;
+		goto err_cmap;
+	}
+
+	/* MMIO mapping setup.  */
+	info->fix.mmio_start = start;
+	par->mmio = ioremap(info->fix.mmio_start, info->fix.mmio_len);
+	if (!par->mmio) {
+		printk(KERN_ERR "%s: Cannot map MMIO\n", dev_name(dev));
+		err = -ENOMEM;
+		goto err_resource;
+	}
+	par->dac = par->mmio + PMAG_BA_BT459;
+
+	/* Frame buffer mapping setup.  */
+	info->fix.smem_start = start + PMAG_BA_FBMEM;
+	info->screen_base = ioremap(info->fix.smem_start,
+					    info->fix.smem_len);
+	if (!info->screen_base) {
+		printk(KERN_ERR "%s: Cannot map FB\n", dev_name(dev));
+		err = -ENOMEM;
+		goto err_mmio_map;
+	}
+	info->screen_size = info->fix.smem_len;
+
+	pmagbafb_erase_cursor(info);
+
+	err = register_framebuffer(info);
+	if (err < 0) {
+		printk(KERN_ERR "%s: Cannot register framebuffer\n",
+		       dev_name(dev));
+		goto err_smem_map;
+	}
+
+	get_device(dev);
+
+	fb_info(info, "%s frame buffer device at %s\n",
+		info->fix.id, dev_name(dev));
+
+	return 0;
+
+
+err_smem_map:
+	iounmap(info->screen_base);
+
+err_mmio_map:
+	iounmap(par->mmio);
+
+err_resource:
+	release_mem_region(start, len);
+
+err_cmap:
+	fb_dealloc_cmap(&info->cmap);
+
+err_alloc:
+	framebuffer_release(info);
+	return err;
+}
+
+static int pmagbafb_remove(struct device *dev)
+{
+	struct tc_dev *tdev = to_tc_dev(dev);
+	struct fb_info *info = dev_get_drvdata(dev);
+	struct pmagbafb_par *par = info->par;
+	resource_size_t start, len;
+
+	put_device(dev);
+	unregister_framebuffer(info);
+	iounmap(info->screen_base);
+	iounmap(par->mmio);
+	start = tdev->resource.start;
+	len = tdev->resource.end - start + 1;
+	release_mem_region(start, len);
+	fb_dealloc_cmap(&info->cmap);
+	framebuffer_release(info);
+	return 0;
+}
+
+
+/*
+ * Initialize the framebuffer.
+ */
+static const struct tc_device_id pmagbafb_tc_table[] = {
+	{ "DEC     ", "PMAG-BA " },
+	{ }
+};
+MODULE_DEVICE_TABLE(tc, pmagbafb_tc_table);
+
+static struct tc_driver pmagbafb_driver = {
+	.id_table	= pmagbafb_tc_table,
+	.driver		= {
+		.name	= "pmagbafb",
+		.bus	= &tc_bus_type,
+		.probe	= pmagbafb_probe,
+		.remove	= pmagbafb_remove,
+	},
+};
+
+static int __init pmagbafb_init(void)
+{
+#ifndef MODULE
+	if (fb_get_options("pmagbafb", NULL))
+		return -ENXIO;
+#endif
+	return tc_register_driver(&pmagbafb_driver);
+}
+
+static void __exit pmagbafb_exit(void)
+{
+	tc_unregister_driver(&pmagbafb_driver);
+}
+
+
+module_init(pmagbafb_init);
+module_exit(pmagbafb_exit);
+
+MODULE_LICENSE("GPL");