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, 251 insertions, 0 deletions

diff --git a/drivers/video/fbdev/hecubafb.c b/drivers/video/fbdev/hecubafb.c
new file mode 100644
index 000000000..dd2af980f
--- /dev/null
+++ b/drivers/video/fbdev/hecubafb.c
@@ -0,0 +1,251 @@
+/*
+ * linux/drivers/video/hecubafb.c -- FB driver for Hecuba/Apollo controller
+ *
+ * Copyright (C) 2006, Jaya Kumar
+ * This work was sponsored by CIS(M) Sdn Bhd
+ *
+ * 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.
+ *
+ * Layout is based on skeletonfb.c by James Simmons and Geert Uytterhoeven.
+ * This work was possible because of apollo display code from E-Ink's website
+ * http://support.eink.com/community
+ * All information used to write this code is from public material made
+ * available by E-Ink on its support site. Some commands such as 0xA4
+ * were found by looping through cmd=0x00 thru 0xFF and supplying random
+ * values. There are other commands that the display is capable of,
+ * beyond the 5 used here but they are more complex.
+ *
+ * This driver is written to be used with the Hecuba display architecture.
+ * The actual display chip is called Apollo and the interface electronics
+ * it needs is called Hecuba.
+ *
+ * It is intended to be architecture independent. A board specific driver
+ * must be used to perform all the physical IO interactions. An example
+ * is provided as n411.c
+ *
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/mm.h>
+#include <linux/vmalloc.h>
+#include <linux/delay.h>
+#include <linux/interrupt.h>
+#include <linux/fb.h>
+#include <linux/init.h>
+#include <linux/platform_device.h>
+#include <linux/list.h>
+#include <linux/uaccess.h>
+
+#include <video/hecubafb.h>
+
+/* Display specific information */
+#define DPY_W 600
+#define DPY_H 800
+
+static const struct fb_fix_screeninfo hecubafb_fix = {
+	.id =		"hecubafb",
+	.type =		FB_TYPE_PACKED_PIXELS,
+	.visual =	FB_VISUAL_MONO01,
+	.xpanstep =	0,
+	.ypanstep =	0,
+	.ywrapstep =	0,
+	.line_length =	DPY_W,
+	.accel =	FB_ACCEL_NONE,
+};
+
+static const struct fb_var_screeninfo hecubafb_var = {
+	.xres		= DPY_W,
+	.yres		= DPY_H,
+	.xres_virtual	= DPY_W,
+	.yres_virtual	= DPY_H,
+	.bits_per_pixel	= 1,
+	.nonstd		= 1,
+};
+
+/* main hecubafb functions */
+
+static void apollo_send_data(struct hecubafb_par *par, unsigned char data)
+{
+	/* set data */
+	par->board->set_data(par, data);
+
+	/* set DS low */
+	par->board->set_ctl(par, HCB_DS_BIT, 0);
+
+	/* wait for ack */
+	par->board->wait_for_ack(par, 0);
+
+	/* set DS hi */
+	par->board->set_ctl(par, HCB_DS_BIT, 1);
+
+	/* wait for ack to clear */
+	par->board->wait_for_ack(par, 1);
+}
+
+static void apollo_send_command(struct hecubafb_par *par, unsigned char data)
+{
+	/* command so set CD to high */
+	par->board->set_ctl(par, HCB_CD_BIT, 1);
+
+	/* actually strobe with command */
+	apollo_send_data(par, data);
+
+	/* clear CD back to low */
+	par->board->set_ctl(par, HCB_CD_BIT, 0);
+}
+
+static void hecubafb_dpy_update(struct hecubafb_par *par)
+{
+	int i;
+	unsigned char *buf = par->info->screen_buffer;
+
+	apollo_send_command(par, APOLLO_START_NEW_IMG);
+
+	for (i=0; i < (DPY_W*DPY_H/8); i++) {
+		apollo_send_data(par, *(buf++));
+	}
+
+	apollo_send_command(par, APOLLO_STOP_IMG_DATA);
+	apollo_send_command(par, APOLLO_DISPLAY_IMG);
+}
+
+/* this is called back from the deferred io workqueue */
+static void hecubafb_dpy_deferred_io(struct fb_info *info, struct list_head *pagereflist)
+{
+	hecubafb_dpy_update(info->par);
+}
+
+static void hecubafb_defio_damage_range(struct fb_info *info, off_t off, size_t len)
+{
+	struct hecubafb_par *par = info->par;
+
+	hecubafb_dpy_update(par);
+}
+
+static void hecubafb_defio_damage_area(struct fb_info *info, u32 x, u32 y,
+				       u32 width, u32 height)
+{
+	struct hecubafb_par *par = info->par;
+
+	hecubafb_dpy_update(par);
+}
+
+FB_GEN_DEFAULT_DEFERRED_SYSMEM_OPS(hecubafb,
+				   hecubafb_defio_damage_range,
+				   hecubafb_defio_damage_area)
+
+static const struct fb_ops hecubafb_ops = {
+	.owner	= THIS_MODULE,
+	FB_DEFAULT_DEFERRED_OPS(hecubafb),
+};
+
+static struct fb_deferred_io hecubafb_defio = {
+	.delay		= HZ,
+	.deferred_io	= hecubafb_dpy_deferred_io,
+};
+
+static int hecubafb_probe(struct platform_device *dev)
+{
+	struct fb_info *info;
+	struct hecuba_board *board;
+	int retval = -ENOMEM;
+	int videomemorysize;
+	unsigned char *videomemory;
+	struct hecubafb_par *par;
+
+	/* pick up board specific routines */
+	board = dev->dev.platform_data;
+	if (!board)
+		return -EINVAL;
+
+	/* try to count device specific driver, if can't, platform recalls */
+	if (!try_module_get(board->owner))
+		return -ENODEV;
+
+	videomemorysize = (DPY_W*DPY_H)/8;
+
+	videomemory = vzalloc(videomemorysize);
+	if (!videomemory)
+		goto err_videomem_alloc;
+
+	info = framebuffer_alloc(sizeof(struct hecubafb_par), &dev->dev);
+	if (!info)
+		goto err_fballoc;
+
+	info->screen_buffer = videomemory;
+	info->fbops = &hecubafb_ops;
+
+	info->var = hecubafb_var;
+	info->fix = hecubafb_fix;
+	info->fix.smem_len = videomemorysize;
+	par = info->par;
+	par->info = info;
+	par->board = board;
+	par->send_command = apollo_send_command;
+	par->send_data = apollo_send_data;
+
+	info->flags = FBINFO_VIRTFB;
+
+	info->fbdefio = &hecubafb_defio;
+	retval = fb_deferred_io_init(info);
+	if (retval)
+		goto err_fbdefio;
+
+	retval = register_framebuffer(info);
+	if (retval < 0)
+		goto err_fbreg;
+	platform_set_drvdata(dev, info);
+
+	fb_info(info, "Hecuba frame buffer device, using %dK of video memory\n",
+		videomemorysize >> 10);
+
+	/* this inits the dpy */
+	retval = par->board->init(par);
+	if (retval < 0)
+		goto err_fbreg;
+
+	return 0;
+err_fbreg:
+	fb_deferred_io_cleanup(info);
+err_fbdefio:
+	framebuffer_release(info);
+err_fballoc:
+	vfree(videomemory);
+err_videomem_alloc:
+	module_put(board->owner);
+	return retval;
+}
+
+static void hecubafb_remove(struct platform_device *dev)
+{
+	struct fb_info *info = platform_get_drvdata(dev);
+
+	if (info) {
+		struct hecubafb_par *par = info->par;
+		fb_deferred_io_cleanup(info);
+		unregister_framebuffer(info);
+		vfree(info->screen_buffer);
+		if (par->board->remove)
+			par->board->remove(par);
+		module_put(par->board->owner);
+		framebuffer_release(info);
+	}
+}
+
+static struct platform_driver hecubafb_driver = {
+	.probe	= hecubafb_probe,
+	.remove	= hecubafb_remove,
+	.driver	= {
+		.name	= "hecubafb",
+	},
+};
+module_platform_driver(hecubafb_driver);
+
+MODULE_DESCRIPTION("fbdev driver for Hecuba/Apollo controller");
+MODULE_AUTHOR("Jaya Kumar");
+MODULE_LICENSE("GPL");