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, 104 insertions, 0 deletions

diff --git a/include/video/udlfb.h b/include/video/udlfb.h
new file mode 100644
index 000000000..ab34790d5
--- /dev/null
+++ b/include/video/udlfb.h
@@ -0,0 +1,104 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef UDLFB_H
+#define UDLFB_H
+
+/*
+ * TODO: Propose standard fb.h ioctl for reporting damage,
+ * using _IOWR() and one of the existing area structs from fb.h
+ * Consider these ioctls deprecated, but they're still used by the
+ * DisplayLink X server as yet - need both to be modified in tandem
+ * when new ioctl(s) are ready.
+ */
+#define DLFB_IOCTL_RETURN_EDID	 0xAD
+#define DLFB_IOCTL_REPORT_DAMAGE 0xAA
+struct dloarea {
+	int x, y;
+	int w, h;
+	int x2, y2;
+};
+
+struct urb_node {
+	struct list_head entry;
+	struct dlfb_data *dlfb;
+	struct urb *urb;
+};
+
+struct urb_list {
+	struct list_head list;
+	spinlock_t lock;
+	struct semaphore limit_sem;
+	int available;
+	int count;
+	size_t size;
+};
+
+struct dlfb_data {
+	struct usb_device *udev;
+	struct fb_info *info;
+	struct urb_list urbs;
+	char *backing_buffer;
+	int fb_count;
+	bool virtualized; /* true when physical usb device not present */
+	atomic_t usb_active; /* 0 = update virtual buffer, but no usb traffic */
+	atomic_t lost_pixels; /* 1 = a render op failed. Need screen refresh */
+	char *edid; /* null until we read edid from hw or get from sysfs */
+	size_t edid_size;
+	int sku_pixel_limit;
+	int base16;
+	int base8;
+	u32 pseudo_palette[256];
+	int blank_mode; /*one of FB_BLANK_ */
+	struct mutex render_mutex;
+	int damage_x;
+	int damage_y;
+	int damage_x2;
+	int damage_y2;
+	spinlock_t damage_lock;
+	struct work_struct damage_work;
+	struct fb_ops ops;
+	atomic_t mmap_count;
+	/* blit-only rendering path metrics, exposed through sysfs */
+	atomic_t bytes_rendered; /* raw pixel-bytes driver asked to render */
+	atomic_t bytes_identical; /* saved effort with backbuffer comparison */
+	atomic_t bytes_sent; /* to usb, after compression including overhead */
+	atomic_t cpu_kcycles_used; /* transpired during pixel processing */
+	struct fb_var_screeninfo current_mode;
+	struct list_head deferred_free;
+};
+
+#define NR_USB_REQUEST_I2C_SUB_IO 0x02
+#define NR_USB_REQUEST_CHANNEL 0x12
+
+/* -BULK_SIZE as per usb-skeleton. Can we get full page and avoid overhead? */
+#define BULK_SIZE 512
+#define MAX_TRANSFER (PAGE_SIZE*16 - BULK_SIZE)
+#define WRITES_IN_FLIGHT (4)
+
+#define MAX_VENDOR_DESCRIPTOR_SIZE 256
+
+#define GET_URB_TIMEOUT	HZ
+#define FREE_URB_TIMEOUT (HZ*2)
+
+#define BPP                     2
+#define MAX_CMD_PIXELS		255
+
+#define RLX_HEADER_BYTES	7
+#define MIN_RLX_PIX_BYTES       4
+#define MIN_RLX_CMD_BYTES	(RLX_HEADER_BYTES + MIN_RLX_PIX_BYTES)
+
+#define RLE_HEADER_BYTES	6
+#define MIN_RLE_PIX_BYTES	3
+#define MIN_RLE_CMD_BYTES	(RLE_HEADER_BYTES + MIN_RLE_PIX_BYTES)
+
+#define RAW_HEADER_BYTES	6
+#define MIN_RAW_PIX_BYTES	2
+#define MIN_RAW_CMD_BYTES	(RAW_HEADER_BYTES + MIN_RAW_PIX_BYTES)
+
+#define DL_DEFIO_WRITE_DELAY    msecs_to_jiffies(HZ <= 300 ? 4 : 10) /* optimal value for 720p video */
+#define DL_DEFIO_WRITE_DISABLE  (HZ*60) /* "disable" with long delay */
+
+/* remove these once align.h patch is taken into kernel */
+#define DL_ALIGN_UP(x, a) ALIGN(x, a)
+#define DL_ALIGN_DOWN(x, a) ALIGN_DOWN(x, a)
+
+#endif