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, 196 insertions, 0 deletions

diff --git a/drivers/input/misc/ibm-panel.c b/drivers/input/misc/ibm-panel.c
new file mode 100644
index 000000000..aa48f62d7
--- /dev/null
+++ b/drivers/input/misc/ibm-panel.c
@@ -0,0 +1,196 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) IBM Corporation 2020
+ */
+
+#include <linux/i2c.h>
+#include <linux/init.h>
+#include <linux/input.h>
+#include <linux/kernel.h>
+#include <linux/limits.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/spinlock.h>
+
+#define DEVICE_NAME		"ibm-panel"
+#define PANEL_KEYCODES_COUNT	3
+
+struct ibm_panel {
+	u8 idx;
+	u8 command[11];
+	u32 keycodes[PANEL_KEYCODES_COUNT];
+	spinlock_t lock;	/* protects writes to idx and command */
+	struct input_dev *input;
+};
+
+static u8 ibm_panel_calculate_checksum(struct ibm_panel *panel)
+{
+	u8 chksum;
+	u16 sum = 0;
+	unsigned int i;
+
+	for (i = 0; i < sizeof(panel->command) - 1; ++i) {
+		sum += panel->command[i];
+		if (sum & 0xff00) {
+			sum &= 0xff;
+			sum++;
+		}
+	}
+
+	chksum = sum & 0xff;
+	chksum = ~chksum;
+	chksum++;
+
+	return chksum;
+}
+
+static void ibm_panel_process_command(struct ibm_panel *panel)
+{
+	u8 button;
+	u8 chksum;
+
+	if (panel->command[0] != 0xff && panel->command[1] != 0xf0) {
+		dev_dbg(&panel->input->dev, "command invalid: %02x %02x\n",
+			panel->command[0], panel->command[1]);
+		return;
+	}
+
+	chksum = ibm_panel_calculate_checksum(panel);
+	if (chksum != panel->command[sizeof(panel->command) - 1]) {
+		dev_dbg(&panel->input->dev,
+			"command failed checksum: %u != %u\n", chksum,
+			panel->command[sizeof(panel->command) - 1]);
+		return;
+	}
+
+	button = panel->command[2] & 0xf;
+	if (button < PANEL_KEYCODES_COUNT) {
+		input_report_key(panel->input, panel->keycodes[button],
+				 !(panel->command[2] & 0x80));
+		input_sync(panel->input);
+	} else {
+		dev_dbg(&panel->input->dev, "unknown button %u\n",
+			button);
+	}
+}
+
+static int ibm_panel_i2c_slave_cb(struct i2c_client *client,
+				  enum i2c_slave_event event, u8 *val)
+{
+	struct ibm_panel *panel = i2c_get_clientdata(client);
+
+	dev_dbg(&panel->input->dev, "event: %u data: %02x\n", event, *val);
+
+	guard(spinlock_irqsave)(&panel->lock);
+
+	switch (event) {
+	case I2C_SLAVE_STOP:
+		if (panel->idx == sizeof(panel->command))
+			ibm_panel_process_command(panel);
+		else
+			dev_dbg(&panel->input->dev,
+				"command incorrect size %u\n", panel->idx);
+		fallthrough;
+	case I2C_SLAVE_WRITE_REQUESTED:
+		panel->idx = 0;
+		break;
+	case I2C_SLAVE_WRITE_RECEIVED:
+		if (panel->idx < sizeof(panel->command))
+			panel->command[panel->idx++] = *val;
+		else
+			/*
+			 * The command is too long and therefore invalid, so set the index
+			 * to it's largest possible value. When a STOP is finally received,
+			 * the command will be rejected upon processing.
+			 */
+			panel->idx = U8_MAX;
+		break;
+	case I2C_SLAVE_READ_REQUESTED:
+	case I2C_SLAVE_READ_PROCESSED:
+		*val = 0xff;
+		break;
+	default:
+		break;
+	}
+
+	return 0;
+}
+
+static int ibm_panel_probe(struct i2c_client *client)
+{
+	struct ibm_panel *panel;
+	int i;
+	int error;
+
+	panel = devm_kzalloc(&client->dev, sizeof(*panel), GFP_KERNEL);
+	if (!panel)
+		return -ENOMEM;
+
+	spin_lock_init(&panel->lock);
+
+	panel->input = devm_input_allocate_device(&client->dev);
+	if (!panel->input)
+		return -ENOMEM;
+
+	panel->input->name = client->name;
+	panel->input->id.bustype = BUS_I2C;
+
+	error = device_property_read_u32_array(&client->dev,
+					       "linux,keycodes",
+					       panel->keycodes,
+					       PANEL_KEYCODES_COUNT);
+	if (error) {
+		/*
+		 * Use gamepad buttons as defaults for compatibility with
+		 * existing applications.
+		 */
+		panel->keycodes[0] = BTN_NORTH;
+		panel->keycodes[1] = BTN_SOUTH;
+		panel->keycodes[2] = BTN_SELECT;
+	}
+
+	for (i = 0; i < PANEL_KEYCODES_COUNT; ++i)
+		input_set_capability(panel->input, EV_KEY, panel->keycodes[i]);
+
+	error = input_register_device(panel->input);
+	if (error) {
+		dev_err(&client->dev,
+			"Failed to register input device: %d\n", error);
+		return error;
+	}
+
+	i2c_set_clientdata(client, panel);
+	error = i2c_slave_register(client, ibm_panel_i2c_slave_cb);
+	if (error) {
+		dev_err(&client->dev,
+			"Failed to register as i2c slave: %d\n", error);
+		return error;
+	}
+
+	return 0;
+}
+
+static void ibm_panel_remove(struct i2c_client *client)
+{
+	i2c_slave_unregister(client);
+}
+
+static const struct of_device_id ibm_panel_match[] = {
+	{ .compatible = "ibm,op-panel" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, ibm_panel_match);
+
+static struct i2c_driver ibm_panel_driver = {
+	.driver = {
+		.name = DEVICE_NAME,
+		.of_match_table = ibm_panel_match,
+	},
+	.probe = ibm_panel_probe,
+	.remove = ibm_panel_remove,
+};
+module_i2c_driver(ibm_panel_driver);
+
+MODULE_AUTHOR("Eddie James <eajames@linux.ibm.com>");
+MODULE_DESCRIPTION("IBM Operation Panel Driver");
+MODULE_LICENSE("GPL");