On Mar 10 2017 or thereabouts, Dmitry Torokhov wrote:

These 2 lines above seem to be the culprit of the
non-working-on-the-t450s-while-in-bzImage :)

First, i2c_probe_func_quick_read() returns a boolean, so the value can
never be negative. And second, if you try to probe the device on the I2C
bus while psmouse_activate has been called, the device won't answer. So
we can simply ignore the call all together and let i2c_new_probed_device()
checks for the validity of the device.

Bonus point, if I remove the call to i2c_probe_func_quick_read(), I seem
to have reliable detection of SMBus even when psmouse in compiled in the
bzImage.

+
+		/* Device seems to be there, let's try switching over */
+		psmouse_dbg(smbdev->psmouse,
+			    "SMBus companion appeared, triggering rescan\n");
+		serio_rescan(smbdev->psmouse->ps2dev.serio);
+	}
+
+	mutex_unlock(&psmouse_smbus_mutex);
+}
+
+static void psmouse_smbus_detach_i2c_client(struct i2c_client *client)
+{
+	struct psmouse_smbus_dev *smbdev;
+
+	mutex_lock(&psmouse_smbus_mutex);
+
+	list_for_each_entry(smbdev, &psmouse_smbus_list, node) {
+		if (smbdev->client == client) {
+			psmouse_dbg(smbdev->psmouse,
+				    "Marking SMBus companion %s as gone\n",
+				    dev_name(&smbdev->client->dev));
+			smbdev->client = NULL;
+			smbdev->dead = true;
+			serio_rescan(smbdev->psmouse->ps2dev.serio);
+		}
+	}
+
+	kfree(client->dev.platform_data);
+	client->dev.platform_data = NULL;
+
+	mutex_unlock(&psmouse_smbus_mutex);
+}
+
+static int psmouse_smbus_notifier_call(struct notifier_block *nb,
+				       unsigned long action, void *data)
+{
+	struct device *dev = data;
+
+	switch (action) {
+	case BUS_NOTIFY_ADD_DEVICE:
+		if (dev->type == &i2c_adapter_type)
+			psmouse_smbus_check_adapter(to_i2c_adapter(dev));
+		break;
+
+	case BUS_NOTIFY_REMOVED_DEVICE:
+		if (dev->type == &i2c_client_type)
+			psmouse_smbus_detach_i2c_client(to_i2c_client(dev));
+		break;
+	}
+
+	return 0;
+}
+
+static struct notifier_block psmouse_smbus_notifier = {
+	.notifier_call = psmouse_smbus_notifier_call,
+};
+
+static psmouse_ret_t psmouse_smbus_process_byte(struct psmouse *psmouse)
+{
+	return PSMOUSE_FULL_PACKET;
+}
+
+static int psmouse_smbus_reconnect(struct psmouse *psmouse)
+{
+	psmouse_deactivate(psmouse);
+
+	return 0;
+}
+
+struct psmouse_smbus_removal_work {
+	struct work_struct work;
+	struct i2c_client *client;
+};
+
+static void psmouse_smbus_remove_i2c_device(struct work_struct *work)
+{
+	struct psmouse_smbus_removal_work *rwork =
+		container_of(work, struct psmouse_smbus_removal_work, work);
+
+	dev_dbg(&rwork->client->dev, "destroying SMBus companion device\n");
+	i2c_unregister_device(rwork->client);
+
+	kfree(rwork);
+}
+
+/*
+ * This schedules removal of SMBus companion device. We have to do
+ * it in a separate tread to avoid deadlocking on psmouse_mutex in
+ * case the device has a trackstick (which is also driven by psmouse).
+ *
+ * Note that this may be racing with i2c adapter removal, but we
+ * can't do anything about that: i2c automatically destroys clients
+ * attached to an adapter that is being removed. This has to be
+ * fixed in i2c core.
+ */
+static void psmouse_smbus_schedule_remove(struct i2c_client *client)
+{
+	struct psmouse_smbus_removal_work *rwork;
+
+	rwork = kzalloc(sizeof(*rwork), GFP_KERNEL);
+	if (rwork) {
+		INIT_WORK(&rwork->work, psmouse_smbus_remove_i2c_device);
+		rwork->client = client;
+
+		schedule_work(&rwork->work);
+	}
+}
+
+static void psmouse_smbus_disconnect(struct psmouse *psmouse)
+{
+	struct psmouse_smbus_dev *smbdev = psmouse->private;
+
+	mutex_lock(&psmouse_smbus_mutex);
+	list_del(&smbdev->node);
+	mutex_unlock(&psmouse_smbus_mutex);
+
+	if (smbdev->client) {
+		psmouse_dbg(smbdev->psmouse,
+			    "posting removal request for SMBus companion %s\n",
+			    dev_name(&smbdev->client->dev));
+		psmouse_smbus_schedule_remove(smbdev->client);
+	}
+
+	kfree(smbdev);
+	psmouse->private = NULL;
+}
+
+static int psmouse_smbus_create_companion(struct device *dev, void *data)
+{
+	struct psmouse_smbus_dev *smbdev = data;
+	unsigned short addr_list[] = { smbdev->board.addr, I2C_CLIENT_END };
+	struct i2c_adapter *adapter;
+
+	adapter = i2c_verify_adapter(dev);
+	if (!adapter)
+		return 0;
+
+	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_HOST_NOTIFY))
+		return 0;
+
+	smbdev->client = i2c_new_probed_device(adapter, &smbdev->board,
+					       addr_list, NULL);
+	if (!smbdev->client)
+		return 0;
+
+	/* We have our(?) device, stop iterating i2c bus. */
+	return 1;
+}
+
+void psmouse_smbus_cleanup(struct psmouse *psmouse)
+{
+	struct psmouse_smbus_dev *smbdev, *tmp;
+
+	mutex_lock(&psmouse_smbus_mutex);
+
+	list_for_each_entry_safe(smbdev, tmp, &psmouse_smbus_list, node) {
+		if (psmouse == smbdev->psmouse) {
+			list_del(&smbdev->node);
+			kfree(smbdev);
+		}
+	}
+
+	mutex_unlock(&psmouse_smbus_mutex);
+}
+
+int psmouse_smbus_init(struct psmouse *psmouse,
+		       const struct i2c_board_info *board,
+		       const void *pdata, size_t pdata_size,
+		       bool leave_breadcrumbs)
+{
+	struct psmouse_smbus_dev *smbdev;
+	int error;
+
+	smbdev = kzalloc(sizeof(*smbdev), GFP_KERNEL);
+	if (!smbdev)
+		return -ENOMEM;
+
+	smbdev->psmouse = psmouse;
+	smbdev->board = *board;
+
+	smbdev->board.platform_data = kmemdup(pdata, pdata_size, GFP_KERNEL);
+	if (!smbdev->board.platform_data) {
+		kfree(smbdev);
+		return -ENOMEM;
+	}
+
+	psmouse->private = smbdev;
+	psmouse->protocol_handler = psmouse_smbus_process_byte;
+	psmouse->reconnect = psmouse_smbus_reconnect;
+	psmouse->fast_reconnect = psmouse_smbus_reconnect;
+	psmouse->disconnect = psmouse_smbus_disconnect;
+	psmouse->resync_time = 0;
+
+	psmouse_deactivate(psmouse);
+
+	mutex_lock(&psmouse_smbus_mutex);
+	list_add_tail(&smbdev->node, &psmouse_smbus_list);
+	mutex_unlock(&psmouse_smbus_mutex);
+
+	/* Bind to already existing adapters right away */
+	error = i2c_for_each_dev(smbdev, psmouse_smbus_create_companion);
+
+	if (smbdev->client) {
+		/* We have our companion device */
+		return 0;
+	}
+
+	/*
+	 * If we did not create i2c device we will not need platform
+	 * data even if we are leaving breadcrumbs.
+	 */
+	kfree(smbdev->board.platform_data);
+	smbdev->board.platform_data = NULL;
+
+	if (error < 0 || !leave_breadcrumbs) {
+		mutex_lock(&psmouse_smbus_mutex);
+		list_del(&smbdev->node);
+		mutex_unlock(&psmouse_smbus_mutex);
+
+		kfree(smbdev);
+	}
+
+	return error < 0 ? error : -EAGAIN;
+}
+
+int __init psmouse_smbus_module_init(void)
+{
+	int error;
+
+	error = bus_register_notifier(&i2c_bus_type, &psmouse_smbus_notifier);
+	if (error) {
+		pr_err("failed to register i2c bus notifier: %d\n", error);
+		return error;
+	}
+
+	return 0;
+}
+
+void __exit psmouse_smbus_module_exit(void)

This raises a new section mismatch warning:

    WARNING: drivers/input/mouse/psmouse.o(.init.text+0x7c): Section mismatch in reference from the function psmouse_init() to the function .exit.text:psmouse_s
    The function __init psmouse_init() references
    a function __exit psmouse_smbus_module_exit().
    This is often seen when error handling in the init function
    uses functionality in the exit path.
    The fix is often to remove the __exit annotation of
    psmouse_smbus_module_exit() so it may be used outside an exit section.

As mentioned, dropping the __exit tag solves the issue.

With these 2 changes, the series is:
Reviewed-by: Benjamin Tissoires <redacted>

Thanks again for the respin!

Cheers,
Benjamin