gpio: introduce utilities for synchronous fake device creation

Both gpio-sim and gpio-virtuser share a mechanism to instantiate a
platform device, wait for probe completion, and retrieve the probe
success or error status synchronously. With gpio-aggregator planned to
adopt this approach for its configfs interface, it's time to factor
out the common code.

Add dev-sync-probe.[ch] to house helper functions used by all such
implementations.

No functional change.

Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Link: https://lore.kernel.org/r/20250221133501.2203897-2-koichiro.den@canonical.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/Kconfig

@@ -1948,3 +1948,6 @@ config GPIO_VIRTUSER
 endmenu
 
 endif
+
+config DEV_SYNC_PROBE
+	tristate

drivers/gpio/Makefile

@@ -19,6 +19,9 @@ obj-$(CONFIG_GPIO_GENERIC)	+= gpio-generic.o
 # directly supported by gpio-generic
 gpio-generic-$(CONFIG_GPIO_GENERIC)	+= gpio-mmio.o
 
+# Utilities for drivers that need synchronous fake device creation
+obj-$(CONFIG_DEV_SYNC_PROBE)		+= dev-sync-probe.o
+
 obj-$(CONFIG_GPIO_104_DIO_48E)		+= gpio-104-dio-48e.o
 obj-$(CONFIG_GPIO_104_IDI_48)		+= gpio-104-idi-48.o
 obj-$(CONFIG_GPIO_104_IDIO_16)		+= gpio-104-idio-16.o

drivers/gpio/dev-sync-probe.c

@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Common code for drivers creating fake platform devices.
+ *
+ * Provides synchronous device creation: waits for probe completion and
+ * returns the probe success or error status to the device creator.
+ *
+ * Copyright (C) 2021 Bartosz Golaszewski <brgl@bgdev.pl>
+ * Copyright (C) 2025 Koichiro Den <koichiro.den@canonical.com>
+ */
+
+#include <linux/device.h>
+#include <linux/slab.h>
+
+#include "dev-sync-probe.h"
+
+static int dev_sync_probe_notifier_call(struct notifier_block *nb,
+					unsigned long action, void *data)
+{
+	struct dev_sync_probe_data *pdata;
+	struct device *dev = data;
+
+	pdata = container_of(nb, struct dev_sync_probe_data, bus_notifier);
+	if (!device_match_name(dev, pdata->name))
+		return NOTIFY_DONE;
+
+	switch (action) {
+	case BUS_NOTIFY_BOUND_DRIVER:
+		pdata->driver_bound = true;
+		break;
+	case BUS_NOTIFY_DRIVER_NOT_BOUND:
+		pdata->driver_bound = false;
+		break;
+	default:
+		return NOTIFY_DONE;
+	}
+
+	complete(&pdata->probe_completion);
+	return NOTIFY_OK;
+}
+
+void dev_sync_probe_init(struct dev_sync_probe_data *data)
+{
+	memset(data, 0, sizeof(*data));
+	init_completion(&data->probe_completion);
+	data->bus_notifier.notifier_call = dev_sync_probe_notifier_call;
+}
+EXPORT_SYMBOL_GPL(dev_sync_probe_init);
+
+int dev_sync_probe_register(struct dev_sync_probe_data *data,
+			    struct platform_device_info *pdevinfo)
+{
+	struct platform_device *pdev;
+	char *name;
+
+	name = kasprintf(GFP_KERNEL, "%s.%d", pdevinfo->name, pdevinfo->id);
+	if (!name)
+		return -ENOMEM;
+
+	data->driver_bound = false;
+	data->name = name;
+	reinit_completion(&data->probe_completion);
+	bus_register_notifier(&platform_bus_type, &data->bus_notifier);
+
+	pdev = platform_device_register_full(pdevinfo);
+	if (IS_ERR(pdev)) {
+		bus_unregister_notifier(&platform_bus_type, &data->bus_notifier);
+		kfree(data->name);
+		return PTR_ERR(pdev);
+	}
+
+	wait_for_completion(&data->probe_completion);
+	bus_unregister_notifier(&platform_bus_type, &data->bus_notifier);
+
+	if (!data->driver_bound) {
+		platform_device_unregister(pdev);
+		kfree(data->name);
+		return -ENXIO;
+	}
+
+	data->pdev = pdev;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(dev_sync_probe_register);
+
+void dev_sync_probe_unregister(struct dev_sync_probe_data *data)
+{
+	platform_device_unregister(data->pdev);
+	kfree(data->name);
+	data->pdev = NULL;
+}
+EXPORT_SYMBOL_GPL(dev_sync_probe_unregister);
+
+MODULE_AUTHOR("Bartosz Golaszewski <brgl@bgdev.pl>");
+MODULE_AUTHOR("Koichiro Den <koichiro.den@canonical.com>");
+MODULE_DESCRIPTION("Utilities for synchronous fake device creation");
+MODULE_LICENSE("GPL");

drivers/gpio/dev-sync-probe.h

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef DEV_SYNC_PROBE_H
+#define DEV_SYNC_PROBE_H
+
+#include <linux/completion.h>
+#include <linux/notifier.h>
+#include <linux/platform_device.h>
+
+struct dev_sync_probe_data {
+	struct platform_device *pdev;
+	const char *name;
+
+	/* Synchronize with probe */
+	struct notifier_block bus_notifier;
+	struct completion probe_completion;
+	bool driver_bound;
+};
+
+void dev_sync_probe_init(struct dev_sync_probe_data *data);
+int dev_sync_probe_register(struct dev_sync_probe_data *data,
+			    struct platform_device_info *pdevinfo);
+void dev_sync_probe_unregister(struct dev_sync_probe_data *data);
+
+#endif /* DEV_SYNC_PROBE_H */