emul: add stub driver for emulators

Not all emulators correspond to a real driver.  Notably some tests don't
require a real driver.  Provide a macro that initializes a stub driver.

Signed-off-by: Keith Short <[email protected]>

Author: keith-zephyr

include/zephyr/drivers/emul_stub_device.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2023 Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_INCLUDE_EMUL_STUB_DEVICE_H_
+#define ZEPHYR_INCLUDE_EMUL_STUB_DEVICE_H_
+
+#include <zephyr/device.h>
+#include <zephyr/devicetree.h>
+
+/*
+ * Needed for emulators without corresponding DEVICE_DT_DEFINE drivers
+ */
+
+struct emul_stub_dev_data {
+	/* Stub */
+};
+struct emul_stub_dev_config {
+	/* Stub */
+};
+struct emul_stub_dev_api {
+	/* Stub */
+};
+
+/* For every instance of a DT_DRV_COMPAT stub out a device for that instance */
+#define EMUL_STUB_DEVICE(n)                                                                        \
+	__maybe_unused static int emul_init_stub_##n(const struct device *dev)                     \
+	{                                                                                          \
+		ARG_UNUSED(dev);                                                                   \
+		return 0;                                                                          \
+	}                                                                                          \
+                                                                                                   \
+	static struct emul_stub_dev_data stub_data_##n;                                            \
+	static struct emul_stub_dev_config stub_config_##n;                                        \
+	static struct emul_stub_dev_api stub_api_##n;                                              \
+	DEVICE_DT_INST_DEFINE(n, &emul_init_stub_##n, NULL, &stub_data_##n, &stub_config_##n,      \
+			      POST_KERNEL, 1, &stub_api_##n);
+
+#endif /* ZEPHYR_INCLUDE_EMUL_STUB_DEVICE_H_ */