drivers: flash_rpc: Lower driver init priority

Some nRF53 initializations are now done after POST_KERNEL init level, so
initialize the serialization at the APPLICATION level.

As DT doesn't allow to initialize devices at the APPLICATION level, make
use of the zephyr,deferred-init property and initialize the device
manually from SYS_INIT().

For whatever reason the property was not recognized if
nordic,rpc-flash-controller binding was no present, so add it.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

drivers/flash/flash_rpc/flash_rpc_common.c

@@ -31,4 +31,4 @@ static int serialization_init(void)
 	return 0;
 }
 
-SYS_INIT(serialization_init, POST_KERNEL, CONFIG_FLASH_RPC_SYS_INIT_PRIORITY);
+SYS_INIT(serialization_init, APPLICATION, CONFIG_FLASH_RPC_SYS_INIT_PRIORITY);

drivers/flash/flash_rpc/flash_rpc_controller.c

@@ -231,3 +231,18 @@ static const struct flash_driver_api flash_driver_rpc_api = {
 
 DEVICE_DT_INST_DEFINE(0, flash_rpc_init, NULL, NULL, NULL, POST_KERNEL,
 		      CONFIG_FLASH_RPC_DRIVER_INIT_PRIORITY, &flash_driver_rpc_api);
+
+static int controller_init(void)
+{
+	int err;
+
+	err = device_init(DEVICE_DT_GET(DT_NODELABEL(rpc_flash_controller)));
+	if (err != 0) {
+		LOG_ERR("Flash RPC controller initialization failed, err %d", err);
+		return err;
+	}
+
+	return 0;
+}
+
+SYS_INIT(controller_init, APPLICATION, CONFIG_FLASH_RPC_DRIVER_INIT_PRIORITY);

dts/bindings/flash/nordic,rpc-flash-controller.yaml

@@ -0,0 +1,8 @@
+# Copyright (c) 2025 Nordic Semiconductor
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+
+description: Flash RPC controller
+
+compatible: "nordic,rpc-flash-controller"
+
+include: flash-controller.yaml

tests/drivers/flash/flash_rpc/app.overlay

@@ -5,8 +5,8 @@
 			reg = <0x00000000 DT_SIZE_K(256)>;
 			#address-cells = <1>;
 			#size-cells = <1>;
-			erase-value = <0xff>;
 			status = "okay";
+			zephyr,deferred-init;
 			flash_rpc: flash_rpc@0 {
 				status = "okay";
 				compatible = "soc-nv-flash";