test: fs: enable fstab devicetree integration for fatfs

Added tests in order to verify the functionality of the
newly added fatfs fstab integration.

Signed-off-by: Carlo Kirchmeier <[email protected]>

samples/subsys/fs/fatfs_fstab/CMakeLists.txt

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+# Copyright (c) 2025 Endress+Hauser AG
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(fatfs_fstab_sample)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

samples/subsys/fs/fatfs_fstab/README.rst

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+.. zephyr:code-sample:: fatfs-fstab
+   :name: Fatfs filesystem fstab
+   :relevant-api: file_system_api
+
+   Define fatfs filesystems in the devicetree.
+
+Overview
+***********
+
+This sample shows how to define a fatfs fstab entry in the devicetree.
+This scenario uses a fatfs on a RAM disk. The sample is run on the
+:zephyr:board:`native_sim` board.
+
+Building and running
+********************
+
+To run this sample, build it for the :zephyr:board:`native_sim` board
+and afterwards run the generated executable file within the build folder.
+
+The RAM disk sample for the :zephyr:board:`native_sim` board can be built as follows:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/fs/fatfs-fstab
+   :host-os: unix
+   :board: native_sim
+   :gen-args: -DDTC_OVERLAY_FILE=fatfs_fstab.overlay
+   :goals: build
+   :compact:
+
+Sample Output
+=============
+
+When the sample runs successfully you should see following message on the screen:
+.. code-block:: console
+  I: Filesystem access successful

samples/subsys/fs/fatfs_fstab/fatfs_fstab.overlay

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+/*
+ * Copyright (c) 2025 Endress+Hauser AG
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	fstab {
+		compatible = "zephyr,fstab";
+		ffs1: ffs1 {
+			compatible = "zephyr,fstab,fatfs";
+			automount;
+			disk-access;
+			mount-point = "/RAM";
+		};
+	};
+
+	ramdisk0 {
+		compatible = "zephyr,ram-disk";
+		disk-name = "RAM";
+		sector-size = <512>;
+		sector-count = <128>;
+	};
+};

samples/subsys/fs/fatfs_fstab/prj.conf

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+CONFIG_DISK_ACCESS=y
+CONFIG_DISK_DRIVERS=y
+
+CONFIG_LOG=y
+CONFIG_LOG_MODE_MINIMAL=y
+
+CONFIG_FILE_SYSTEM=y
+CONFIG_FILE_SYSTEM_MKFS=y
+CONFIG_FAT_FILESYSTEM_ELM=y

samples/subsys/fs/fatfs_fstab/sample.yaml

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+sample:
+  name: fatfs fstab sample
+tests:
+  sample.filesystem.fat_fs.fstab:
+    platform_allow:
+      - native_sim
+    extra_args:
+      - EXTRA_DTC_OVERLAY_FILE="fatfs_fstab.overlay"
+    tags: filesystem

samples/subsys/fs/fatfs_fstab/src/main.c

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+/*
+ * Copyright (c) 2025 Endress+Hauser AG
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <ff.h>
+#include <zephyr/device.h>
+#include <zephyr/fs/fs.h>
+#include <zephyr/kernel.h>
+#include <zephyr/logging/log.h>
+#include <zephyr/storage/disk_access.h>
+
+LOG_MODULE_REGISTER(main, LOG_LEVEL_INF);
+
+int main(void)
+{
+	struct fs_file_t file;
+	int rc;
+	static const char data[] = "Hello";
+
+	fs_file_t_init(&file);
+	rc = fs_open(&file, "/RAM:/hello.txt", FS_O_CREATE | FS_O_WRITE);
+	if (rc != 0) {
+		LOG_ERR("Accessing filesystem failed");
+		return rc;
+	}
+	rc = fs_write(&file, data, strlen(data));
+	if (rc != strlen(data)) {
+		LOG_ERR("Writing filesystem failed");
+		return rc;
+	}
+	rc = fs_close(&file);
+	if (rc != 0) {
+		LOG_ERR("Closing file failed");
+		return rc;
+	}
+
+	LOG_INF("Filesystem access successful");
+	return 0;
+}