samples: littlefs: use DT-defined mount information

Extend the nrf52840dk_nrf52840 sample overlay with a fstab entry for a
littlefs file system on the storage partition.

This eliminates the need for application configuration of the file
system parameters and mount data.

Signed-off-by: Peter Bigot <[email protected]>

Author: pabigot

samples/subsys/fs/littlefs/boards/nrf52840dk_nrf52840.overlay

@@ -6,13 +6,30 @@
 
 /delete-node/ &storage_partition;
 
+/ {
+	fstab {
+		compatible = "zephyr,fstab";
+		lfs1: lfs1 {
+			compatible = "zephyr,fstab,littlefs";
+			mount-point = "/lfs1";
+			partition = <&lfs1_part>;
+			automount;
+			read-size = <16>;
+			prog-size = <16>;
+			cache-size = <64>;
+			lookahead-size = <32>;
+			block-cycles = <512>;
+		};
+	};
+};
+
 &mx25r64 {
 	partitions {
 		compatible = "fixed-partitions";
 		#address-cells = <1>;
 		#size-cells = <1>;
 
-		partition@0 {
+		lfs1_part: partition@0 {
 			label = "storage";
 			reg = <0x00000000 0x00010000>;
 		};

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

@@ -17,17 +17,29 @@
 /* Matches LFS_NAME_MAX */
 #define MAX_PATH_LEN 255
 
+#define PARTITION_NODE DT_NODELABEL(lfs1)
+
+#if DT_NODE_EXISTS(PARTITION_NODE)
+FS_FSTAB_DECLARE_ENTRY(PARTITION_NODE);
+#else /* PARTITION_NODE */
 FS_LITTLEFS_DECLARE_DEFAULT_CONFIG(storage);
 static struct fs_mount_t lfs_storage_mnt = {
 	.type = FS_LITTLEFS,
 	.fs_data = &storage,
 	.storage_dev = (void *)FLASH_AREA_ID(storage),
 	.mnt_point = "/lfs",
 };
+#endif /* PARTITION_NODE */
 
 void main(void)
 {
-	struct fs_mount_t *mp = &lfs_storage_mnt;
+	struct fs_mount_t *mp =
+#if DT_NODE_EXISTS(PARTITION_NODE)
+		&FS_FSTAB_ENTRY(PARTITION_NODE)
+#else
+		&lfs_storage_mnt
+#endif
+		;
 	unsigned int id = (uintptr_t)mp->storage_dev;
 	char fname[MAX_PATH_LEN];
 	struct fs_statvfs sbuf;