fs: add fs_readdir()

Needed to support efi file protocol.  The fallback.efi loader wants
to be able to read the contents of the /EFI directory to find an OS
to boot.

Modelled after POSIX opendir()/readdir()/closedir().  Unlike the other
fs APIs, this is stateful (ie. state is held in the FS_DIR "directory
stream"), to avoid re-traversing of the directory structure at each
step.  The directory stream must be released with closedir() when it
is no longer needed.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Łukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/fs.h

@@ -26,6 +26,17 @@
  */
 int fs_set_blk_dev(const char *ifname, const char *dev_part_str, int fstype);
 
+/*
+ * fs_set_blk_dev_with_part - Set current block device + partition
+ *
+ * Similar to fs_set_blk_dev(), but useful for cases where you already
+ * know the blk_desc and part number.
+ *
+ * Returns 0 on success.
+ * Returns non-zero if invalid partition or error accessing the disk.
+ */
+int fs_set_blk_dev_with_part(struct blk_desc *desc, int part);
+
 /*
  * Print the list of files on the partition previously set by fs_set_blk_dev(),
  * in directory "dirname".
@@ -78,6 +89,62 @@ int fs_read(const char *filename, ulong addr, loff_t offset, loff_t len,
 int fs_write(const char *filename, ulong addr, loff_t offset, loff_t len,
 	     loff_t *actwrite);
 
+/*
+ * Directory entry types, matches the subset of DT_x in posix readdir()
+ * which apply to u-boot.
+ */
+#define FS_DT_DIR  4         /* directory */
+#define FS_DT_REG  8         /* regular file */
+#define FS_DT_LNK  10        /* symbolic link */
+
+/*
+ * A directory entry, returned by fs_readdir().  Returns information
+ * about the file/directory at the current directory entry position.
+ */
+struct fs_dirent {
+	unsigned type;       /* one of FS_DT_x (not a mask) */
+	loff_t size;         /* size in bytes */
+	char name[256];
+};
+
+/* Note: fs_dir_stream should be treated as opaque to the user of fs layer */
+struct fs_dir_stream {
+	/* private to fs. layer: */
+	struct blk_desc *desc;
+	int part;
+};
+
+/*
+ * fs_opendir - Open a directory
+ *
+ * @filename: the path to directory to open
+ * @return a pointer to the directory stream or NULL on error and errno
+ *    set appropriately
+ */
+struct fs_dir_stream *fs_opendir(const char *filename);
+
+/*
+ * fs_readdir - Read the next directory entry in the directory stream.
+ *
+ * Works in an analogous way to posix readdir().  The previously returned
+ * directory entry is no longer valid after calling fs_readdir() again.
+ * After fs_closedir() is called, the returned directory entry is no
+ * longer valid.
+ *
+ * @dirs: the directory stream
+ * @return the next directory entry (only valid until next fs_readdir() or
+ *    fs_closedir() call, do not attempt to free()) or NULL if the end of
+ *    the directory is reached.
+ */
+struct fs_dirent *fs_readdir(struct fs_dir_stream *dirs);
+
+/*
+ * fs_closedir - close a directory stream
+ *
+ * @dirs: the directory stream
+ */
+void fs_closedir(struct fs_dir_stream *dirs);
+
 /*
  * Common implementation for various filesystem commands, optionally limited
  * to a specific filesystem type via the fstype parameter.