fs: Add a header file for I/O iteration

Provide a struct iov_iter and some related functions to encapsulate an
iterator used when reading and writing files. This will be used with the
new 'file' uclass.

While it is modelled on Linux, I/O vectors have been omitted for now.
These can be added later without updating the interface used by the
uclass.

Signed-off-by: Simon Glass <sjg@chromium.org>

MAINTAINERS

@@ -1200,6 +1200,7 @@ F:	fs/dir-uclass.c
 F:	include/dir.h
 F:	include/fpga.h
 F:	include/fs.h
+F:	include/iovec.h
 F:	test/dm/fpga.c
 
 FLATTENED DEVICE TREE

include/iovec.h

@@ -0,0 +1,85 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Iterator for I/O, modelled on Linux but significantly simplified
+ *
+ * Copyright 2025 Simon Glass <sjg@chromium.org>
+ */
+
+#ifndef __IOVEC_H
+#define __IOVEC_H
+
+#include <linux/types.h>
+
+/**
+ * enum iov_type_t - defines the type of this I/O vector
+ *
+ * @OIV_BUF: Simple buffer with a size
+ */
+enum iov_type_t {
+	OIV_BUF,
+};
+
+/**
+ * struct iov_iter - holds an interator for I/O
+ *
+ * Do not use this directly. Instead, call copy_to_iter()
+ *
+ * Note that this is much simpler than the Linux version, but can be extended
+ * later as needed. It is introduced so that we can use the desired API for
+ * read(), etc. It is also commented properly.
+ *
+ * @type: Type of the iterator (always OIV_BUF)
+ * @data_source: true if this iterator produces data, false if it consumes it
+ * @offset: Current offset within the buffer
+ * @buf: Contiguous data buffer to use (other things could be added to the
+ *	union later)
+ * @count: Size of data buffer
+ */
+struct iov_iter {
+	enum iov_type_t type;
+	bool data_source;
+	ssize_t offset;
+	union {
+		void *ubuf;
+	};
+	ssize_t count;
+};
+
+/**
+ * iter_iov_ptr() - Get a pointer to the current position
+ *
+ * @iter: Iterator to examine
+ * Return: pointer to the start of the buffer portion to read/write
+ */
+static inline void *iter_iov_ptr(const struct iov_iter *iter)
+{
+	return iter->ubuf + iter->offset;
+};
+
+/**
+ * iter_iov_avail() - Get the number of bytes available at the current postiion
+ *
+ * @iter: iterator to examine
+ * Return: number of bytes which can be read/written
+ */
+static inline ssize_t iter_iov_avail(const struct iov_iter *iter)
+{
+	return iter->count - iter->offset;
+};
+
+static inline void iter_ubuf(struct iov_iter *iter, bool data_source, void *buf,
+			     size_t count)
+{
+	*iter = (struct iov_iter) {
+		.data_source = data_source,
+		.ubuf = buf,
+		.count = count,
+	};
+}
+
+static inline void iter_advance(struct iov_iter *iter, size_t len)
+{
+	iter->offset += len;
+}
+
+#endif