[iobuf] Add iob_split() to split an I/O buffer into portions

RNDIS devices may provide multiple packets encapsulated into a single
message.  Provide an API to allow the RNDIS driver to split an I/O
buffer into smaller portions.

The current implementation will always copy the underlying data,
rather than splitting the buffer in situ.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/iobuf.c

@@ -200,3 +200,33 @@ struct io_buffer * iob_concatenate ( struct list_head *list ) {
 
 	return concatenated;
 }
+
+/**
+ * Split I/O buffer
+ *
+ * @v iobuf		I/O buffer
+ * @v len		Length to split into a new I/O buffer
+ * @ret split		New I/O buffer, or NULL on allocation failure
+ *
+ * Split the first @c len bytes of the existing I/O buffer into a
+ * separate I/O buffer.  The resulting buffers are likely to have no
+ * headroom or tailroom.
+ *
+ * If this call fails, then the original buffer will be unmodified.
+ */
+struct io_buffer * iob_split ( struct io_buffer *iobuf, size_t len ) {
+	struct io_buffer *split;
+
+	/* Sanity checks */
+	assert ( len <= iob_len ( iobuf ) );
+
+	/* Allocate new I/O buffer */
+	split = alloc_iob ( len );
+	if ( ! split )
+		return NULL;
+
+	/* Copy in data */
+	memcpy ( iob_put ( split, len ), iobuf->data, len );
+	iob_pull ( iobuf, len );
+	return split;
+}

src/include/ipxe/iobuf.h

@@ -217,5 +217,6 @@ extern void free_iob ( struct io_buffer *iobuf );
 extern void iob_pad ( struct io_buffer *iobuf, size_t min_len );
 extern int iob_ensure_headroom ( struct io_buffer *iobuf, size_t len );
 extern struct io_buffer * iob_concatenate ( struct list_head *list );
+extern struct io_buffer * iob_split ( struct io_buffer *iobuf, size_t len );
 
 #endif /* _IPXE_IOBUF_H */