Separate the "is data ready" function of xfer_seek() into an

xfer_window() function, which can return a scalar rather than a
boolean.

src/core/downloader.c

 	.close		= downloader_xfer_close,
 	.vredirect	= xfer_vopen,
 	.seek		= downloader_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= downloader_xfer_deliver_raw,
 };

src/core/hw.c

 	.close		= hw_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,
 };
 	struct hw *hw = container_of ( process, struct hw, process );
 	int rc;
 
-	if ( xfer_ready ( &hw->xfer ) == 0 ) {
+	if ( xfer_window ( &hw->xfer ) ) {
 		rc = xfer_deliver_raw ( &hw->xfer, hw_msg, sizeof ( hw_msg ) );
 		hw_finished ( hw, rc );
 	}

src/core/posix_io.c

 	.close		= posix_file_xfer_close,
 	.vredirect	= xfer_vopen,
 	.seek		= posix_file_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= posix_file_xfer_deliver_iob,
 	.deliver_raw	= xfer_deliver_as_iob,

src/core/resolv.c

 	int have_local;
 };
 
-/**
- * Handle seek() event
- *
- * @v xfer		Data transfer interface
- * @v offset		Offset to new position
- * @v whence		Basis for new position
- * @ret rc		Return status code
- */
-static int resolv_xfer_seek ( struct xfer_interface *xfer __unused,
-			      off_t offset __unused, int whence __unused ) {
-	/* Never ready to accept data */
-	return -EAGAIN;
-}
-
 /** Named socket opener data transfer interface operations */
 static struct xfer_interface_operations named_xfer_ops = {
 	.close		= ignore_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
-	.seek		= resolv_xfer_seek,
+	.seek		= ignore_xfer_seek,
+	.window		= no_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,

src/core/xfer.c

 	return rc;
 }
 
+/**
+ * Check flow control window
+ *
+ * @v xfer		Data transfer interface
+ * @ret len		Length of window
+ */
+size_t xfer_window ( struct xfer_interface *xfer ) {
+	struct xfer_interface *dest = xfer_get_dest ( xfer );
+	size_t len;
+
+	len = dest->op->window ( dest );
+
+	xfer_put ( dest );
+	return len;
+}
+
 /**
  * Test to see if interface is ready to accept data
  *
 	return 0;
 }
 
+/**
+ * Unlimited flow control window
+ *
+ * @v xfer		Data transfer interface
+ * @ret len		Length of window
+ *
+ * This handler indicates that the interface is always ready to accept
+ * data.
+ */
+size_t unlimited_xfer_window ( struct xfer_interface *xfer __unused ) {
+	return ~( ( size_t ) 0 );
+}
+
+/**
+ * No flow control window
+ *
+ * @v xfer		Data transfer interface
+ * @ret len		Length of window
+ *
+ * This handler indicates that the interface is never ready to accept
+ * data.
+ */
+size_t no_xfer_window ( struct xfer_interface *xfer __unused ) {
+	return 0;
+}
+
 /**
  * Allocate I/O buffer
  *
 	.close		= ignore_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,

src/include/gpxe/xfer.h

 	 * @v whence		Basis for new position
 	 * @ret rc		Return status code
 	 *
-	 * @c whence must be one of @c SEEK_SET or @c SEEK_CUR.  A
-	 * successful return indicates that the interface is ready to
-	 * immediately accept datagrams; return -EAGAIN if this is not
-	 * the case.
+	 * @c whence must be one of @c SEEK_SET or @c SEEK_CUR.
 	 */
 	int ( * seek ) ( struct xfer_interface *xfer, off_t offset,
 			 int whence );
+	/** Check flow control window
+	 *
+	 * @v xfer		Data transfer interface
+	 * @ret len		Length of window
+	 *
+	 * Flow control is regarded as advisory but not mandatory.
+	 * Users who have control over their own rate of data
+	 * generation should perform a flow control check before
+	 * generating new data.  Users who have no control (such as
+	 * NIC drivers or filter layers) are not obliged to check.
+	 *
+	 * Data transfer interfaces must be prepared to accept
+	 * datagrams even if they are advertising a window of zero
+	 * bytes.
+	 */
+	size_t ( * window ) ( struct xfer_interface *xfer );
 	/** Allocate I/O buffer
 	 *
 	 * @v xfer		Data transfer interface
 	 * A data transfer interface that wishes to support only raw
 	 * data delivery should set this method to
 	 * xfer_deliver_as_raw().
-	 *
-	 * Interfaces may not temporarily refuse to accept data by
-	 * returning -EAGAIN; such a response may be treated as a
-	 * fatal error.
 	 */
 	int ( * deliver_iob ) ( struct xfer_interface *xfer,
 				struct io_buffer *iobuf,
 	 * A data transfer interface that wishes to support only I/O
 	 * buffer delivery should set this method to
 	 * xfer_deliver_as_iob().
-	 *
-	 * Interfaces may not temporarily refuse to accept data by
-	 * returning -EAGAIN; such a response may be treated as a
-	 * fatal error.
 	 */
 	int ( * deliver_raw ) ( struct xfer_interface *xfer,
 				const void *data, size_t len );
 			    va_list args );
 extern int xfer_redirect ( struct xfer_interface *xfer, int type, ... );
 extern int xfer_seek ( struct xfer_interface *xfer, off_t offset, int whence );
-extern int xfer_ready ( struct xfer_interface *xfer );
+extern size_t xfer_window ( struct xfer_interface *xfer );
 extern struct io_buffer * xfer_alloc_iob ( struct xfer_interface *xfer,
 					   size_t len );
 extern int xfer_deliver_iob ( struct xfer_interface *xfer,
 				   int type, va_list args );
 extern int ignore_xfer_seek ( struct xfer_interface *xfer, off_t offset,
 			      int whence );
+extern size_t unlimited_xfer_window ( struct xfer_interface *xfer );
+extern size_t no_xfer_window ( struct xfer_interface *xfer );
 extern struct io_buffer * default_xfer_alloc_iob ( struct xfer_interface *xfer,
 						   size_t len );
 extern int xfer_deliver_as_raw ( struct xfer_interface *xfer,

src/interface/pxe/pxe_udp.c

 	.close		= ignore_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= pxe_udp_deliver_iob,
 	.deliver_raw	= xfer_deliver_as_iob,

src/net/tcp.c

 }
 
 /**
- * Seek to position
+ * Check flow control window
  *
  * @v xfer		Data transfer interface
- * @v offset		Offset to new position
- * @v whence		Basis for new position
- * @ret rc		Return status code
+ * @ret len		Length of window
  */
-static int tcp_xfer_seek ( struct xfer_interface *xfer, off_t offset,
-			   int whence ) {
+static size_t tcp_xfer_window ( struct xfer_interface *xfer ) {
 	struct tcp_connection *tcp =
 		container_of ( xfer, struct tcp_connection, xfer );
 
-	/* TCP doesn't support seeking to arbitrary positions */
-	if ( ( whence != SEEK_CUR ) || ( offset != 0 ) )
-		return -EINVAL;
-
 	/* Not ready if we're not in a suitable connection state */
 	if ( ! TCP_CAN_SEND_DATA ( tcp->tcp_state ) )
-		return -EAGAIN;
+		return 0;
 
-	/* Not ready if data queue is non-empty */
+	/* Not ready if data queue is non-empty.  This imposes a limit
+	 * of only one unACKed packet in the TX queue at any time; we
+	 * do this to conserve memory usage.
+	 */
 	if ( ! list_empty ( &tcp->queue ) )
-		return -EAGAIN;
+		return 0;
 
-	return 0;
+	/* Return TCP window length */
+	return tcp->snd_win;
 }
 
 /**
 static struct xfer_interface_operations tcp_xfer_operations = {
 	.close		= tcp_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
-	.seek		= tcp_xfer_seek,
+	.seek		= ignore_xfer_seek,
+	.window		= tcp_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= tcp_xfer_deliver_iob,
 	.deliver_raw	= xfer_deliver_as_iob,

src/net/tcp/ftp.c

 	.close		= ftp_control_close,
 	.vredirect	= xfer_vopen,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ftp_control_deliver_raw,
 	.close		= ftp_data_closed,
 	.vredirect	= xfer_vopen,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= ftp_data_deliver_iob,
 	.deliver_raw	= xfer_deliver_as_iob,
 	.close		= ftp_xfer_closed,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,

src/net/tcp/http.c

 	const char *query = http->uri->query;
 	int rc;
 
-	if ( xfer_ready ( &http->socket ) == 0 ) {
+	if ( xfer_window ( &http->socket ) ) {
 		process_del ( &http->process );
 		if ( ( rc = xfer_printf ( &http->socket,
 					  "GET %s%s%s HTTP/1.1\r\n"
 	.close		= http_socket_close,
 	.vredirect	= xfer_vopen,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= http_socket_deliver_iob,
 	.deliver_raw	= xfer_deliver_as_iob,
 	.close		= http_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,

src/net/udp.c

 	.close		= udp_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= udp_alloc_iob,
 	.deliver_iob	= udp_xfer_deliver_iob,
 	.deliver_raw	= xfer_deliver_as_iob,

src/net/udp/dhcp.c

 	.close		= ignore_xfer_close,
 	.vredirect	= xfer_vopen,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= dhcp_deliver_raw,
 };

src/net/udp/dns.c

 	.close		= dns_xfer_close,
 	.vredirect	= xfer_vopen,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= dns_xfer_deliver_raw,

src/net/udp/tftp.c

 	.close		= tftp_socket_close,
 	.vredirect	= xfer_vopen,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= tftp_socket_deliver_iob,
 	.deliver_raw	= xfer_deliver_as_iob,
 	.close		= tftp_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
 	.seek		= ignore_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,