[xfer] Add xfer_window_changed()

xfer_window_changed() can be used to notify peers that an interface is
now ready to accept data.  This can potentially be used to eliminate
the need for wasteful processes that simply poll xfer_window() until
the window becomes non-zero.

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

src/core/xfer.c

 	return len;
 }
 
+/**
+ * Report change of flow control window
+ *
+ * @v intf		Data transfer interface
+ *
+ * Note that this method is used to indicate only unsolicited changes
+ * in the flow control window.  In particular, this method must not be
+ * called as part of the response to xfer_deliver(), since that could
+ * easily lead to an infinite loop.  Callers of xfer_deliver() should
+ * assume that the flow control window will have changed without
+ * generating an xfer_window_changed() message.
+ */
+void xfer_window_changed ( struct interface *intf ) {
+	struct interface *dest;
+	xfer_window_changed_TYPE ( void * ) *op =
+		intf_get_dest_op ( intf, xfer_window_changed, &dest );
+	void *object = intf_object ( dest );
+
+	if ( op ) {
+		op ( object );
+	} else {
+		/* Default is to do nothing */
+	}
+
+	intf_put ( dest );
+}
+
 /**
  * Allocate I/O buffer
  *

src/include/ipxe/xfer.h

 #define xfer_window_TYPE( object_type ) \
 	typeof ( size_t ( object_type ) )
 
+extern void xfer_window_changed ( struct interface *intf );
+#define xfer_window_changed_TYPE( object_type ) \
+	typeof ( void ( object_type ) )
+
 extern struct io_buffer * xfer_alloc_iob ( struct interface *intf,
 					   size_t len );
 #define xfer_alloc_iob_TYPE( object_type ) \