[int13] Provide a permanently closed window via the control interface

Allow objects to support both streaming and block device protocols, by
starting streaming data only when the data transfer window opens.

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

src/arch/i386/interface/pcbios/int13.c

@@ -1333,6 +1333,20 @@ static void int13_unhook_vector ( void ) {
 				&int13_vector );
 }
 
+/**
+ * Check INT13 emulated drive flow control window
+ *
+ * @v int13		Emulated drive
+ */
+static size_t int13_block_window ( struct int13_drive *int13 __unused ) {
+
+	/* We are never ready to receive data via this interface.
+	 * This prevents objects that support both block and stream
+	 * interfaces from attempting to send us stream data.
+	 */
+	return 0;
+}
+
 /**
  * Handle INT 13 emulated drive underlying block device closing
  *
@@ -1357,6 +1371,7 @@ static void int13_block_close ( struct int13_drive *int13, int rc ) {
 
 /** INT 13 drive interface operations */
 static struct interface_operation int13_block_op[] = {
+	INTF_OP ( xfer_window, struct int13_drive *, int13_block_window ),
 	INTF_OP ( intf_close, struct int13_drive *, int13_block_close ),
 };