Intel Extensible Firmware Interface Specification page 239

Only the REQUEST_TO_SEND, DATA_TERMINAL_READY, HARDWARE_LOOPBACK_ENABLE,
SOFTWARE_LOOPBACK_ENABLE, and
with SetControl(). All the bits can be read with GetControl().
Status Codes Returned
EFI_SUCCESS
EFI_UNSUPPORTED
EFI_DEVICE_ERROR
Version 1.02
HARDWARE_FLOW_CONTROL_ENABLE
The new control bits were set on the serial device.
The serial device does not support this operation.
The serial device is not functioning correctly.
12/12/00
Serial I/O Protocol
bits can be set
221