This DPI module provides a “virtual” JTAG connection between an simulated chip and OpenOCD. It makes use of the remote_bitbang JTAG driver shipped with OpenOCD, which forwards JTAG requests over TCP to a remote server. The jtagdpi module is instantiated in the hardware simulation to receive the JTAG requests from OpenOCD and drive the JTAG pins (TCK, TMS, TDI, etc.) from it.
The remote_bitbang protocol is documented in the OpenOCD source tree at doc/manual/jtag/drivers/remote_bitbang.txt, or online at https://repo.or.cz/openocd.git/blob/HEAD:/doc/manual/jtag/drivers/remote_bitbang.txt