Drop RTI cycle in case DMI delay is zero

[en-sc]

Currently when DMI delay is zero the Run-Test/Idle state is still visited.
Bypassing it and going immediately from Update DR to Capture DR will save a clock per scan.
This is allowed. See [6.1.4. DTM Control and Status (dtmcs, at 0x10)], idle field description:

0: It is not necessary to enter Run-Test/Idle at all.

Note, that this is not possible for the last scan, that should end in a "stable" JTAG state.

[Biancaa-R]

@en-sc I am trying to do this ,

  uint32_t dmcs;
    if (riscv_dmi_read(target, 0x10, &dmcs) != ERROR_OK)
        LOG_ERROR("Failed to read DMCS");

    cmd->cmd.runtest->dmi_delay = (dmcs & 0x1) ? 1 : 0;

I added another parameter as dmi_delay
But how do I get access to target , in

 static void openjtag_execute_runtest(struct jtag_command *cmd)
{

	enum tap_state end_state = cmd->cmd.runtest->end_state;
	tap_set_end_state(end_state);

	/* only do a state_move when we're not already in IDLE */
	if (tap_get_state() != TAP_IDLE) {
		openjtag_set_state(TAP_IDLE);
		tap_set_state(TAP_IDLE);
	}

	if (openjtag_variant != OPENJTAG_VARIANT_CY7C65215 ||
		cmd->cmd.runtest->num_cycles) {
		uint8_t command;
		unsigned int num_cycles = cmd->cmd.runtest->num_cycles;

		do {
			const unsigned int num_cycles_cmd = MIN(num_cycles, 16);

			command = 7;
			command |= ((num_cycles_cmd - 1) & 0x0F) << 4;

			openjtag_add_byte(command);
			num_cycles -= num_cycles_cmd;
		} while (num_cycles > 0);
	}

	tap_set_end_state(end_state);
	if (tap_get_end_state() != tap_get_state()) {
		openjtag_set_state(end_state);
		tap_set_state(end_state);
	}
}

we are not accessing it ,can I pass it also as an arg?

Finally did ,

    // Determine if we can skip TAP_IDLE
    int skip_idle = (cmd->cmd.runtest->dmi_delay == 0 &&
                     !cmd->cmd.runtest->is_last_scan);

    if (!skip_idle && tap_get_state() != TAP_IDLE) {
        openjtag_set_state(TAP_IDLE);
        tap_set_state(TAP_IDLE);
    }

and also can you please suggest me a good way to keep track of if its last_scan :/

[en-sc]

@Biancaa-R, I'd suggest attempting to fix this in batch.c.
Take a look at this call to jtag_add_dr_scan():

riscv-openocd/src/target/riscv/batch.c

Line 300 in eb01c63

jtag_add_dr_scan(batch->target->tap, 1, batch->fields + i, TAP_IDLE);

The last argument specifies the state the TAP should be left in after the scan completes. Currently it is always TAP_IDLE, therefore at least one TCK cycle is spent in Run-Test/Idle.

The suggestion is to move the call to get_delay() up and to check the delay before adding a scan.

If the delay is zero and the scan is not the last one in the batch (can be checked based on the index and batch->used_scans), there is no need to enter Run-Test/Idle and the scan can end in Select DR instead (Exit1 DR -> Update DR -> Run-Test/Idle -> Select DR becomes Exit1 DR -> Update DR -> Select DR).