Fix missing ND response to zero-length ND request

[colluca]

Issue

The idma_nd_midend module fails to generate a response to zero-length ND requests.

How it arises

nd_rsp_valid_o is never asserted due to the last signal in the burst response from the backend not being asserted:

iDMA/src/midend/idma_nd_midend.sv

Line 208 in 5ebff11

nd_rsp_valid_o = burst_rsp_valid_i & burst_rsp_i.last;

This is in turn due to how the backend handles zero-length transfers, setting most of the response signals to zero:

iDMA/src/backend/tpl/idma_backend.sv.tpl

Lines 394 to 403 in 5ebff11

	// a zero transfer happens
	if (is_length_zero & req_valid_i & req_ready_o) begin
	// block backend
	rsp_ready = 1'b0;
	// generate new response
	rsp_valid_o = 1'b1;
	idma_rsp_o = '0;
	idma_rsp_o.error = 1'b1;
	idma_rsp_o.pld.err_type = idma_pkg::BACKEND;
	end

What it implies

As a consequence of the missing response from the idma_nd_midend, the frontend does not properly keep track of the completed_id, as it never asserts retire_id for zero-length transfers:

iDMA/src/frontend/inst64/idma_inst64_top.sv

Lines 255 to 272 in 5ebff11

	//--------------------------------------
	// ID gen
	//--------------------------------------
	idma_transfer_id_gen #(
	.IdWidth ( TfIdWidth )
	) i_idma_transfer_id_gen (
	.clk_i,
	.rst_ni,
	.issue_i ( issue_id ),
	.retire_i ( retire_id ),
	.next_o ( next_id ),
	.completed_o ( completed_id )
	);
	// we are always ready to accept responses
	assign idma_nd_rsp_ready = 1'b1;
	assign issue_id = idma_nd_req_valid & idma_nd_req_ready;
	assign retire_id = idma_nd_rsp_valid & idma_nd_rsp_ready;

Fix

This PR proposes to correct this condition in the backend, by always asserting the last signal in response to zero-length transfers.

[thommythomaso]

Thanks for the bugfix. 👍