axi_dmac: Add support for zero latency transfer switching

Right now there is always a period of one clock cycle where we can not transfer
any data when switching between two transfers. This patch modifies the data
mover to allow for zero latency. This fixes problems on the FMCOMMS1 platform

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

library/axi_dmac/data_mover.v

@@ -58,7 +58,7 @@ module dmac_data_mover (
 	output m_axi_last,
 
 	input req_valid,
-	output reg req_ready,
+	output req_ready,
 	input [3:0] req_last_burst_length
 );
 
@@ -72,20 +72,25 @@ reg [3:0] last_burst_length;
 reg [C_ID_WIDTH-1:0] id = 'h00;
 reg [C_ID_WIDTH-1:0] id_next;
 reg [3:0] beat_counter = 'h00;
-wire [3:0] beat_counter_next;
 wire last;
+wire last_load;
 reg pending_burst;
+reg active;
 
 assign response_id = id;
 
-assign beat_counter_next = s_axi_ready && s_axi_valid ? beat_counter + 1'b1 : beat_counter;
 assign last = beat_counter == (eot ? last_burst_length : 4'hf);
 
-assign s_axi_ready = m_axi_ready & pending_burst & ~req_ready;
-assign m_axi_valid = s_axi_valid & pending_burst & ~req_ready;
+assign s_axi_ready = m_axi_ready & pending_burst & active;
+assign m_axi_valid = s_axi_valid & pending_burst & active;
 assign m_axi_data = s_axi_data;
 assign m_axi_last = last;
 
+// If we want to support zero delay between transfers we have to assert
+// req_ready on the same cycle on which the last load happens.
+assign last_load = s_axi_ready && s_axi_valid && last && eot;
+assign req_ready = last_load || ~active;
+
 always @(posedge clk) begin
 	if (resetn == 1'b0) begin
 		enabled <= 1'b0;
@@ -111,20 +116,31 @@ end
 always @(posedge clk) begin
 	if (resetn == 1'b0) begin
 		beat_counter <= 'h0;
-		req_ready <= 1'b1;
+	end else begin
+		if (req_ready && req_valid) begin
+			beat_counter <= 'h0;
+		end else if (s_axi_ready && s_axi_valid) begin
+			beat_counter <= beat_counter + 1'b1;
+		end
+	end
+end
+
+always @(posedge clk) begin
+	if (req_ready && req_valid) begin
+		last_burst_length <= req_last_burst_length;
+	end
+end
+
+always @(posedge clk) begin
+	if (resetn == 1'b0) begin
+		active <= 1'b0;
 	end else begin
 		if (~enabled) begin
-			req_ready <= 1'b1;
-		end else if (req_ready) begin
-			if (req_valid && enabled) begin
-				last_burst_length <= req_last_burst_length;
-				req_ready <= 1'b0;
-				beat_counter <= 'h0;
-			end
-		end else if (s_axi_ready && s_axi_valid) begin
-			if (last && eot)
-				req_ready <= 1'b1;
-			beat_counter <= beat_counter + 1'b1;
+			active <= 1'b0;
+		end else if (req_ready && req_valid) begin
+			active <= 1'b1;
+		end else if (last_load) begin
+			active <= 1'b0;
 		end
 	end
 end
@@ -141,6 +157,7 @@ end
 always @(posedge clk) begin
 	if (resetn == 1'b0) begin
 		id <= 'h0;
+		pending_burst <= 1'b0;
 	end else begin
 		id <= id_next;
 		pending_burst <= id_next != request_id;