simple_ram: Add pipelining support

The generic PIPELINE_DEPTH can be set to 0 to keep it operating
as a non-pipelined slave, or a larger value indicating
the amount of extra cycles between requests and acks.

It will always generate a valid stall signal, so it can be used
in either mode with a pipelined master (but only in non-pipelined
mode with a non-pipelined master).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

simple_ram_behavioural.vhdl

@@ -10,7 +10,8 @@ use work.simple_ram_behavioural_helpers.all;
 entity mw_soc_memory is
     generic (
         RAM_INIT_FILE  : string;
-        MEMORY_SIZE   : integer
+        MEMORY_SIZE    : integer;
+	PIPELINE_DEPTH : integer := 0
         );
 
     port (
@@ -29,48 +30,101 @@ architecture behave of mw_soc_memory is
     signal ret_ack    : std_ulogic := '0';
     signal identifier : integer := behavioural_initialize(filename => RAM_INIT_FILE, size => MEMORY_SIZE);
     signal reload     : integer := 0;
+    signal ret_dat    : wishbone_data_type;
+
+    subtype pipe_idx_t is integer range 0 to PIPELINE_DEPTH-1;
+    type pipe_ack_t is array(pipe_idx_t) of std_ulogic;
+    type pipe_dat_t is array(pipe_idx_t) of wishbone_data_type;
 begin
-    wishbone_process: process(clk)
-        variable ret_dat: std_ulogic_vector(63 downto 0) := (others => '0');
-	variable adr: std_ulogic_vector(63 downto 0);
+
+    pipe_big: if PIPELINE_DEPTH > 1 generate
+	signal pipe_ack : pipe_ack_t;
+	signal pipe_dat : pipe_dat_t;
+    begin
+	wishbone_out.stall <= '0';
+	wishbone_out.ack <= pipe_ack(0);
+	wishbone_out.dat <= pipe_dat(0);
+
+	pipe_big_sync: process(clk)
+	begin	
+	    if rising_edge(clk) then
+		pipe_stages: for i in 0 to PIPELINE_DEPTH-2 loop
+		    pipe_ack(i) <= pipe_ack(i+1);
+		    pipe_dat(i) <= pipe_dat(i+1);
+		end loop;
+		pipe_ack(PIPELINE_DEPTH-1) <= ret_ack;
+		pipe_dat(PIPELINE_DEPTH-1) <= ret_dat;
+	    end if;
+	end process;
+    end generate;
+
+    pipe_one: if PIPELINE_DEPTH = 1 generate
+	signal pipe_ack : std_ulogic;
+	signal pipe_dat : wishbone_data_type;
+    begin
+	wishbone_out.stall <= '0';
+	wishbone_out.ack <= pipe_ack;
+	wishbone_out.dat <= pipe_dat;
+
+	pipe_one_sync: process(clk)
 	begin
-        wishbone_out.ack <= ret_ack and wishbone_in.cyc and wishbone_in.stb;
+	    if rising_edge(clk) then
+		pipe_ack <= ret_ack;
+		pipe_dat <= ret_dat;
+	    end if;
+	end process;
+    end generate;
+
+    pipe_none: if PIPELINE_DEPTH = 0 generate
+    begin
+	wishbone_out.ack <= ret_ack;
 	wishbone_out.dat <= ret_dat;
+	wishbone_out.stall <= wishbone_in.cyc and not ret_ack;
+    end generate;
     
+    wishbone_process: process(clk)
+	variable ret_dat_v : wishbone_data_type;
+	variable adr       : std_ulogic_vector(63 downto 0);
+    begin
         if rising_edge(clk) then
             if rst = '1' then
                 state <= IDLE;
                 ret_ack <= '0';
             else
-                ret_dat := x"FFFFFFFFFFFFFFFF";
+                ret_dat <= x"FFFFFFFFFFFFFFFF";
+		ret_ack <= '0';
 
                 -- Active
                 if wishbone_in.cyc = '1' then
                     case state is
                         when IDLE =>
                             if wishbone_in.stb = '1' then
+				adr := (wishbone_in.adr'left downto 0 => wishbone_in.adr,
+					others => '0');
                                 -- write
-				adr := (wishbone_in.adr'left downto 0 => wishbone_in.adr, others => '0');
                                 if wishbone_in.we = '1' then
                                     assert not(is_x(wishbone_in.dat)) and not(is_x(wishbone_in.adr)) severity failure;
                                     report "RAM writing " & to_hstring(wishbone_in.dat) & " to " & to_hstring(wishbone_in.adr);
                                     behavioural_write(wishbone_in.dat, adr, to_integer(unsigned(wishbone_in.sel)), identifier);
                                     reload <= reload + 1;
                                     ret_ack <= '1';
+				    if PIPELINE_DEPTH = 0 then
 					state <= ACK;
+				    end if;
                                 else
-                                    behavioural_read(ret_dat, adr, to_integer(unsigned(wishbone_in.sel)), identifier, reload);
-                                    report "RAM reading from " & to_hstring(wishbone_in.adr) & " returns " & to_hstring(ret_dat);
+                                    behavioural_read(ret_dat_v, adr, to_integer(unsigned(wishbone_in.sel)), identifier, reload);
+                                    report "RAM reading from " & to_hstring(wishbone_in.adr) & " returns " & to_hstring(ret_dat_v);
+				    ret_dat <= ret_dat_v;
                                     ret_ack <= '1';
+				    if PIPELINE_DEPTH = 0 then
 					state <= ACK;
 				    end if;
                                 end if;
+                            end if;
                         when ACK =>
-                            ret_ack <= '0';
                             state <= IDLE;
                     end case;
                 else
-                    ret_ack <= '0';
                     state <= IDLE;
                 end if;
             end if;