ram: Add block RAM pipelining

This adds an output buffer to help with timing and allows the BRAMs
to actually pipeline.

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

fpga/mw_soc_memory.vhdl

@@ -4,6 +4,7 @@
 
 library ieee;
 use ieee.std_logic_1164.all;
+use ieee.std_logic_unsigned.all;
 use ieee.numeric_std.all;
 use std.textio.all;
 
@@ -29,9 +30,10 @@ entity mw_soc_memory is
 end entity mw_soc_memory;
 
 architecture behaviour of mw_soc_memory is
-    signal wb_adr_in : std_logic_vector(log2(MEMORY_SIZE) - 1 downto 0);
+    -- RAM type definition
     type ram_t is array(0 to (MEMORY_SIZE / 8) - 1) of std_logic_vector(63 downto 0);
 
+    -- RAM loading
     impure function init_ram(name : STRING) return ram_t is
         file ram_file : text open read_mode is name;
         variable ram_line : line;
@@ -48,58 +50,61 @@ architecture behaviour of mw_soc_memory is
         return temp_ram;
     end function;
 
+    -- RAM instance
     signal memory : ram_t := init_ram(RAM_INIT_FILE);
-
     attribute ram_style : string;
     attribute ram_style of memory : signal is "block";
-
     attribute ram_decomp : string;
-    attribute ram_decomp of memory : signal is "power"; 
+    attribute ram_decomp of memory : signal is "power";
-
+
-    type state_type is (IDLE, ACK);
+    -- RAM interface
-    signal state : state_type;
+    constant ram_addr_bits : integer := log2(MEMORY_SIZE) - 3;
-
+    signal ram_addr : std_logic_vector(ram_addr_bits - 1 downto 0);
-    signal read_ack : std_logic;
+    signal ram_di   : std_logic_vector(63 downto 0);
-
+    signal ram_do   : std_logic_vector(63 downto 0);
+    signal ram_sel  : std_logic_vector(7 downto 0);
+    signal ram_we   : std_ulogic;
+
+    -- Others
+    signal ram_obuf      : std_logic_vector(63 downto 0);
+    signal ack, ack_obuf : std_ulogic;
 begin
 
-    wb_adr_in <= wishbone_in.adr(log2(MEMORY_SIZE) - 1 downto 0);
+    -- Actual RAM template    
+    memory_0: process(clk)
+    begin
+	if rising_edge(clk) then
+	    if ram_we = '1' then
+		for i in 0 to 7 loop
+		    if ram_sel(i) = '1' then
+			memory(conv_integer(ram_addr))((i + 1) * 8 - 1 downto i * 8) <=
+			    ram_di((i + 1) * 8 - 1 downto i * 8);
+		    end if;
+		end loop;
+	    end if;
+	    ram_do <= memory(conv_integer(ram_addr));
+	    ram_obuf <= ram_do;
+	end if;
+    end process;
 
-    wishbone_out.ack <= read_ack and wishbone_in.cyc and wishbone_in.stb;
+    -- Wishbone interface
-    wishbone_out.stall <= '0' when wishbone_in.cyc = '0' else not wishbone_out.ack;
+    ram_addr <= wishbone_in.adr(ram_addr_bits + 2 downto 3);
+    ram_di <= wishbone_in.dat;
+    ram_sel <= wishbone_in.sel;
+    ram_we <= wishbone_in.we and wishbone_in.stb and wishbone_in.cyc;
+    wishbone_out.stall <= '0';
+    wishbone_out.ack <= ack_obuf;
+    wishbone_out.dat <= ram_obuf;
 
-    memory_0: process(clk)
+    wb_0: process(clk)
     begin
 	if rising_edge(clk) then
-	    if rst = '1' then
+	    if rst = '1' or wishbone_in.cyc = '0' then
-		read_ack <= '0';
+		ack_obuf <= '0';
-		state <= IDLE;
+		ack <= '0';
 	    else
-		if wishbone_in.cyc = '1' then
+		ack <= wishbone_in.stb;
-		    case state is
+		ack_obuf <= ack;
-		    when IDLE =>
-			if wishbone_in.stb = '1' and wishbone_in.we = '1' then
-			    for i in 0 to 7 loop
-				if wishbone_in.sel(i) = '1' then
-				    memory(to_integer(unsigned(wb_adr_in(wb_adr_in'left downto 3))))(((i + 1) * 8) - 1 downto i * 8)
-					<= wishbone_in.dat(((i + 1) * 8) - 1 downto i * 8);
-				end if;
-			    end loop;
-			    read_ack <= '1';
-			    state <= ACK;
-			elsif wishbone_in.stb = '1' then
-			    wishbone_out.dat <= memory(to_integer(unsigned(wb_adr_in(wb_adr_in'left downto 3))));
-			    read_ack <= '1';
-			    state <= ACK;
-			end if;
-		    when ACK =>
-			read_ack <= '0';
-			state <= IDLE;
-		    end case;
-		else
-		    state <= IDLE;
-		    read_ack <= '0';
-		end if;
 	    end if;
 	end if;
     end process;