core: Add alternate reset address

An external signal can control whether the core will start
executing at the standard or the alternate reset address.

This will be used when litedram is initialized by microwatt
itself, to route the reset to the built-in init code secondary
block RAM.

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

core.vhdl

@@ -10,12 +10,17 @@ entity core is
     generic (
         SIM : boolean := false;
 	DISABLE_FLATTEN : boolean := false;
-        EX1_BYPASS : boolean := true
+        EX1_BYPASS : boolean := true;
+	ALT_RESET_ADDRESS : std_ulogic_vector(63 downto 0) := (others => '0')
         );
     port (
-        clk          : in std_logic;
-        rst          : in std_logic;
+        clk          : in std_ulogic;
+        rst          : in std_ulogic;
 
+	-- Alternate reset (0xffff0000) for use by DRAM init fw
+	alt_reset    : in std_ulogic;
+
+	-- Wishbone interface
         wishbone_insn_in  : in wishbone_slave_out;
         wishbone_insn_out : out wishbone_master_out;
 
@@ -125,11 +130,13 @@ begin
 
     fetch1_0: entity work.fetch1
         generic map (
-            RESET_ADDRESS => (others => '0')
+            RESET_ADDRESS => (others => '0'),
+	    ALT_RESET_ADDRESS => ALT_RESET_ADDRESS
             )
         port map (
             clk => clk,
             rst => core_rst,
+	    alt_reset_in => alt_reset,
             stall_in => fetch1_stall_in,
             flush_in => flush,
 	    stop_in => dbg_core_stop,

core_tb.vhdl

@@ -27,7 +27,8 @@ begin
 	    rst => rst,
 	    system_clk => clk,
 	    uart0_rxd => '0',
-	    uart0_txd => open
+	    uart0_txd => open,
+	    alt_reset => '0'
 	    );
 
     clk_process: process

fetch1.vhdl

@@ -7,7 +7,8 @@ use work.common.all;
 
 entity fetch1 is
     generic(
-	RESET_ADDRESS : std_logic_vector(63 downto 0) := (others => '0')
+	RESET_ADDRESS     : std_logic_vector(63 downto 0) := (others => '0');
+	ALT_RESET_ADDRESS : std_logic_vector(63 downto 0) := (others => '0')
 	);
     port(
 	clk           : in std_ulogic;
@@ -17,6 +18,7 @@ entity fetch1 is
 	stall_in      : in std_ulogic;
 	flush_in      : in std_ulogic;
 	stop_in       : in std_ulogic;
+	alt_reset_in  : in std_ulogic;
 
 	-- redirect from execution unit
 	e_in          : in Execute1ToFetch1Type;
@@ -60,7 +62,11 @@ begin
 	v_int := r_int;
 
 	if rst = '1' then
+	    if alt_reset_in = '1' then
+		v.nia :=  ALT_RESET_ADDRESS;
+	    else
 		v.nia :=  RESET_ADDRESS;
+	    end if;
 	    v_int.stop_state := RUNNING;
 	elsif e_in.redirect = '1' then
 	    v.nia := e_in.redirect_nia;

soc.vhdl

@@ -27,7 +27,8 @@ entity soc is
 
 	-- UART0 signals:
 	uart0_txd    : out std_ulogic;
-	uart0_rxd    : in  std_ulogic
+	uart0_rxd    : in  std_ulogic;
+	alt_reset    : in std_ulogic
 	);
 end entity soc;
 
@@ -89,11 +90,13 @@ begin
     processor: entity work.core
 	generic map(
 	    SIM => SIM,
-	    DISABLE_FLATTEN => DISABLE_FLATTEN_CORE
+	    DISABLE_FLATTEN => DISABLE_FLATTEN_CORE,
+	    ALT_RESET_ADDRESS => (15 downto 0 => '0', others => '1')
 	    )
 	port map(
 	    clk => system_clk,
 	    rst => rst,
+	    alt_reset => alt_reset,
 	    wishbone_insn_in => wishbone_icore_in,
 	    wishbone_insn_out => wishbone_icore_out,
 	    wishbone_data_in => wishbone_dcore_in,