fetch1: Fix debug stop

The ability to stop the core using the debug interface has been broken
since commit bb4332b7e6b5 ("Remove fetch2 pipeline stage"), which
removed a statement that cleared the valid bit on instructions when
their stop_mark was 1.

Fix this by clearing r.req coming out of fetch1 when r.stop_mark = 1.
This has the effect of making i_out.valid be 0 from the icache.  We
also fix a bug in icache.vhdl where it was not honouring i_in.req when
use_previous = 1.

It turns out that the logic in fetch1.vhdl to handle stopping and
restarting was not correct, with the effect that stopping the core
would leave NIA pointing to the last instruction executed, not the
next instruction to be executed.  In fact the state machine is
unnecessary and the whole thing can be simplified enormously - we
need to increment NIA whenever stop_in = 0 in the previous cycle.

Fixes: bb4332b7e6b5 ("Remove fetch2 pipeline stage")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

fetch1.vhdl

@@ -35,9 +35,7 @@ entity fetch1 is
 end entity fetch1;
 
 architecture behaviour of fetch1 is
-    type stop_state_t is (RUNNING, STOPPED, RESTARTING);
     type reg_internal_t is record
-	stop_state: stop_state_t;
         mode_32bit: std_ulogic;
     end record;
     signal r, r_next : Fetch1ToIcacheType;
@@ -70,7 +68,6 @@ begin
     comb : process(all)
 	variable v : Fetch1ToIcacheType;
 	variable v_int : reg_internal_t;
-	variable increment : boolean;
     begin
 	v := r;
 	v_int := r_int;
@@ -85,7 +82,6 @@ begin
             v.virt_mode := '0';
             v.priv_mode := '1';
             v.big_endian := '0';
-	    v_int.stop_state := RUNNING;
             v_int.mode_32bit := '0';
 	elsif e_in.redirect = '1' then
 	    v.nia := e_in.redirect_nia(63 downto 2) & "00";
@@ -103,49 +99,9 @@ begin
             end if;
 	elsif stall_in = '0' then
 
-	    -- For debug stop/step to work properly we need a little bit of
-	    -- trickery here. If we just stop incrementing and send stop marks
-	    -- when stop_in is set, then we'll increment on the cycle it clears
-	    -- and end up never executing the instruction we were stopped on.
-	    --
-	    -- Avoid this along with the opposite issue when stepping (stop is
-	    -- cleared for only one cycle) is handled by the state machine below
-	    --
-	    -- By default, increment addresses
-	    increment := true;
-	    case v_int.stop_state is
-	    when RUNNING =>
-		-- If we are running and stop_in is set, then stop incrementing,
-		-- we are now stopped.
-		if stop_in = '1' then
-		    increment := false;
-		    v_int.stop_state := STOPPED;
-		end if;
-	    when STOPPED =>
-		-- When stopped, never increment. If stop is cleared, go to state
-		-- "restarting" but still don't increment that cycle. stop_in is
-		-- now 0 so we'll send the NIA down without a stop mark.
-		increment := false;
-		if stop_in = '0' then
-		    v_int.stop_state := RESTARTING;
-		end if;
-	    when RESTARTING =>
-		-- We have just sent the NIA down, we can start incrementing again.
-		-- If stop_in is still not set, go back to running normally.
-		-- If stop_in is set again (that was a one-cycle "step"), go
-		-- back to "stopped" state which means we'll stop incrementing
-		-- on the next cycle. This ensures we increment the PC once after
-		-- sending one instruction without a stop mark. Since stop_in is
-		-- now set, the new PC will be sent with a stop mark and thus not
-		-- executed.
-		if stop_in = '0' then
-		    v_int.stop_state := RUNNING;
-		else
-		    v_int.stop_state := STOPPED;
-		end if;
-	    end case;
-
-	    if increment then
+            -- If the last NIA value went down with a stop mark, it didn't get
+            -- executed, and hence we shouldn't increment NIA.
+	    if r.stop_mark = '0' then
                 if r_int.mode_32bit = '0' then
                     v.nia := std_ulogic_vector(unsigned(r.nia) + 4);
                 else
@@ -155,7 +111,7 @@ begin
 	    end if;
 	end if;
 
-	v.req := not rst;
+	v.req := not rst and not stop_in;
 	v.stop_mark := stop_in;
 
 	r_next <= v;

icache.vhdl

@@ -499,7 +499,7 @@ begin
         -- last cycle, and we don't want the first 32-bit chunk, then we can
         -- keep the data we read last cycle and just use that.
         if unsigned(i_in.nia(INSN_BITS+2-1 downto 2)) /= 0 then
-            use_previous <= i_in.sequential and r.hit_valid;
+            use_previous <= i_in.req and i_in.sequential and r.hit_valid;
         else
             use_previous <= '0';
         end if;