FPU: Add stage-2 stall ability to FPU

This makes the FPU able to stall other units at execute stage 2 and be
stalled by other units (specifically the LSU).

This means that the completion and writeback for an instruction can
now end up being deferred until the second cycle of a following
instruction, i.e. the cycle when the state machine has gone through
IDLE state into one of the DO_* states, which means we need to latch
the destination FPR number, CR mask, etc. from the previous
instruction so that we present the correct information to writeback.

The advantage of this is that we can get rid of the in_progress signal
from the LSU.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

common.vhdl

@@ -480,7 +480,6 @@ package common is
     type Loadstore1ToExecute1Type is record
         busy : std_ulogic;
         l2stall : std_ulogic;
-        in_progress : std_ulogic;
     end record;
 
     type Loadstore1ToDcacheType is record
@@ -640,16 +639,18 @@ package common is
         frt     : gspr_index_t;
         rc      : std_ulogic;
         out_cr  : std_ulogic;
+        stall   : std_ulogic;
     end record;
     constant Execute1ToFPUInit : Execute1ToFPUType := (valid => '0', op => OP_ILLEGAL, nia => (others => '0'),
                                                        itag => instr_tag_init,
                                                        insn  => (others => '0'), fe_mode => "00", rc => '0',
                                                        fra => (others => '0'), frb => (others => '0'),
                                                        frc => (others => '0'), frt => (others => '0'),
-                                                       single => '0', out_cr => '0');
+                                                       single => '0', out_cr => '0', stall => '0');
 
     type FPUToExecute1Type is record
         busy      : std_ulogic;
+        f2stall   : std_ulogic;
         exception : std_ulogic;
     end record;
     constant FPUToExecute1Init : FPUToExecute1Type := (others => '0');

core.vhdl

@@ -384,6 +384,7 @@ begin
             port map (
                 clk => clk,
                 rst => rst_fpu,
+                flush_in => flush,
                 e_in => execute1_to_fpu,
                 e_out => fpu_to_execute1,
                 w_out => fpu_to_writeback

execute1.vhdl

@@ -442,9 +442,9 @@ begin
     -- writeback, unless a pipeline flush has happened in the meantime.
     xerc_in <= ex1.xerc when ex1.xerc_valid = '1' else e_in.xerc;
 
-    with e_in.unit select busy_out <=
+    -- N.B. the busy signal from each source includes the
-        l_in.busy or l_in.in_progress or ex1.e.valid or ex1.busy or fp_in.busy when FPU,
+    -- stage2 stall from that source in it.
-        l_in.busy or ex1.busy or fp_in.busy when others;
+    busy_out <= l_in.busy or ex1.busy or fp_in.busy;
 
     valid_in <= e_in.valid and not (busy_out or flush_in or ex1.e.redirect or ex1.e.interrupt);
 
@@ -1299,8 +1299,7 @@ begin
             end if;
         end if;
 
-        v.no_instr_avail := not (e_in.valid or l_in.busy or l_in.in_progress or
+        v.no_instr_avail := not (e_in.valid or l_in.busy or ex1.busy or fp_in.busy);
-                                 ex1.busy or fp_in.busy);
 
         go := valid_in and not exception;
         v.instr_dispatch := go;
@@ -1436,7 +1435,7 @@ begin
         lv.is_32bit := e_in.is_32bit;
         lv.repeat := e_in.repeat;
         lv.second := e_in.second;
-        lv.e2stall := '0';
+        lv.e2stall := fp_in.f2stall;
 
         -- Outputs to FPU
         fv.op := e_in.insn_type;
@@ -1451,6 +1450,7 @@ begin
         fv.frt := e_in.write_reg;
         fv.rc := e_in.rc;
         fv.out_cr := e_in.output_cr;
+        fv.stall := l_in.l2stall;
 
 	-- Update registers
 	ex1in <= v;
@@ -1472,7 +1472,7 @@ begin
         ctrl.cfar when SPRSEL_CFAR,
         assemble_xer(ex1.e.xerc, ctrl.xer_low) when others;
 
-    stage2_stall <= l_in.l2stall or fp_in.busy;
+    stage2_stall <= l_in.l2stall or fp_in.f2stall;
 
     -- Second execute stage control
     execute2_1: process(all)

fpu.vhdl

@@ -15,6 +15,7 @@ entity fpu is
     port (
         clk : in std_ulogic;
         rst : in std_ulogic;
+        flush_in : in std_ulogic;
 
         e_in  : in  Execute1ToFPUType;
         e_out : out FPUToExecute1Type;
@@ -35,7 +36,7 @@ architecture behaviour of fpu is
         mantissa : std_ulogic_vector(63 downto 0);      -- 10.54 format
     end record;
 
-    type state_t is (IDLE,
+    type state_t is (IDLE, DO_ILLEGAL,
                      DO_MCRFS, DO_MTFSB, DO_MTFSFI, DO_MFFS, DO_MTFSF,
                      DO_FMR, DO_FMRG, DO_FCMP, DO_FTDIV, DO_FTSQRT,
                      DO_FCFID, DO_FCTI,
@@ -71,7 +72,9 @@ architecture behaviour of fpu is
     type reg_type is record
         state        : state_t;
         busy         : std_ulogic;
+        f2stall      : std_ulogic;
         instr_done   : std_ulogic;
+        complete     : std_ulogic;
         do_intr      : std_ulogic;
         illegal      : std_ulogic;
         op           : insn_type_t;
@@ -83,7 +86,9 @@ architecture behaviour of fpu is
         rc           : std_ulogic;
         is_cmp       : std_ulogic;
         single_prec  : std_ulogic;
+        sp_result    : std_ulogic;
         fpscr        : std_ulogic_vector(31 downto 0);
+        comm_fpscr   : std_ulogic_vector(31 downto 0);  -- committed FPSCR value
         a            : fpu_reg_type;
         b            : fpu_reg_type;
         c            : fpu_reg_type;
@@ -96,13 +101,17 @@ architecture behaviour of fpu is
         result_class : fp_number_class;
         result_exp   : signed(EXP_BITS-1 downto 0);
         shift        : signed(EXP_BITS-1 downto 0);
-        writing_back : std_ulogic;
+        writing_fpr  : std_ulogic;
+        write_reg    : gspr_index_t;
+        complete_tag : instr_tag_t;
+        writing_cr   : std_ulogic;
         int_result   : std_ulogic;
         cr_result    : std_ulogic_vector(3 downto 0);
         cr_mask      : std_ulogic_vector(7 downto 0);
         old_exc      : std_ulogic_vector(4 downto 0);
         update_fprf  : std_ulogic;
         quieten_nan  : std_ulogic;
+        nsnan_result : std_ulogic;
         tiny         : std_ulogic;
         denorm       : std_ulogic;
         round_mode   : std_ulogic_vector(2 downto 0);
@@ -542,17 +551,30 @@ begin
     fpu_0: process(clk)
     begin
         if rising_edge(clk) then
-            if rst = '1' then
+            if rst = '1' or flush_in = '1' then
                 r.state <= IDLE;
                 r.busy <= '0';
+                r.f2stall <= '0';
                 r.instr_done <= '0';
+                r.complete <= '0';
+                r.illegal <= '0';
                 r.do_intr <= '0';
+                r.writing_fpr <= '0';
+                r.writing_cr <= '0';
                 r.fpscr <= (others => '0');
-                r.writing_back <= '0';
+                r.write_reg <= (others =>'0');
-                r.dest_fpr <= (others =>'0');
+                r.complete_tag.valid <= '0';
                 r.cr_mask <= (others =>'0');
                 r.cr_result <= (others =>'0');
                 r.instr_tag.valid <= '0';
+                if rst = '1' then
+                    r.fpscr <= (others => '0');
+                    r.comm_fpscr <= (others => '0');
+                elsif r.do_intr = '0' then
+                    -- flush_in = 1 and not due to us generating an interrupt,
+                    -- roll back to committed fpscr
+                    r.fpscr <= r.comm_fpscr;
+                end if;
             else
                 assert not (r.state /= IDLE and e_in.valid = '1') severity failure;
                 r <= rin;
@@ -577,14 +599,19 @@ begin
     end process;
 
     e_out.busy <= r.busy;
+    e_out.f2stall <= r.f2stall;
     e_out.exception <= r.fpscr(FPSCR_FEX);
 
-    w_out.valid <= r.instr_done and not r.do_intr;
+    -- Note that the cycle where r.complete = 1 for an instruction can be as
-    w_out.instr_tag <= r.instr_tag;
+    -- late as the second cycle of the following instruction (i.e. in the state
-    w_out.write_enable <= r.writing_back;
+    -- following IDLE state).  Hence it is important that none of the fields of
-    w_out.write_reg <= r.dest_fpr;
+    -- r that are used below are modified in IDLE state.
+    w_out.valid <= r.complete;
+    w_out.instr_tag <= r.complete_tag;
+    w_out.write_enable <= r.writing_fpr and r.complete;
+    w_out.write_reg <= r.write_reg;
     w_out.write_data <= fp_result;
-    w_out.write_cr_enable <= r.instr_done and (r.rc or r.is_cmp);
+    w_out.write_cr_enable <= r.writing_cr and r.complete;
     w_out.write_cr_mask <= r.cr_mask;
     w_out.write_cr_data <= r.cr_result & r.cr_result & r.cr_result & r.cr_result &
                            r.cr_result & r.cr_result & r.cr_result & r.cr_result;
@@ -599,7 +626,6 @@ begin
         variable bdec        : fpu_reg_type;
         variable cdec        : fpu_reg_type;
         variable fpscr_mask  : std_ulogic_vector(31 downto 0);
-        variable illegal     : std_ulogic;
         variable j, k        : integer;
         variable flm         : std_ulogic_vector(7 downto 0);
         variable int_input   : std_ulogic;
@@ -644,12 +670,22 @@ begin
         variable maddend     : std_ulogic_vector(127 downto 0);
         variable sum         : std_ulogic_vector(63 downto 0);
         variable round_inc   : std_ulogic_vector(63 downto 0);
+        variable int_result  : std_ulogic;
+        variable illegal     : std_ulogic;
     begin
         v := r;
-        illegal := '0';
+        v.complete := '0';
-        v.busy := '0';
+        v.do_intr := '0';
         int_input := '0';
 
+        if r.complete = '1' or r.do_intr = '1' then
+            v.instr_done := '0';
+            v.writing_fpr := '0';
+            v.writing_cr := '0';
+            v.comm_fpscr := r.fpscr;
+            v.illegal := '0';
+        end if;
+
         -- capture incoming instruction
         if e_in.valid = '1' then
             v.insn := e_in.insn;
@@ -660,14 +696,8 @@ begin
             v.dest_fpr := e_in.frt;
             v.single_prec := e_in.single;
             v.longmask := e_in.single;
-            v.int_result := '0';
             v.rc := e_in.rc;
             v.is_cmp := e_in.out_cr;
-            if e_in.out_cr = '0' then
-                v.cr_mask := num_to_fxm(1);
-            else
-                v.cr_mask := num_to_fxm(to_integer(unsigned(insn_bf(e_in.insn))));
-            end if;
             int_input := '0';
             if e_in.op = OP_FPOP_I then
                 int_input := '1';
@@ -741,8 +771,6 @@ begin
             pcmpb_lt := '1';
         end if;
 
-        v.writing_back := '0';
-        v.instr_done := '0';
         v.update_fprf := '0';
         v.shift := to_signed(0, EXP_BITS);
         v.first := '0';
@@ -777,6 +805,8 @@ begin
         pshift := '0';
         renorm_sqrt := '0';
         shiftin := '0';
+        int_result := '0';
+        illegal := '0';
         case r.state is
             when IDLE =>
                 v.use_a := '0';
@@ -785,6 +815,7 @@ begin
                 v.invalid := '0';
                 v.negate := '0';
                 if e_in.valid = '1' then
+                    v.busy := '1';
                     case e_in.insn(5 downto 1) is
                         when "00000" =>
                             if e_in.insn(8) = '1' then
@@ -876,13 +907,17 @@ begin
                             end if;
                             v.state := DO_FMADD;
                         when others =>
-                            illegal := '1';
+                            v.state := DO_ILLEGAL;
                     end case;
                 end if;
                 v.x := '0';
                 v.old_exc := r.fpscr(FPSCR_VX downto FPSCR_XX);
                 set_s := '1';
 
+            when DO_ILLEGAL =>
+                illegal := '1';
+                v.instr_done := '1';
+
             when DO_MCRFS =>
                 j := to_integer(unsigned(insn_bfa(r.insn)));
                 for i in 0 to 7 loop
@@ -894,11 +929,9 @@ begin
                 end loop;
                 v.fpscr := r.fpscr and (fpscr_mask or x"6007F8FF");
                 v.instr_done := '1';
-                v.state := IDLE;
 
             when DO_FTDIV =>
                 v.instr_done := '1';
-                v.state := IDLE;
                 v.cr_result := "0000";
                 if r.a.class = INFINITY or r.b.class = ZERO or r.b.class = INFINITY or
                     (r.b.class = FINITE and r.b.mantissa(53) = '0') then
@@ -917,7 +950,6 @@ begin
 
             when DO_FTSQRT =>
                 v.instr_done := '1';
-                v.state := IDLE;
                 v.cr_result := "0000";
                 if r.b.class = ZERO or r.b.class = INFINITY or
                     (r.b.class = FINITE and r.b.mantissa(53) = '0') then
@@ -932,7 +964,6 @@ begin
                 -- fcmp[uo]
                 -- r.opsel_a = AIN_B
                 v.instr_done := '1';
-                v.state := IDLE;
                 update_fx := '1';
                 v.result_exp := r.b.exponent;
                 if (r.a.class = NAN and r.a.mantissa(53) = '0') or
@@ -993,7 +1024,6 @@ begin
                     end if;
                 end loop;
                 v.instr_done := '1';
-                v.state := IDLE;
 
             when DO_MTFSFI =>
                 -- mtfsfi
@@ -1007,20 +1037,17 @@ begin
                     end loop;
                 end if;
                 v.instr_done := '1';
-                v.state := IDLE;
 
             when DO_FMRG =>
                 -- fmrgew, fmrgow
                 opsel_r <= RES_MISC;
                 misc_sel <= "01" & r.insn(8) & '0';
-                v.int_result := '1';
+                int_result := '1';
-                v.writing_back := '1';
+                v.writing_fpr := '1';
                 v.instr_done := '1';
-                v.state := IDLE;
 
             when DO_MFFS =>
-                v.int_result := '1';
+                v.writing_fpr := '1';
-                v.writing_back := '1';
                 opsel_r <= RES_MISC;
                 case r.insn(20 downto 16) is
                     when "00000" =>
@@ -1044,10 +1071,11 @@ begin
                         -- mffsl
                         fpscr_mask := x"0007F0FF";
                     when others =>
-                        illegal := '1';
+                        v.illegal := '1';
+                        v.writing_fpr := '0';
                 end case;
+                int_result := '1';
                 v.instr_done := '1';
-                v.state := IDLE;
 
             when DO_MTFSF =>
                 if r.insn(25) = '1' then
@@ -1064,7 +1092,6 @@ begin
                     end if;
                 end loop;
                 v.instr_done := '1';
-                v.state := IDLE;
 
             when DO_FMR =>
                 -- r.opsel_a = AIN_B
@@ -1082,9 +1109,8 @@ begin
                 else
                     v.result_sign := r.a.negative;     -- fcpsgn
                 end if;
-                v.writing_back := '1';
+                v.writing_fpr := '1';
                 v.instr_done := '1';
-                v.state := IDLE;
 
             when DO_FRI =>    -- fri[nzpm]
                 -- r.opsel_a = AIN_B
@@ -1153,7 +1179,7 @@ begin
                     invalid := '1';
                 end if;
 
-                v.int_result := '1';
+                int_result := '1';
                 case r.b.class is
                     when ZERO =>
                         arith_done := '1';
@@ -1671,7 +1697,6 @@ begin
                 end if;
                 v.fpscr(FPSCR_FL downto FPSCR_FU) := v.cr_result;
                 v.instr_done := '1';
-                v.state := IDLE;
 
             when MULT_1 =>
                 f_to_multiply.valid <= r.first;
@@ -1849,7 +1874,6 @@ begin
                 v.cr_result(1) := exp_tiny or exp_huge;
                 if exp_tiny = '1' or exp_huge = '1' or r.a.class = ZERO or r.first = '0' then
                     v.instr_done := '1';
-                    v.state := IDLE;
                 else
                     v.shift := r.a.exponent;
                     v.doing_ftdiv := "10";
@@ -2054,6 +2078,7 @@ begin
                     when others =>      -- fctidu[z]
                         need_check := r.r(63);
                 end case;
+                int_result := '1';
                 if need_check = '1' then
                     v.state := INT_CHECK;
                 else
@@ -2080,6 +2105,7 @@ begin
                         v.fpscr(FPSCR_XX) := '1';
                     end if;
                 end if;
+                int_result := '1';
                 arith_done := '1';
 
             when INT_OFLOW =>
@@ -2090,6 +2116,7 @@ begin
                 end if;
                 v.fpscr(FPSCR_VXCVI) := '1';
                 invalid := '1';
+                int_result := '1';
                 arith_done := '1';
 
             when FRI_1 =>
@@ -2306,11 +2333,10 @@ begin
             -- Neither does enabled zero-divide exception
             if (v.invalid and r.fpscr(FPSCR_VE)) = '0' and
                 (zero_divide and r.fpscr(FPSCR_ZE)) = '0' then
-                v.writing_back := '1';
+                v.writing_fpr := '1';
                 v.update_fprf := '1';
             end if;
             v.instr_done := '1';
-            v.state := IDLE;
             update_fx := '1';
         end if;
 
@@ -2530,12 +2556,6 @@ begin
             v.shift := resize(signed('0' & clz) - 9, EXP_BITS);
         end if;
 
-        if r.int_result = '1' then
-            fp_result <= r.r;
-        else
-            fp_result <= pack_dp(r.result_sign, r.result_class, r.result_exp, r.r,
-                                 r.single_prec, r.quieten_nan);
-        end if;
         if r.update_fprf = '1' then
             v.fpscr(FPSCR_C downto FPSCR_FU) := result_flags(r.result_sign, r.result_class,
                                                              r.r(54) and not r.denorm);
@@ -2549,24 +2569,49 @@ begin
             (v.fpscr(FPSCR_VX downto FPSCR_XX) and not r.old_exc) /= "00000" then
             v.fpscr(FPSCR_FX) := '1';
         end if;
-        if r.rc = '1' then
-            v.cr_result := v.fpscr(FPSCR_FX downto FPSCR_OX);
-        end if;
 
-        v.illegal := illegal;
+        if v.instr_done = '1' then
-        if illegal = '1' then
+            if r.state /= IDLE then
-            v.instr_done := '0';
+                v.state := IDLE;
-            v.do_intr := '1';
+                v.busy := '0';
-            v.writing_back := '0';
+                v.f2stall := '0';
-            v.busy := '0';
+                if r.rc = '1' then
-            v.state := IDLE;
+                    v.cr_result := v.fpscr(FPSCR_FX downto FPSCR_OX);
+                end if;
+                v.sp_result := r.single_prec;
+                v.int_result := int_result;
+                v.illegal := illegal;
+                v.nsnan_result := v.quieten_nan;
+                if r.is_cmp = '0' then
+                    v.cr_mask := num_to_fxm(1);
+                else
+                    v.cr_mask := num_to_fxm(to_integer(unsigned(insn_bf(r.insn))));
+                end if;
+                v.writing_cr := r.is_cmp or r.rc;
+                v.write_reg := r.dest_fpr;
+                v.complete_tag := r.instr_tag;
+            end if;
+            if e_in.stall = '0' then
+                v.complete := not v.illegal;
+                v.do_intr := (v.fpscr(FPSCR_FEX) and r.fe_mode) or v.illegal;
+            end if;
+            -- N.B. We rely on execute1 to prevent any new instruction
+            -- coming in while e_in.stall = 1, without us needing to
+            -- have busy asserted.
         else
-            v.do_intr := v.instr_done and v.fpscr(FPSCR_FEX) and r.fe_mode;
+            if r.state /= IDLE and e_in.stall = '0' then
-            if v.state /= IDLE or v.do_intr = '1' then
+                v.f2stall := '1';
-                v.busy := '1';
             end if;
         end if;
 
+        -- This mustn't depend on any fields of r that are modified in IDLE state.
+        if r.int_result = '1' then
+            fp_result <= r.r;
+        else
+            fp_result <= pack_dp(r.result_sign, r.result_class, r.result_exp, r.r,
+                                 r.sp_result, r.nsnan_result);
+        end if;
+
         rin <= v;
     end process;
 

loadstore1.vhdl

@@ -159,7 +159,6 @@ architecture behave of loadstore1 is
     signal flush    : std_ulogic;
     signal busy     : std_ulogic;
     signal complete : std_ulogic;
-    signal in_progress : std_ulogic;
     signal flushing : std_ulogic;
 
     signal store_sp_data : std_ulogic_vector(31 downto 0);
@@ -523,7 +522,6 @@ begin
 
     busy <= dc_stall or d_in.error or r1.busy or r2.busy;
     complete <= r2.one_cycle or (r2.wait_dc and d_in.valid) or r3.complete;
-    in_progress <= r1.req.valid or (r2.req.valid and not complete);
 
     -- Processing done in the first cycle of a load/store instruction
     loadstore1_1: process(all)
@@ -981,7 +979,6 @@ begin
         -- update busy signal back to execute1
         e_out.busy <= busy;
         e_out.l2stall <= dc_stall or d_in.error or r2.busy;
-        e_out.in_progress <= in_progress;
 
         events <= r3.events;