single_step test : only drive debug_req_i after stepping finishes

This addresses a current testbench issue where asserting debug_req_i close to
when single_stepping over an instruction causes an incorrect 'cause' to be
recorded within DCSR.

dv/uvm/core_ibex/env/core_ibex_dut_probe_if.sv

@@ -19,6 +19,7 @@ interface core_ibex_dut_probe_if(input logic clk);
   logic                    debug_req;
   ibex_pkg::priv_lvl_e     priv_mode;
   ibex_pkg::ctrl_fsm_e     ctrl_fsm_cs;
+  logic                    debug_mode;
 
   clocking dut_cb @(posedge clk);
     output fetch_enable;
@@ -36,6 +37,7 @@ interface core_ibex_dut_probe_if(input logic clk);
     input alert_major_bus;
     input priv_mode;
     input ctrl_fsm_cs;
+    input debug_mode;
   endclocking
 
   initial begin

dv/uvm/core_ibex/tb/core_ibex_tb_top.sv

@@ -199,15 +199,16 @@ module core_ibex_tb_top;
   // Irq interface connections
   assign irq_vif.reset = ~rst_n;
   // Dut_if interface connections
-  assign dut_if.ecall            = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.ecall_insn;
-  assign dut_if.wfi              = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.wfi_insn;
-  assign dut_if.ebreak           = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.ebrk_insn;
-  assign dut_if.illegal_instr    = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.illegal_insn_d;
-  assign dut_if.dret             = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.dret_insn;
-  assign dut_if.mret             = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.mret_insn;
-  assign dut_if.reset            = ~rst_n;
-  assign dut_if.priv_mode        = dut.u_ibex_top.u_ibex_core.priv_mode_id;
-  assign dut_if.ctrl_fsm_cs      = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.ctrl_fsm_cs;
+  assign dut_if.ecall         = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.ecall_insn;
+  assign dut_if.wfi           = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.wfi_insn;
+  assign dut_if.ebreak        = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.ebrk_insn;
+  assign dut_if.illegal_instr = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.illegal_insn_d;
+  assign dut_if.dret          = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.dret_insn;
+  assign dut_if.mret          = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.mret_insn;
+  assign dut_if.reset         = ~rst_n;
+  assign dut_if.priv_mode     = dut.u_ibex_top.u_ibex_core.priv_mode_id;
+  assign dut_if.ctrl_fsm_cs   = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.ctrl_fsm_cs;
+  assign dut_if.debug_mode    = dut.u_ibex_top.u_ibex_core.id_stage_i.controller_i.debug_mode_q;
   // Instruction monitor connections
   assign instr_monitor_if.reset        = ~rst_n;
   assign instr_monitor_if.valid_id     = dut.u_ibex_top.u_ibex_core.id_stage_i.instr_valid_i;

dv/uvm/core_ibex/tests/core_ibex_test_lib.sv

@@ -1125,12 +1125,38 @@ class core_ibex_debug_single_step_test extends core_ibex_directed_test;
   `uvm_component_utils(core_ibex_debug_single_step_test)
   `uvm_component_new
 
+  uvm_event e1;
+  int       cnt;
+  int       debug_mode_end_dwell_cycles = 3000;
+
   virtual task check_stimulus();
-    forever begin
-      clk_vif.wait_clks(2000);
-      vseq.start_debug_single_seq();
-      wait_ret("dret", 50000);
-    end
+    e1 = new();
+    fork
+      begin
+        forever begin
+          // Create an event (e1) whenever we are out of debug_mode for a configurable length of time.
+          // This allows us to detect when the system has stopped single-stepping.
+          cnt = 0;
+          @(negedge dut_vif.dut_cb.debug_mode);
+          while (dut_vif.dut_cb.debug_mode == '0) begin
+            clk_vif.wait_clks(1);
+            cnt++;
+            if (cnt == debug_mode_end_dwell_cycles) begin
+              e1.trigger();
+              break;
+            end
+          end
+        end
+      end
+      begin
+        forever begin
+          clk_vif.wait_clks(2000);
+          vseq.start_debug_single_seq();
+          // Wait for the above event (e1) before sending another debug_req
+          e1.wait_trigger();
+        end
+      end
+    join_none
   endtask
 
 endclass