[rtl] Add plusarg to disable trace log

Option to control the state of the Ibex tracer.
Defaults to enable, but allows a setting to disable the trace log.

In long running simulations this file can get quite big and the contents
might not be needed. Use the argument `+ibex_tracer_enable=0` to prevent
the creation of the file.

doc/03_reference/tracer.rst

@@ -16,6 +16,15 @@ The file name base, defaulting to ``trace_core`` can be set using the ``ibex_tra
 For example, ``+ibex_tracer_file_base=ibex_my_trace`` will produce log files named ``ibex_my_trace_<HARTID>.log``.
 The exact syntax of passing plusargs to a simulation depends on the simulator.
 
+Disabling the tracer
+--------------------
+
+If the instruction log is not needed for a specific simulation run, the tracer can be disabled.
+
+The plusarg ``ibex_tracer_enable`` controls the tracer.
+The tracer is enabled by default.
+To disable the tracer use ``ibex_tracer_enable=0`` with the correct plusarg syntax of the simulator.
+
 Trace output format
 -------------------
 

rtl/ibex_tracer.sv

@@ -16,6 +16,10 @@
  * plusarg passed to the simulation, e.g. "+ibex_tracer_file_base=ibex_my_trace". The exact syntax
  * of passing plusargs to a simulation depends on the simulator.
  *
+ * The creation of the instruction trace is enabled by default but can be disabled for a simulation.
+ * This behaviour is controlled by the plusarg "ibex_tracer_enable". Use "ibex_tracer_enable=0" to
+ * disable the tracer.
+ *
  * The trace contains six columns, separated by tabs:
  * - The simulation time
  * - The clock cycle count since reset
@@ -92,6 +96,17 @@ module ibex_tracer (
   localparam logic [4:0] MEM = (1 << 4);
   logic [4:0] data_accessed;
 
+  logic trace_log_enable;
+  initial begin
+    if ($value$plusargs("ibex_tracer_enable=%b", trace_log_enable)) begin
+      if (trace_log_enable == 1'b0) begin
+        $display("%m: Instruction trace disabled.");
+      end
+    end else begin
+      trace_log_enable = 1'b1;
+    end
+  end
+
   function automatic void printbuffer_dumpline();
     string rvfi_insn_str;
 
@@ -730,7 +745,7 @@ module ibex_tracer (
 
   // log execution
   always_ff @(posedge clk_i) begin
-    if (rvfi_valid) begin
+    if (rvfi_valid && trace_log_enable) begin
       printbuffer_dumpline();
     end
   end