code refactoring for Vivado compatibility

hw/rtl/libs/VX_fair_arbiter.sv

@@ -0,0 +1,60 @@
+`include "VX_platform.vh"
+
+`TRACING_OFF
+module VX_fair_arbiter #(
+    parameter NUM_REQS     = 1,
+    parameter LOCK_ENABLE  = 0,
+    parameter LOG_NUM_REQS = $clog2(NUM_REQS)
+) (
+    input  wire                     clk,
+    input  wire                     reset,
+    input  wire                     enable,
+    input  wire [NUM_REQS-1:0]      requests, 
+    output wire [LOG_NUM_REQS-1:0]  grant_index,
+    output wire [NUM_REQS-1:0]      grant_onehot,   
+    output wire                     grant_valid
+  );
+
+    if (NUM_REQS == 1)  begin                
+        
+        `UNUSED_VAR (clk)
+        `UNUSED_VAR (reset)
+        assign grant_index  = 0;
+        assign grant_onehot = requests;
+        assign grant_valid  = requests[0];
+
+    end else begin    
+
+        reg [NUM_REQS-1:0] buffer;
+        reg use_buffer;     
+
+        wire [NUM_REQS-1:0] requests_qual = use_buffer ? buffer : requests;
+        wire [NUM_REQS-1:0] buffer_n = requests_qual & ~grant_onehot;
+
+        always @(posedge clk) begin
+            if (reset) begin
+                use_buffer <= 0;
+            end else if (!LOCK_ENABLE || enable) begin
+                use_buffer <= (buffer_n != 0);
+            end
+            if (!LOCK_ENABLE || enable) begin
+                buffer <= buffer_n;
+            end
+        end
+               
+        VX_fixed_arbiter #(
+            .NUM_REQS    (NUM_REQS),
+            .LOCK_ENABLE (LOCK_ENABLE)
+        ) fixed_arbiter (
+            .clk          (clk),
+            .reset        (reset),
+            .enable       (enable),
+            .requests     (requests_qual), 
+            .grant_index  (grant_index),
+            .grant_onehot (grant_onehot),
+            .grant_valid  (grant_valid)
+        );
+    end
+    
+endmodule
+`TRACING_ON