fifo: synchronize read-domain reset handling

Synchronize legacy and XPM reset indications into the FIFO read clock domain instead of feeding cross-domain reset logic into asynchronous PRE/CLR paths.

This removes the FIFO-induced CDC-10 findings while preserving legacy reset behavior.

lib/fifo/fifo_2clk_xpm_core.v

@@ -65,23 +65,80 @@ module fifo_2clk_xpm_core #(
   wire [COUNT_WIDTH-1:0] xpm_wr_data_count;
   wire [COUNT_WIDTH-1:0] xpm_rd_data_count;
 
-  // Keep the read-side legacy interface quiescent from the instant the old
+  // ------------------------------------------------------------------------
-  // asynchronous reset is asserted until both XPM reset propagation and the
+  // Read-domain reset/hold handling
-  // read-domain reset-busy interval have completed. Assertion is asynchronous;
+  // ------------------------------------------------------------------------
-  // release is synchronized to rd_clk.
+  // Do NOT combine xpm_rst (wr_clk domain) with rd-domain signals and feed the
-  wire rd_hold_req = rst | xpm_rst | xpm_rd_rst_busy;
+  // result into an asynchronous PRE/CLR. Vivado report_cdc correctly reports
-  (* ASYNC_REG = "TRUE", SHREG_EXTRACT = "NO" *)
+  // that topology as CDC-10 (combinational logic before a synchronizer).
-  reg [1:0] rd_hold_sync = 2'b11;
+  //
-  always @(posedge rd_clk or posedge rd_hold_req) begin
+  // Instead, synchronize each reset indication into rd_clk using dedicated XPM
-    if (rd_hold_req)
+  // CDC structures, then make all hold/release decisions synchronously in the
-      rd_hold_sync <= 2'b11;
+  // read domain.
-    else begin
+
-      rd_hold_sync[0] <= 1'b0;
+  // Preserve the legacy FIFO Generator behavior at the external interface:
-      rd_hold_sync[1] <= rd_hold_sync[0];
+  // assertion of rst is visible immediately in the read domain, while release
+  // is synchronized to rd_clk.
+  wire legacy_rd_rst;
+  xpm_cdc_async_rst #(
+    .DEST_SYNC_FF   (2),
+    .INIT_SYNC_FF   (1),
+    .RST_ACTIVE_HIGH(1)
+  ) legacy_rd_rst_sync_i (
+    .src_arst (rst),
+    .dest_clk (rd_clk),
+    .dest_arst(legacy_rd_rst)
+  );
+
+  // xpm_rst is generated synchronously in the write domain. Synchronize that
+  // reset request into rd_clk so the read interface can remain held until the
+  // XPM reset has actually propagated through the write side. INIT=1 keeps the
+  // read side conservative during FPGA configuration/startup.
+  wire xpm_rst_rd;
+  xpm_cdc_sync_rst #(
+    .DEST_SYNC_FF (2),
+    .INIT         (1),
+    .INIT_SYNC_FF (1),
+    .SIM_ASSERT_CHK(1)
+  ) xpm_rst_to_rd_i (
+    .src_rst (xpm_rst),
+    .dest_clk(rd_clk),
+    .dest_rst(xpm_rst_rd)
+  );
+
+  // A reset event is complete only after the read domain has observed the XPM
+  // reset sequence (xpm_rst_rd and/or rd_rst_busy asserted) and both indicators
+  // have subsequently gone low. This prevents a short external reset from
+  // creating a gap where the legacy interface is released before XPM has
+  // propagated the reset from wr_clk into rd_clk.
+  reg rd_reset_pending = 1'b1;
+  reg rd_reset_seen    = 1'b0;
+  reg legacy_rd_rst_d  = 1'b1;
+  always @(posedge rd_clk) begin
+    legacy_rd_rst_d <= legacy_rd_rst;
+
+    // Start a fresh acknowledgement cycle for each external reset event.
+    if (legacy_rd_rst && !legacy_rd_rst_d) begin
+      rd_reset_pending <= 1'b1;
+      rd_reset_seen    <= 1'b0;
+    end else if (legacy_rd_rst) begin
+      // The XPM reset may assert and even complete while the legacy reset is
+      // still being synchronously released. Remember that it was observed.
+      rd_reset_pending <= 1'b1;
+      if (xpm_rst_rd || xpm_rd_rst_busy)
+        rd_reset_seen <= 1'b1;
+    end else if (rd_reset_pending) begin
+      if (xpm_rst_rd || xpm_rd_rst_busy)
+        rd_reset_seen <= 1'b1;
+
+      if (rd_reset_seen && !xpm_rst_rd && !xpm_rd_rst_busy)
+        rd_reset_pending <= 1'b0;
     end
   end
 
-  wire rd_hold = rd_hold_sync[1];
+  // All terms below are now local to rd_clk (or an XPM-synchronized reset).
+  wire rd_hold = legacy_rd_rst | rd_reset_pending |
+                 xpm_rst_rd | xpm_rd_rst_busy;
   wire wr_hold = rst | xpm_rst | xpm_wr_rst_busy;
 
   // XPM requires wr_en/rd_en low while reset or the corresponding reset-busy