[prim,rtl] Rephrase a term in prim_reg_cdc.sv

If DstWrReq is false then the src_update signal cannot be true. This
is because it is driven by the src_update_o port on a prim_reg_cdc_arb
instance. If DstWrReq is false then the instance wires src_update_o to
zero (in the gen_passthru generate block).

This rewrite will behave equivalently in the same way as before but
will no longer generate any conditional coverage terms that depend on
src_update if DstWrReq is false.

Note that if DstWrReq is zero then the src_update and busy signals are
constant zero and are no longer actually used. To avoid a lint error
from the unused signals in this situation, they get "sampled" in the
gen_passthru block.

Signed-off-by: Rupert Swarbrick <[email protected]>

hw/ip/prim/rtl/prim_reg_cdc.sv

@@ -111,6 +111,24 @@ module prim_reg_cdc #(
   // This is the current destination value
   logic [DataWidth-1:0] dst_qs;
   logic src_update;
+
+  // The dst_to_src signal (on the src clock) means that data should be moving from the dst clock
+  // domain to the src clock domain on this cycle.
+  //
+  // This either means that an operation from the src side is being acknowledged or (if it's
+  // possible for the value to change on the dst side) it means that a register read response is
+  // coming from the dst clock domain. The register value may have changed from last time so should
+  // be copied back.
+  logic dst_to_src;
+  if (DstWrReq) begin : gen_wr_req
+    assign dst_to_src = src_busy_q && src_ack || src_update && !busy;
+  end else begin : gen_passthru
+    assign dst_to_src = src_busy_q && src_ack;
+
+    logic unused_dst_wr;
+    assign unused_dst_wr = src_update ^ busy;
+  end
+
   always_ff @(posedge clk_src_i or negedge rst_src_ni) begin
     if (!rst_src_ni) begin
       src_q <= ResetVal;
@@ -122,7 +140,7 @@ module prim_reg_cdc #(
       // change for the duration of the synchronization operation.
       src_q <= src_wd_i & BitMask;
       txn_bits_q <= {src_we_i, src_re_i, src_regwen_i};
-    end else if (src_busy_q && src_ack || src_update && !busy) begin
+    end else if (dst_to_src) begin
       // sample data whenever a busy transaction finishes OR
       // when an update pulse is seen.
       // TODO: We should add a cover group to test different sync timings