[otbn,rtl] Fix timing on instruction memory request

This stops errors factoring into the outputs for the instruction memory
request. Now on error the next imem request will still occur but OTBN
otherwise immediately halts are previously.

Signed-off-by: Greg Chadwick <gac@lowrisc.org>

hw/ip/otbn/rtl/otbn_controller.sv

@@ -490,7 +490,6 @@ module otbn_controller
     // On any error immediately halt, either going to OtbnStateLocked or OtbnStateHalt depending on
     // whether it was a fatal error.
     if (err) begin
-      prefetch_en_o           = 1'b0;
       insn_fetch_resp_clear_o = 1'b1;
 
       if (fatal_err) begin

hw/ip/otbn/rtl/otbn_instruction_fetch.sv

@@ -102,8 +102,10 @@ module otbn_instruction_fetch
   logic [NWdr-1:0] rf_bignum_wr_sec_wipe_onehot;
 
   // The prefetch has failed if a fetch is requested and either no prefetch has done or was done to
-  // the wrong address.
-  assign insn_prefetch_fail = insn_fetch_req_valid_i &
+  // the wrong address. The `insn_fetch_req_valid_raw_i` signal doesn't factor in errors which is
+  // important to avoid timing issues here as this signal factors into the muxing for the
+  // imem_addr_o output.
+  assign insn_prefetch_fail = insn_fetch_req_valid_raw_i &
                               (~imem_rvalid_i || (insn_fetch_req_addr_i != insn_prefetch_addr));
 
   // Fetch response is valid when prefetch has matched what was requested. Otherwise if no fetch is
@@ -315,7 +317,9 @@ module otbn_instruction_fetch
     // By default prefetch the next instruction
     imem_addr_o = insn_prefetch_addr + 'd4;
 
-    if (!insn_fetch_req_valid_i) begin
+    // Use the `insn_fetch_req_valid_raw_i` signal here as it doesn't factor in errors. This is
+    // important for timing reasons so errors don't factor into the `imem_addr_o` signal.
+    if (!insn_fetch_req_valid_raw_i) begin
       // Keep prefetching the same instruction when a new one isn't being requested. In this
       // scenario OTBN is stalled and will eventually want the prefetched instruction.
       imem_addr_o = insn_prefetch_addr;