Fix demux R channel backpressure

src/obi_demux.sv

@@ -40,6 +40,7 @@ module obi_demux #(
   logic [CounterWidth-1:0] in_flight;
   logic sbr_port_gnt;
   logic sbr_port_rready;
+  logic rsp_phase_stalled;
 
   select_t select_d, select_q;
 
@@ -53,7 +54,9 @@ module obi_demux #(
     sbr_port_gnt = 1'b0;
 
     if (!overflow) begin
-      if (sbr_port_select_i == select_q || in_flight == '0 || (in_flight == 1 && cnt_down)) begin
+      // R-4.1.1: block source changes while a stalled R phase is active
+      if (sbr_port_select_i == select_q || (!rsp_phase_stalled &&
+          (in_flight == '0 || (in_flight == 1 && cnt_down)))) begin
         mgr_ports_req_o[sbr_port_select_i].req = sbr_port_req_i.req;
         mgr_ports_req_o[sbr_port_select_i].a   = sbr_port_req_i.a;
         sbr_port_gnt                           = mgr_ports_rsp_i[sbr_port_select_i].gnt;
@@ -72,14 +75,18 @@ module obi_demux #(
 
   if (ObiCfg.UseRReady) begin : gen_rready
     assign sbr_port_rready = sbr_port_req_i.rready;
+    assign rsp_phase_stalled = sbr_port_rsp_o.rvalid && !sbr_port_rready;
+
     for (genvar i = 0; i < NumMgrPorts; i++) begin : gen_rready
       assign mgr_ports_req_o[i].rready = sbr_port_req_i.rready;
     end
   end else begin : gen_no_rready
     assign sbr_port_rready = 1'b1;
+    assign rsp_phase_stalled = 1'b0;
   end
 
-  assign cnt_down = mgr_ports_rsp_i[select_q].rvalid && sbr_port_rready;
+  // R-6: retire the active response only after its R phase transfer completes
+  assign cnt_down = sbr_port_rsp_o.rvalid && sbr_port_rready;
 
   delta_counter #(
     .WIDTH           ( CounterWidth ),
@@ -164,4 +171,3 @@ module obi_demux_intf #(
   );
 
 endmodule
-

src/obi_mux.sv

@@ -58,6 +58,8 @@ module obi_mux #(
   sbr_port_a_chan_t mgr_port_a_in_sbr;
   logic [RequiredExtraIdWidth-1:0] selected_id, response_id;
   logic mgr_port_req, fifo_full, fifo_pop;
+  logic [NumSbrPorts-1:0] sbr_rsp_rvalid;
+  sbr_port_r_chan_t [NumSbrPorts-1:0] sbr_rsp_r;
 
   rr_arb_tree #(
     .NumIn     ( NumSbrPorts       ),
@@ -135,10 +137,17 @@ module obi_mux #(
   end
 
   if (MgrPortObiCfg.UseRReady) begin : gen_rready_connect
-    assign mgr_port_req_o.rready = sbr_ports_req_i[response_id].rready;
+    logic routed_rsp_ready;
+
+    always_comb begin
+      // R-4.1.1: route readiness from the active response owner
+      routed_rsp_ready = sbr_ports_req_i[response_id].rready;
+    end
+
+    assign mgr_port_req_o.rready = routed_rsp_ready;
+  end else begin : gen_no_rready_connect
   end
-  logic [NumSbrPorts-1:0] sbr_rsp_rvalid;
-  sbr_port_r_chan_t [NumSbrPorts-1:0] sbr_rsp_r;
+
   always_comb begin : proc_sbr_rsp
     for (int i = 0; i < NumSbrPorts; i++) begin
       sbr_rsp_r[i] = '0;
@@ -154,6 +163,7 @@ module obi_mux #(
   end
 
   if (MgrPortObiCfg.UseRReady) begin : gen_fifo_pop
+    // R-6: pop the response owner only after its R phase transfer completes
     assign fifo_pop = mgr_port_rsp_i.rvalid && mgr_port_req_o.rready;
   end else begin : gen_fifo_pop
     assign fifo_pop = mgr_port_rsp_i.rvalid;