[i2c,dv] Extend i2c_agent to verify host clock stretching

  Host can stretch its clock (scl_o) in case device needs more
  time to proceed a received byte. Meaning that, the device
  pulls down scl_i after clock cycle 9th then the host can hold
  its scl_o at high until the device releases scl_i.

  To enable clock stretching feature for the host,
  the TIMEOUT_CTRL.EN must be set, and TIMEOUT_CTRL.VAL must
  be programmed to a value which is higher than the basic clock pulse.
  Once the host clock is stretched longer than the basic clock pulse,
  the intr_stretch_timeout_o will be asserted.

Signed-off-by: Tung Hoang <tung.hoang.290780@gmail.com>

hw/dv/sv/i2c_agent/i2c_device_driver.sv

diff --git a/hw/dv/sv/i2c_agent/i2c_device_driver.sv b/hw/dv/sv/i2c_agent/i2c_device_driver.sv
index 362880e..fc500d8 100644
--- a/hw/dv/sv/i2c_agent/i2c_device_driver.sv
+++ b/hw/dv/sv/i2c_agent/i2c_device_driver.sv
@@ -11,7 +11,9 @@
   constraint rd_data_c { rd_data inside {[0 : 127]}; }
 
   virtual task get_and_drive();
+    int num_stretch_host_clks;
     i2c_item rsp_item;
+
     @(posedge cfg.vif.rst_ni);
     forever begin
       cfg.vif.scl_o = 1'b1;
@@ -20,7 +22,23 @@
       seq_item_port.get_next_item(rsp_item);
       unique case (rsp_item.drv_type)
         DevAck: begin
-          cfg.vif.device_send_ack(cfg.timing_cfg);
+          fork
+            begin
+              // host clock stretching allows a high-speed host to communicate
+              // with a low-speed device by setting TIMEOUT_CTRL.EN bit
+              // the device ask host clock stretching its clock scl_i by pulling down scl_o
+              // the host clock pulse is extended until device scl_o is pulled up
+              // once scl_o is pulled down longer than TIMEOUT_CTRL.VAL field,
+              // intr_stretch_timeout_o is asserted (ref. https://www.i2c-bus.org/clock-stretching)
+              if (cfg.timing_cfg.enbTimeOut) begin
+                num_stretch_host_clks = gen_num_stretch_host_clks(cfg.timing_cfg);
+                cfg.vif.device_stretch_host_clk(cfg.timing_cfg, num_stretch_host_clks);
+              end
+            end
+            begin
+              cfg.vif.device_send_ack(cfg.timing_cfg);
+            end
+          join
         end
         RdData: begin
           `DV_CHECK_MEMBER_RANDOMIZE_FATAL(rd_data)
@@ -38,5 +56,13 @@
     end
   endtask : get_and_drive
 
+  function int gen_num_stretch_host_clks(ref timing_cfg_t tc);
+    // By randomly pulling down scl_o "offset" within [0:2*tc.tTimeOut],
+    // intr_stretch_timeout_o interrupt would be generated uniformly
+    // To test this feature more regressive, there might need a dedicated vseq (V2)
+    // in which TIMEOUT_CTRL.EN is always set.
+    return $urandom_range(tc.tClockPulse, tc.tClockPulse + 2*tc.tTimeOut);
+  endfunction : gen_num_stretch_host_clks
+
 endclass : i2c_device_driver
 

hw/dv/sv/i2c_agent/i2c_if.sv

diff --git a/hw/dv/sv/i2c_agent/i2c_if.sv b/hw/dv/sv/i2c_agent/i2c_if.sv
index a96a1f8..cdd872a 100644
--- a/hw/dv/sv/i2c_agent/i2c_if.sv
+++ b/hw/dv/sv/i2c_agent/i2c_if.sv
@@ -138,7 +138,6 @@
   endtask: wait_for_device_ack
 
   task automatic device_send_ack(ref timing_cfg_t tc);
-    device_stretch_clk(tc);
     sda_o = 1'b1;
     wait_for_dly(tc.tClockLow);
     sda_o = 1'b0;
@@ -150,7 +149,6 @@
 
   task automatic device_send_bit(ref timing_cfg_t tc,
                                  input bit bit_i);
-    device_stretch_clk(tc);
     sda_o = 1'b1;
     wait_for_dly(tc.tClockLow);
     sda_o = bit_i;
@@ -160,13 +158,13 @@
     sda_o = 1'b1;
   endtask: device_send_bit
 
-  task automatic device_stretch_clk(ref timing_cfg_t tc);
-    if (tc.enbTimeOut) begin
-      scl_o = 1'b0;
-      wait_for_dly(tc.tTimeOut);
-      scl_o = 1'b1;
-    end
-  endtask : device_stretch_clk
+  task automatic device_stretch_host_clk(ref timing_cfg_t tc,
+                                         input int num_stretch_host_clks);
+    wait_for_dly(tc.tClockLow + tc.tSetupBit);
+    scl_o = 1'b0;
+    wait_for_dly(num_stretch_host_clks);
+    scl_o = 1'b1;
+  endtask : device_stretch_host_clk
 
   task automatic get_bit_data(string src = "host",
                               ref timing_cfg_t tc,