[i2c] Add StretchAcqSetup state to preserve ACK setup time

Add StretchAcqSetup state to follow the release of the hold from
StretchAcqFull after the ACQ FIFO gains the necessary space. Continue to
stretch SCL so the change to SDA doesn't violate setup time, then
proceed to the normal ACK setup state.

Remove the immediate write to the ACQ FIFO in the StretchAcqFull state
when we're proceeding to a normal ACK. This created extra ACQ FIFO
writes when it transitioned to the AcquireAckWait state, and the FSM
gets simpler and less error-prone if only error cases operate on FIFOs
outside of the non-stretch states.

Signed-off-by: Alexander Williams <[email protected]>

hw/ip/i2c/dv/env/seq_lib/i2c_glitch_vseq.sv

@@ -49,20 +49,24 @@ class i2c_glitch_vseq extends i2c_target_smoke_vseq;
     // Target function clock stretch handling.
     StretchAddr,
     StretchTx, StretchTxSetup,
-    StretchAcqFull
+    StretchAcqFull, StretchAcqSetup
   } state_e;
 
   // initialize the states in which glitch is to be introduced
   // Ignore state transitions covered in other testcases
   // WaitForStop -> Idle and WaitForStop -> AcquireStart
   // TransmitAckPulse -> Idle and TransmitAckPulse -> AcquireStart
   // AcquireByte -> Idle and AcquireByte -> AcquireStart
+  // StretchAcqSetup is also an invalid start, as a control symbol is impossible
+  // in this state.
   state_e read_states[]  = '{TransmitWait, TransmitSetup, TransmitPulse, TransmitHold, TransmitAck,
                             TransmitAckPulse, StretchTx, StretchTxSetup};
   state_e write_states[] = '{AcquireAckWait, AcquireAckSetup, AcquireAckPulse, AcquireAckHold,
                              StretchAcqFull};
   state_e addr_states[]  = '{StretchAddr, AddrAckWait, AddrAckSetup, AddrAckPulse, AddrAckHold};
-  state_e scl_high_states[]  = '{TransmitPulse, AcquireAckPulse, AddrAckPulse};
+  // AddrAckSetup is here because SCL and SDA can change simultaneously,
+  // leading to failures after the modeled CDC random insertion delay.
+  state_e scl_high_states[]  = '{TransmitPulse, AcquireAckPulse, AddrAckSetup, AddrAckPulse};
 
   // Common steps for DUT initialization
   virtual task setup();

hw/ip/i2c/rtl/i2c_target_fsm.sv

@@ -333,7 +333,7 @@ module i2c_target_fsm import i2c_pkg::*;
     // Target function clock stretch handling.
     StretchAddr,
     StretchTx, StretchTxSetup,
-    StretchAcqFull
+    StretchAcqFull, StretchAcqSetup
   } state_e;
 
   state_e state_q, state_d;
@@ -585,12 +585,15 @@ module i2c_target_fsm import i2c_pkg::*;
           nack_transaction_d = 1'b1;
           acq_fifo_wvalid_o = !acq_fifo_full_or_last_space;
           acq_fifo_wdata_o = {AcqNack, input_byte};
-        end else if (!stretch_rx) begin
-          // When space becomes available, deposit entry
-          acq_fifo_wvalid_o = 1'b1;                 // assert that acq_fifo has space
-          acq_fifo_wdata_o = {AcqData, input_byte}; // transfer data to acq_fifo
         end
       end
+      // StretchAcqSetup: Drive the ACK and wait for tSetupData before
+      // releasing SCL
+      StretchAcqSetup : begin
+        target_idle_o = 1'b0;
+        scl_d = 1'b0;
+        sda_d = 1'b0;
+      end
       // default
       default : begin
         target_idle_o = 1'b1;
@@ -882,15 +885,23 @@ module i2c_target_fsm import i2c_pkg::*;
         end
       end
       // StretchAcqFull: target stretches the clock when acq_fifo is full
-      // When space becomes available, deposit the entry into the acqusition fifo
-      // and move on to receive the next byte.
-      // If we hit our NACK timeout we must continue and unconditionally NACK
-      // the next one.
+      // When space becomes available, move on to prepare to ACK. If we hit
+      // our NACK timeout we must continue and unconditionally NACK the next
+      // one.
       StretchAcqFull : begin
         if (nack_timeout) begin
           state_d = WaitForStop;
         end else if (~stretch_rx) begin
-          state_d = AcquireByte;
+          state_d = StretchAcqSetup;
+          load_tcount = 1'b1;
+          tcount_sel = tSetupData;
+        end
+      end
+      // StretchAcqSetup: Drive the ACK and wait for tSetupData before
+      // releasing SCL
+      StretchAcqSetup : begin
+        if (tcount_q == 20'd1) begin
+          state_d = AcquireAckSetup;
         end
       end
       // default