[pattgen,dv] Reformat and document pattgen_cnt_rollover_vseq

Unfortunately, it doesn't actually do what it's supposed to do! Add a
big TODO notice linked to an issue that tracks the problem.

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

hw/ip/pattgen/dv/env/seq_lib/pattgen_cnt_rollover_vseq.sv

@@ -2,33 +2,51 @@
 // Licensed under the Apache License, Version 2.0, see LICENSE for details.
 // SPDX-License-Identifier: Apache-2.0
 
-// Count rollover test vseq
+// TODO(#26317):
+//
+//   This sequence doesn't really satisfy the cnt_rollover testpoint. The maximum prediv value is
+//   (1<<32-1), which is very large! The code below actually constrains to a 16-bit prediv and also
+//   overly constrains len and reps.
+//
+//   Unfortunately, the clk_cnt_q signal in pattgen_chan isn't actually exposed to other blocks. If
+//   we want to work with large values, we'll probably need to force its value to "just below
+//   prediv_q")
+
 class pattgen_cnt_rollover_vseq extends pattgen_base_vseq;
   `uvm_object_utils(pattgen_cnt_rollover_vseq)
 
-  // reduce num_trans due to long running simulations
-  constraint num_trans_c        { num_trans inside {[2 : 3]}; }
-
-  function new (string name="");
-    super.new(name);
-    pattgen_max_dly = 0;
-  endfunction
-
-  // override this function for pattgen_cnt_rollover test
-  function pattgen_channel_cfg get_random_channel_config(uint channel);
-    bit [1:0] is_max;
-    pattgen_channel_cfg ch_cfg;
-    ch_cfg = pattgen_channel_cfg::type_id::create($sformatf("channel_cfg_%0d", channel));
-    `DV_CHECK_RANDOMIZE_WITH_FATAL(ch_cfg,
-      prediv dist {0 :/ 1, [1 : 'hfffe] :/ 1, 'hffff :/ 1};
-      len    dist {0 :/ 1, [1 : 'he] :/ 1, 'hf :/ 1};
-      reps   dist {0 :/ 1, [1 : 'h3e] :/ 1, 'h3f :/ 1};
-      ((prediv+1) * (len+1) * (reps+1)) <= 'h1_0000;
-      // dependent constraints
-      solve len before data;
-      data inside {[0 : (1 << (len + 1)) - 1]};
-    )
-    return ch_cfg;
-  endfunction : get_random_channel_config
-
-endclass : pattgen_cnt_rollover_vseq
+  // Because the constraint on the sizes below is quite large, a simulation takes a long time.
+  // Constrain num_trans to be reasonably small, in order to keep reasonable run times.
+  extern constraint num_trans_c;
+
+  extern function new(string name="");
+
+  // This function is defined in pattgen_base_vseq, but we override it here to constrain the channel
+  // configuration to be reasonably large.
+  extern function pattgen_channel_cfg get_random_channel_config(uint channel);
+endclass
+
+constraint pattgen_cnt_rollover_vseq::num_trans_c {
+  num_trans inside {[2 : 3]};
+}
+
+function pattgen_cnt_rollover_vseq::new(string name="");
+  super.new(name);
+  pattgen_max_dly = 0;
+endfunction
+
+function pattgen_channel_cfg pattgen_cnt_rollover_vseq::get_random_channel_config(uint channel);
+  bit [1:0] is_max;
+  pattgen_channel_cfg ch_cfg;
+  ch_cfg = pattgen_channel_cfg::type_id::create($sformatf("channel_cfg_%0d", channel));
+  `DV_CHECK_RANDOMIZE_WITH_FATAL(ch_cfg,
+    prediv dist {0 :/ 1, [1 : 'hfffe] :/ 1, 'hffff :/ 1};
+    len    dist {0 :/ 1, [1 : 'he] :/ 1, 'hf :/ 1};
+    reps   dist {0 :/ 1, [1 : 'h3e] :/ 1, 'h3f :/ 1};
+    ((prediv+1) * (len+1) * (reps+1)) <= 'h1_0000;
+    // dependent constraints
+    solve len before data;
+    data inside {[0 : (1 << (len + 1)) - 1]};
+  )
+  return ch_cfg;
+endfunction