uvm中transaction的response和id的解读

在公司写代码的时候发现前辈有一段这样的代码：

....//其他transaction
`uvm_create(trans);
    ....
    ....
`uvm_send(trans);
tmp_id = trans.get_transaction_id();
get_response(rsp,tmp_id);

如果前面有其他transaction，这段代码里的get_response不带id的话，就会错误地get到前面transaction的response，有点好奇原理，就去看了看源码。

先从driver入手，如果要返回response的话，要先new一个rsp，然后set_id_info，再put_response：

//参考张强白皮书
seq_item_port.get_next_item(req);
drive_one_pkt(req);
rsp = new("rsp");
rsp.set_id_info(req);
seq_item_port.put_response(rsp);
seq_item_port.item_done();

先看set_id_info这个函数，在uvm_sequence_item.svh里面，作用就是将req的两个id set到rsp里，我们主要关注transaction_id，继续往后走。

  function void set_id_info(uvm_sequence_item item);
    if (item == null) begin
      uvm_report_fatal(get_full_name(), "set_id_info called with null parameter", UVM_NONE);
    end
    this.set_transaction_id(item.get_transaction_id());
    this.set_sequence_id(item.get_sequence_id());
  endfunction

get_transaction_id在uvm_transaction.svh里面，返回了m_transaction_id，这个id是uvm_transaction里面的一个local integer变量，默认值为-1。这个id的更改在其他地方，稍后再讲解。

function void uvm_transaction::set_transaction_id(integer id);
    m_transaction_id = id;
endfunction
function integer uvm_transaction::get_transaction_id();
    return (m_transaction_id);
endfunction

然后看put_response函数，在uvm_sequence.svh里，调用了put_base_response函数。

  virtual function void put_response(uvm_sequence_item response_item);
    RSP response;
    if (!$cast(response, response_item)) begin
      uvm_report_fatal("PUTRSP", "Failure to cast response in put_response", UVM_NONE);
    end
    put_base_response(response_item);
  endfunction

put_base_response在uvm_sequence_base.svh里，作用是将response压入队列中，队列默认长度为8，即response_queue_depth=8，当response在队列中的数量为8还继续压入队列后就会报错，一般发生在没有取response的情况下。

  virtual function void put_base_response(input uvm_sequence_item response);
    if ((response_queue_depth == -1) ||
        (response_queue.size() < response_queue_depth)) begin
      response_queue.push_back(response);
      return;
    end
    if (response_queue_error_report_disabled == 0) begin
      uvm_report_error(get_full_name(), "Response queue overflow, response was dropped", UVM_NONE);
    end
  endfunction

接下来到sequence端，我们看看transaction的id和response是怎么get的。transaction_id，在uvm_transaction.svh里，就是返回m_transaction_id。get_response在uvm_sequence.svh里，调用了get_base_response，这里的参数列表里就有transaction_id了。

  virtual task get_response(output RSP response, input int transaction_id = -1);
    uvm_sequence_item rsp;
    get_base_response( rsp, transaction_id);
    $cast(response,rsp);
  endtask

get_base_response在uvm_sequence_base.svh里，可以看到在没有指定transaction_id(id为默认值)的时候，返回的response是从队列直接pop的，指定了以后就遍历队列找id对应的response，这里就是最前面代码的机制。

  virtual task get_base_response(output uvm_sequence_item response, input int transaction_id = -1);
 
    int queue_size, i;
 
    if (response_queue.size() == 0)
      wait (response_queue.size() != 0);
 
    if (transaction_id == -1) begin
      response = response_queue.pop_front();
      return;
    end
 
    forever begin
      queue_size = response_queue.size();
      for (i = 0; i < queue_size; i++) begin
        if (response_queue[i].get_transaction_id() == transaction_id) 
          begin
            $cast(response,response_queue[i]);
            response_queue.delete(i);
            return;
          end
      end
      wait (response_queue.size() != queue_size);
    end
  endtask

但是到目前为止，response阶段没有看到id是怎么来的，我们从transaction的create和send阶段找一下。这些宏在uvm_sequence_defines.svh里，可以看到create阶段不涉及id的操作，send里面调了函数，可能会有。因为我们send的是transaction，是uvm_sequence_item类型，uvm_sequence_base是其子类，所以在uvm_send_pri里case不会成功，首先进入start_item里。

`define uvm_create(SEQ_OR_ITEM) \
  `uvm_create_on(SEQ_OR_ITEM, m_sequencer)
`define uvm_create_on(SEQ_OR_ITEM, SEQR) \
  begin \
  uvm_object_wrapper w_; \
  w_ = SEQ_OR_ITEM.get_type(); \
  $cast(SEQ_OR_ITEM , create_item(w_, SEQR, `"SEQ_OR_ITEM`"));\
  end
 
`define uvm_send(SEQ_OR_ITEM) \
  `uvm_send_pri(SEQ_OR_ITEM, -1)
`define uvm_send_pri(SEQ_OR_ITEM, PRIORITY) \
  begin \
  uvm_sequence_base __seq; \
  if (!$cast(__seq,SEQ_OR_ITEM)) begin \
     start_item(SEQ_OR_ITEM, PRIORITY);\
     finish_item(SEQ_OR_ITEM, PRIORITY);\
  end \
  else __seq.start(__seq.get_sequencer(), this, PRIORITY, 0);\
  end

start_item在uvm_sequence_base.svh里，貌似也不涉及id的操作，再看finish_item，也在vm_sequence_base.svh里。finish_item调用了sequencer的send_request函数，进去看看。

 virtual task start_item (uvm_sequence_item item,
                           int set_priority = -1,
                           uvm_sequencer_base sequencer=null);
    uvm_sequence_base seq;
     
    if(item == null) begin
      uvm_report_fatal("NULLITM",
         {"attempting to start a null item from sequence '",
          get_full_name(), "'"}, UVM_NONE);
      return;
    end
          
    if($cast(seq, item)) begin
      uvm_report_fatal("SEQNOTITM",
         {"attempting to start a sequence using start_item() from sequence '",
          get_full_name(), "'. Use seq.start() instead."}, UVM_NONE);
      return;
    end
          
    if (sequencer == null)
        sequencer = item.get_sequencer();
        
    if(sequencer == null)
        sequencer = get_sequencer();   
        
    if(sequencer == null) begin
        uvm_report_fatal("SEQ",{"neither the item's sequencer nor dedicated sequencer has been supplied to start item in ",get_full_name()},UVM_NONE);
       return;
    end
 
    item.set_item_context(this, sequencer);
 
    if (set_priority < 0)
      set_priority = get_priority();
    
    sequencer.wait_for_grant(this, set_priority);
 
    `ifndef UVM_DISABLE_AUTO_ITEM_RECORDING
      void'(sequencer.begin_child_tr(item, m_tr_handle, item.get_root_sequence_name()));
    `endif
 
    pre_do(1);
 
  endtask  
 
  virtual task finish_item (uvm_sequence_item item,
                            int set_priority = -1);
 
    uvm_sequencer_base sequencer;
    
    sequencer = item.get_sequencer();
 
    if (sequencer == null) begin
        uvm_report_fatal("STRITM", "sequence_item has null sequencer", UVM_NONE);
    end
 
    mid_do(item);
    sequencer.send_request(this, item);
    sequencer.wait_for_item_done(this, -1);
    `ifndef UVM_DISABLE_AUTO_ITEM_RECORDING
    sequencer.end_tr(item);
    `endif
    post_do(item);
 
  endtask

send_request在uvm_sequencer_param_base里，可以看到req的transaction_id就是在这里设置的，值为m_next_transaction_id，且每次设置完就自加1，这个值在uvm_sequence_base.svh里，初值为1。

function void uvm_sequencer_param_base::send_request(uvm_sequence_base sequence_ptr,
                                                     uvm_sequence_item t,
                                                     bit rerandomize = 0);
  REQ param_t;
 
  if (sequence_ptr == null) begin
    uvm_report_fatal("SNDREQ", "Send request sequence_ptr is null", UVM_NONE);
  end
 
  if (sequence_ptr.m_wait_for_grant_semaphore < 1) begin
    uvm_report_fatal("SNDREQ", "Send request called without wait_for_grant", UVM_NONE);
  end
  sequence_ptr.m_wait_for_grant_semaphore--;
  
  if ($cast(param_t, t)) begin
    if (rerandomize == 1) begin
      if (!param_t.randomize()) begin
        uvm_report_warning("SQRSNDREQ", "Failed to rerandomize sequence item in send_request");
      end
    end
    if (param_t.get_transaction_id() == -1) begin
      param_t.set_transaction_id(sequence_ptr.m_next_transaction_id++);
    end
    m_last_req_push_front(param_t);
  end else begin
    uvm_report_fatal(get_name(),$sformatf("send_request failed to cast sequence item"), UVM_NONE);
  end
 
  param_t.set_sequence_id(sequence_ptr.m_get_sqr_sequence_id(m_sequencer_id, 1));
  t.set_sequencer(this);
  if (m_req_fifo.try_put(param_t) != 1) begin
    uvm_report_fatal(get_full_name(), "Concurrent calls to get_next_item() not supported. Consider using a semaphore to ensure that concurrent processes take turns in the driver", UVM_NONE);
  end
 
  m_num_reqs_sent++;
  // Grant any locks as soon as possible
  grant_queued_locks();
endfunction

这下搞清楚了，transaction_id初值为1，且一个sequence里面每个transaction_id的值均不相同，每发一个req，id都会加一，通过这个id可以来区分不同的transaction！