Casting the driver's response item into request item

UVM
,
1

Hi
In the following block of code:


// Somewhere in a driver - request and response items
bus_seq_item req_item;
bus_seq_item rsp_item;
 
task run_phase( uvm_phase phase );
 
  forever begin
    seq_item_port.get(req_item);
    assert($cast(rsp_item, req_item.clone()); // This does not copy the id info
    rsp_item.set_id_info(req_item); // This sets the rsp_item id to the req_item id
    //
    // Do the pin level transaction, populate the response fields
    //
    // Return the response:
    seq_item_port.put(rsp_item);
    //
  end
endtask: run

I understand that we need to clone the response so that successive responses point to different objects. But why are we casting a clone of req_item into the rsp_item? What does req_item have to do with the response, apart from the id which is handled separately using set_id_info?
From what I understand, we could replace the above code with the following.


// Somewhere in a driver - request and response items
bus_seq_item req_item;
bus_seq_item rsp_item;
 
task run_phase( uvm_phase phase );
 
  forever begin
    seq_item_port.get(req_item);
    //assert($cast(rsp_item, req_item.clone()); // This does not copy the id info
    rsp_item.set_id_info(req_item); // This sets the rsp_item id to the req_item id
    //
    // Do the pin level transaction, populate the response fields
    //
    // Return the response:
    seq_item_port.put(rsp_item.clone());
    //
  end
endtask: run

Please enlighten me about what is actually missed in the second piece of code.
Thank you

2

In reply to Farhad:

Both the uvm_sequencer and uvm_driver allow you to specify different types for both the request and response, so it is possible that there is no relationship between either item. When this is the case, the only common item that correlates the response to the request is the request id. Also, since the request and response types may be different, you can’t rely on the clone() function being usable, so UVM added the set_id_info() function to make it easy for the user.

However, it is very common that the request and response types are the same. Often, the user wants the response to have the same data (i.e. address, read/write, etc.) as the request, so the clone() function is used.

In your second code snippet, you will get a fatal error since you don’t create() the rsp_item. You will need to do this prior to calling set_id_info().

Also, if the request and response types are the same, it is common that the request itself is updated with the transaction response data and status, and no response is used. Since only the handle of the request is passed to the driver, the updates can be seen by the sequence when the sequence_item is finished.