RE: [sv-ac] Fwd: sv-ac: virtual interface application example

No, it does not mean that this is not an important issue.

Also, there can be a difference between what the LRM actually says and what it was intended to mean. Sometimes the written word is stronger than what it was intended to say. I'd not qualified to judge on this specific case.

Regards,
Shalom

From: owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] On Behalf Of Kulshrestha, Manisha
Sent: Monday, July 05, 2010 2:23 PM
To: Bresticker, Shalom; sv-ac@server.eda.org
Subject: RE: [sv-ac] Fwd: sv-ac: virtual interface application example

Hi Shalom,

This mantis is not assigned to anyone and it is quite old. Does that mean this is not an important issue and whatever explanation is there in the notes is sufficient ? It looks like there is no restriction on using virtual interfaces explicitly in the sensivitily lists as per the notes.

Thanks.
Manisha

________________________________
From: owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] On Behalf Of Bresticker, Shalom
Sent: Monday, July 05, 2010 1:50 PM
To: sv-ac@server.eda.org
Subject: RE: [sv-ac] Fwd: sv-ac: virtual interface application example
See Mantis 1441 (http://www.eda-stds.org/mantis/view.php?id=1441).

Shalom

From: owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] On Behalf Of Kulshrestha, Manisha
Sent: Monday, July 05, 2010 10:19 AM
To: ben@systemverilog.us
Cc: Eduard Cerny; sv-ac@eda.org; Korchemny, Dmitry
Subject: RE: [sv-ac] Fwd: sv-ac: virtual interface application example

Hi,

Yes it is. I do not know what exactly they mean by sensitivity lists. I think LRM is not clear on this. I'll see if I can get opinion on this from somebody in sv-ec.

Manisha
________________________________
From: owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] On Behalf Of ben cohen
Sent: Monday, July 05, 2010 11:32 AM
To: Kulshrestha, Manisha
Cc: Eduard Cerny; sv-ac@eda.org; Korchemny, Dmitry
Subject: Re: [sv-ac] Fwd: sv-ac: virtual interface application example
Manisha,
2. Using class variables in clock expression. Currently usage of class variables in clock expressions is not well defined. In your example, the expect needs to be explicitly clocked (it does not infer clock). We have seen usage of class variables in clock expressions earlier and I filed a mantis item (001513: LRM does not clarify if class variables can be used in clock expressions under SV-EC). Here is something from LRM which prohibits use of virtual interface in sensitivity lists. If we use virtual interface in the clock expression for expect, we are effectively using it in sensitivity list:

Once a virtual interface has been initialized, all the components of the underlying interface instance are
directly available to the virtual interface via the dot notation. These components can only be used in
procedural statements; they cannot be used in continuous assignments or sensitivity lists

[Ben] I sort of disagree that If we use virtual interface in the clock expression for expect, we are effectively using it in sensitivity list:, The LRM that you quote just says that the following is illegal:
  assign some_virutal_if.w = some_virutal_if.q;
 always (@ some_virutal_if.w) ???? Illegal But per LRM example ???
task wait_for_bus(); // wait for the bus to be granted
@(posedge bus.grant); [Ben] Isn't that a sensitivity list?
endtask

Can I get some clarification as to why the LRM example @(posedge bus.grant); is legal ?
Ben
On Wed, Jun 23, 2010 at 1:08 AM, Kulshrestha, Manisha <Manisha_Kulshrestha@mentor.com<mailto:Manisha_Kulshrestha@mentor.com>> wrote:
Hi Ben,

There are few issues here that we need to address.

1. The virtual interface is attached to an instance of a class (or class object). So, different class objects (of the same class) can have different interface objects attached to them. Even same class object can refer to different virtual interfaces at different times. It is possible that in the middle of execution of expect (while it is waiting), the virtual interface object changes.
Here is some text from LRM:

Virtual interface variables may be passed as arguments to tasks, functions, or methods. A single virtual interface variable can thus represent different interface instances at different times throughout the simulation. A virtual interface shall be initialized before referencing a component of the virtual interface; it has the value null before it is initialized. Attempting to use a null virtual interface shall result in a fatal run-time error.

2. Using class variables in clock expression. Currently usage of class variables in clock expressions is not well defined. In your example, the expect needs to be explicitly clocked (it does not infer clock). We have seen usage of class variables in clock expressions earlier and I filed a mantis item (001513: LRM does not clarify if class variables can be used in clock expressions under SV-EC). Here is something from LRM which prohibits use of virtual interface in sensitivity lists. If we use virtual interface in the clock expression for expect, we are effectively using it in sensitivity list:

Once a virtual interface has been initialized, all the components of the underlying interface instance are
directly available to the virtual interface via the dot notation. These components can only be used in
procedural statements; they cannot be used in continuous assignments or sensitivity lists.

2. Usage of automatic variables in expect. Although LRM currently allows usage of automatic variables in expect, the assumption is that the automatic variable will not change in the middle of execution of expect. Even procedural concurrent assertions are allowed to use automatic variables but they are considered constants. The value of these variables is captured at the time of attempt and the same value is used through out the evaluation of that attempt. Is that how you see the usage of automatic signals in your example, or you would like the expect to use the latest values.

Thanks.
Manisha

________________________________
From: owner-sv-ac@eda.org<mailto:owner-sv-ac@eda.org> [mailto:owner-sv-ac@eda.org<mailto:owner-sv-ac@eda.org>] On Behalf Of ben cohen
Sent: Wednesday, June 23, 2010 3:29 AM
To: Eduard Cerny; sv-ac@eda.org<mailto:sv-ac@eda.org>; Korchemny, Dmitry
Subject: Re: [sv-ac] Fwd: sv-ac: virtual interface application example
Ed,
That particular example is from my VMM book, and does not use the expect. Here are 2 examples; One with a modification that could use the expect, and another example.
class Fifo_cmd_xactor extends vmm_xactor;
  virtual fifo_if.fdrvr_if_mp f_if;
  virtual fifo_if.fslave_if_mp s_if;
  ...
  function new(...
               virtual fifo_if.fdrvr_if_mp new_vir_if,
               virtual fifo_if.fslave_if_mp new_svir_if;
               ... );
      this.f_if = new_vir_if;
      this.s_if = new_svir_if;
     ...
  endfunction : new
endclass: Fifo_cmd_xactor

 task Fifo_cmd_xactor::push_task (word_t data);
    @ ( f_if.driver_cb);
    expect (s_if.xxx ##[1:5} s_if.yyy); // <----
    f_if.driver_cb.data_in <= data;
    f_if.driver_cb.push <= 1'b1;
    f_if.driver_cb.pop <= 1'b0;
    @ ( f_if.driver_cb);
    f_if.driver_cb.push <= 1'b0;

  endtask : push_task

// NEW EXAMPLE with Action Blocks
However, here is another example
class mst_bfm; // driver class
  virtual master_if.fdrvr_if_mp v_if;
  function new( ...
  virtual master_if.fdrvr_if_mp new_vir_if, ..);
this.v_if = new_vir_if;
..
endfunction : new
...
task continue2() .. endtask : continue2 // What to do if response to abort is the ABORTED
task abort_error(); ... endtask : abort_error // What to do if response to abort is not ABORTED
task driver(pkt_c in_pkt);
// code..
  vif.abort <= 1'b1;
  expect ( @(posedge vif.clk) vif.mst_abort ##[5:8] vif.slv_resp == ABORTED)
     continue2(); // pass action block
    else abort_error() ); // fail action block
endtask : driver
endclass : mst_bfm

In fact, this is how envisioned the application of the "expect" in a class.
Ben

On Tue, Jun 22, 2010 at 12:28 PM, Eduard Cerny <Eduard.Cerny@synopsys.com<mailto:Eduard.Cerny@synopsys.com>> wrote:
Hi Ben,

did you say that the example uses an expect? I do not see one.

Thanks,
ed

From: owner-sv-ac@eda.org<mailto:owner-sv-ac@eda.org> [mailto:owner-sv-ac@eda.org<mailto:owner-sv-ac@eda.org>] On Behalf Of ben cohen
Sent: Tuesday, June 22, 2010 1:30 PM
To: Korchemny, Dmitry; sv-ac@eda.org<mailto:sv-ac@eda.org>
Subject: [sv-ac] Fwd: sv-ac: virtual interface application example

---------- Forwarded message ----------
From: ben cohen <hdlcohen@gmail.com<mailto:hdlcohen@gmail.com>>
Date: Tue, Jun 22, 2010 at 9:58 AM
Subject: sv-ac: virtual interface application example
To: sv-ac@eda.org<mailto:sv-ac@eda.org>
 `define TOP fifo_tb
module fifo_tb;
  ...
  fifo_if f_if(.*); // instantiation of fifo interface
endmodule : fifo_tb

class Fifo_env extends vmm_env; // ENVIRONMENT
  ....
  Fifo_cmd_xactor fifo_cmd_xactor_0; // command-layer declaration
  ...
endclass : Fifo_env

 function void Fifo_env::build();
  ...
    this.fifo_cmd_xactor_0 = new("cmd_xactor",
                        0,
                        `TOP.f_if, // <----- actual interface passed
                        fifo_channel_0,
                        fifo_response_chan0
                        );
  ...

endfunction : build

class Fifo_cmd_xactor extends vmm_xactor;
  virtual fifo_if.fdrvr_if_mp f_if;
  ...
  function new(...
               virtual fifo_if.fdrvr_if_mp new_vir_if,
               ... );
      this.f_if = new_vir_if;
     ...
  endfunction : new
endclass: Fifo_cmd_xactor

  task Fifo_cmd_xactor::push_task (word_t data);
    f_if.driver_cb.data_in <= data;
    f_if.driver_cb.push <= 1'b1;
    f_if.driver_cb.pop <= 1'b0;
    @ ( f_if.driver_cb);
    f_if.driver_cb.push <= 1'b0;
  endtask : push_task

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