Re: [POSSIBLE VIRUS:###] Re: [sv-ac] Fwd: sv-ac: virtual interface application example

[Ben] See my comments:

On Wed, Jun 23, 2010 at 10:01 PM, Kulshrestha, Manisha <
Manisha_Kulshrestha@mentor.com> wrote:

> Hi,
>
> Please see my comments:
>
> ------------------------------
> *From:* owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] *On Behalf Of *ben
> cohen
> *Sent:* Thursday, June 24, 2010 3:41 AM
> *To:* Kulshrestha, Manisha
> *Cc:* Eduard Cerny; sv-ac@eda.org; Korchemny, Dmitry
> *Subject:* [POSSIBLE VIRUS:###] Re: [sv-ac] Fwd: sv-ac: virtual interface
> application example
>
> Manisha,
> Thanks for your detailed explanations. Please bear with me on some
> comments, see below:
>
> 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.
>>
> Consider this example: (complete VMM model is attached, but without the
> expect. In previous email (further down in this email is a snippet with the
> expect). In the environment (the Fifo_env class), I can see the
> possibility of assigning to null the object where the expect were to be used
> in a class, and the expect was in the wait state. Specifically, one can do
> after a certain run time the following:
> this.fifo_cmd_xactor_0 = null;
> ... some time later
> this.fifo_cmd_xactor_0 = new("cmd_xactor",
>
> 0,
>
> * `TOP.f_if,* // Same or different interface <----- actual interface passed
>
> fifo_channel_0,
>
> fifo_response_chan0
>
> );
>
> *`define TOP 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
>
>>
>>
>>
> So far, so good! But what are the complications of doing such a thing? If
> the class instance is nulled in the middle of an expect, then everything in
> that class goes away. I don't understand the implications here.
>
> MK: What I wanted to say was that we need to define what happens in such
> cases. If interface object changes or becomes null (not the class
> object), how does expect behave. Is it allowed or tools gives error in this
> case ?
>
[Ben] How can the interface object change without a new of the class object?
 Isn't this where the new of the class links the virtual interface to the
actual? Again, please bear with me with those questions, They might be
obvious to you, but not to me.

>
> 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.
>>
> That kills the application of something like:
> *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*
> Is there a way to fix this? Do we want that? If one vendor implemented
> such application, what assumptions did he do?
>
> MK: I do not know what assumptions any specific vendor made. What I am
> saying is that this restriction in the virtual interfaces has to go away and
> the behaviour needs to be defined if we want to support virtual interfaces
> in the clock expression.
>
[Ben] I really would like us to support the virtual interfaces for the
expect.

>
>> 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.
>>
>> Can you sho an example of the expect with automatic variables?
>>
>> MK: There is an example in the LRM in expect section. Here value is an
>> automatic variable.
>>
>> *
>>
>> integer
>> *data;
>>
>> ...
>> *
>>
>> task automatic
>> *wait_for( *integer *value, *output bit *success );*
>>
>> expect
>> *( @(*posedge *clk) ##[1:10] data == value ) success = 1;*
>>
>> else
>> *success = 0;*
>>
>> endtask
>>
>> initial begin
>>
>> bit
>> *ok;
>>
>> wait_for( 23, ok ); // wait for the value 23
>>
>> ...
>> *
>>
>> end
>> *
>>
> [Ben] Now, I want:
*

integer data;
*
*

task automatic wait_for( integer value, output bit success );
**

expect
*( @(*posedge *clk) ##[1:10] data == my_virtual_interface_object.data)
success = 1;*

else
*success = 0;
*

endtask
*

>
> Ben
>
>> Thanks.
>> Manisha
>>
>> ------------------------------
>> *From:* 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; 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
>> > 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] *On Behalf Of *ben
>>> cohen
>>> *Sent:* Tuesday, June 22, 2010 1:30 PM
>>> *To:* Korchemny, Dmitry; sv-ac@eda.org
>>> *Subject:* [sv-ac] Fwd: sv-ac: virtual interface application example
>>>
>>>
>>>
>>>
>>>
>>> ---------- Forwarded message ----------
>>> From: *ben cohen* <hdlcohen@gmail.com>
>>> Date: Tue, Jun 22, 2010 at 9:58 AM
>>> Subject: sv-ac: virtual interface application example
>>> To: 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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>>
>>
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