Unit Testing UVM Components: The Making Of

Today we take another step into the practical with a demonstration of how SVUnit can be used to test UVM components.

In the 3rd installment of the SVUnit Demo Series, I take people through a simple – yet complete – example of what’s required to test a UVM component within the SVUnit framework (if you haven’t seen the video yet, I’d recommend watching it here before reading on). The example I put together, and more importantly the plumbing under the hood required to make it work took me quite a while to put together for reasons that I don’t really get into in the video… but I will talk a bit about them here.

The usage model for SVUnit involves sequentially running a series of classes or modules through a corresponding list of unit test methods. In UVM however, due to the tight coupling between the phase methods in all uvm components and the instance of the uvm_root that ultimately drives the invocation of each phase method, uvm components are run in parallel.

One usage model wanting to be sequential and the other coded for parallelism gives us two models that are fundamentally at odds… so for unit testing UVM components, I had to get creative :).

I wouldn’t consider myself a UVM expert so the best way forward wasn’t immediately obvious. First I attempted to disable the components I wasn’t interested in by temporarily removing them from the component hierarchy. That was a dead end however because of the local access restrictions. My second idea was to replace components with a dud that effectively had no implementation… which means if the dud ran in place of an actual component, nothing would end up happening. That too was a dead end because of the same local access restrictions of the component hierarchy (in hindsight I should have known right away this was a no-go but I was still learning!).

The last implementation that ended up working involved creating and adding a new uvm_domain that ran in parallel with the existing UVM domain (where the run phases are stored). In the video, I talk about idle components that do nothing and a single unit under test that is driven through the run phases, iteratively if necessary. The idle components end up being idle because the newly created domain they’re assigned to – after of course going through the common phases of build, connect, end of elaboration and start of simulation – ends up raising an objection in the pre_reset_phase. That objection effectively means that components assigned to the domain never advance further. That’s what idle means… never advancing further than pre_reset.

I was pleasantly surprised to see that components could be assigned to different domains at any time which means I could activate and deactivate components whenever I wanted simply by changing which domain they’re assigned to.

When a component is deactivated, it is assigned to the new idle domain by…

function void svunit_deactivate_uvm_component(uvm_component c);
  c.set_domain(svunit_idle_uvm_domain::get_svunit_domain(), 1);

Similarly, where I show a component being activated, it is being assigned to the normal uvm domain by…

function void svunit_activate_uvm_component(uvm_component c);
  c.set_domain(uvm_domain::get_uvm_domain(), 1);

That seems to be the trick to being able to run uvm components sequentially in SVUnit without adding any new control or requirements to the components themselves, something I was looking to avoid from the outset.

If you want to take a closer look at the code, you can become an SVUnit project member and early adopter by getting a hold of me at neil.johnson@agilesoc.com.

Bonus marks go to the person that looks at the code and can suggest a better way of doing the same thing!


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