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DVCon OVM Panelists: Easing The Debug Challenge

Comments(0)Filed under: Industry Insights, DVCon, specman, OVM, verification, Debug

The good news about the Open Verification Methodology (OVM) and the advanced verification techniques it supports is that verification engineers are now finding more bugs than ever. The bad news is that the bottleneck is shifting to debugging. What are we going to do with all those bugs? User and vendor representatives grappled with that question at a Cadence-sponsored lunch panel at DVCon Wednesday, Feb. 24.

Moderator Mike Stellfox, principal verification solutions architect at Cadence, noted that OVM and techniques such as constrained-random test generation have dramatically shortened the time it takes to build a testbench. "We're finding a lot more bugs than we could with traditional directed-test approaches, but now we're getting more bugs than we can debug in a timely manner," he said. In a recent Cadence survey, he noted, advanced verification users said that debug now takes about 50 percent of the overall verification effort.

Panelists included Steve Jorgenson, verification specialist at Hewlett-Packard; Umer Yousafzai, solutions architect at Cadence; Alicia Strang, principal verification engineer at Marvell; and Yuchin Hsu, vice president of R&D for verification products at Springsoft.

With an emphasis on the user presenters, here are several perspectives that I thought were interesting.

How OVM helps with debug

"We find that OVM is very useful," said Strang. "If you just use that methodology, and use the naming conventions, everybody is on the same page."

In a conventional directed-test environment, Yousafzai said, users spend a lot of time debugging test stimulus. In OVM, which supports up-front planning, users spend less time tracking down problems in the testbench and more time finding bugs in the design. Jorgenson noted that OVM supports higher levels of abstraction, and said that "I really think abstraction is what will save us in the end from our debugging problems."

Testbenches need debugging, too

Jorgenson commented that verification teams can easily spend half their time just debugging problems in the testbench. One antidote is to move up in abstraction and reuse as much as possible from previous verification environments. Strang had another suggestion. "In a modern software environment like a testbench, you can do whatever you want. Keep it simple, and you can avoid a lot of bugs in the testbench."

Constrained-random stimulus may obscure bug causes

When a bug is found in a directed-test environment, Jorgenson said, a designer can very likely help identify the root cause. But with constrained-random techniques, "you don't always know what the test did. One thing we've had to do is go in there and get the test to tell us what it did."

Assertions are important

Strang noted that some kinds of bugs can be found only with assertions. These include bugs that do not affect data. "If it's data in, garbage out, I know it's wrong, but if it's data in, data out I don't see anything wrong," she said. Jorgenson said his group has been using assertions for 8 or 9 years, long before SystemVerilog assertions were standardized.

Yousafzai told of a customer who ran a single test 30,000 times on a block, and still had to do a respin because all these tests couldn't find a "trivial" bug - a signal that wasn't toggling correctly under some conditions. He noted that simple assertions that can be added easily are often the most effective ones.

How to find software bugs

Strang uses transaction monitors that generate waveforms that show what firmware is doing in parallel with hardware signals. From this, she said, "I can tell whether software issued a wrong command, or hardware is behaving incorrectly."

A conclusion

As I noted in a blog last year, based on part on an interview with Mike Stellfox, the bug diagnosis-and-repair cycle is becoming the biggest bottleneck in the verification flow. If debug really takes 50 percent of the verification effort - and verification takes 60 to 70 percent of the total design effort, as we've repeatedly heard - do the math and you can see this is an area that deserves far more attention. The OVM lunch panel was thus a very timely event.

Richard Goering

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