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Cadence last week
announced the publication of a new book entitled TLM-Driven Design and
Verification Methodology. Available on-line
(ordering information and preview here), the book
describes in very practical terms what's needed to implement a
transaction-level modeling (TLM) based design and verification flow. In this
roundtable interview, four Cadence co-authors - Michael McNamara, Guy Mosenson,
Mike Stellfox, and Yosinori Watanabe - join with another co-author, consultant
Brian Bailey, to answer questions related to the book and its contents.
Q: There are many
books about ESL and ESL-related topics. Why do we need a new one? What does
this book say that hasn't been said before?
Bailey: As author
of two of those other [ESL] books, I can say that most of the other books out
there deal with tools or technologies. None of them have described a complete
methodology that you can use with those tools. And that is one of the big
things this book helps you achieve.
Mosenson: I think
there is no other book that addresses design and verification together in such
a thorough way. Also, the methodology is backed up with examples that
demonstrate the whole methodology for design and verification. I think that is
McNamara: The book addresses what's
necessary to move to the transaction level, and looks at both design and
verification. By describing both of those and how they work together, this is a
stand-on-its-own textbook that shows you how to move design to the next level.
Q: Many arguments
have been raised for moving from RTL up to a TLM level of abstraction. What are
the strongest arguments? Is reduced SoC development cost one of them?
Stellfox: I think
the biggest reason people want to move [to TLM] is the size of the designs we
have to both implement and verify today. You need more abstraction just to deal
with the size and complexity. On the design side, we see significant
productivity improvements compared to the manual hand capturing of RTL, and on
the verification side, we're seeing similar orders of magnitude improvements in
run-time performance and verification throughput.
In the past a lot of the focus was only on the design side. By
addressing both design and verification concurrently, we can have a significant
impact on the overall flow, which will translate into reduced cost.
Watanabe: I would
like to add two more arguments. The key words are "localization" and "reuse." Localization
means that as soon as you make design decisions you verify them. In the current
flow, design decisions can be made at different levels of abstraction, but
verification is done only at RTL. Here [TLM flow], at each level of abstraction
you make certain decisions and then you verify them. The verification assets
used to verify those decisions can be reused at lower levels of abstraction.
Reuse occurs not only within a single project, but reuse
across multiple projects, because we've raised the level of abstraction. Cost
is not only about single projects -- it's about how to reuse assets across
coming out of this global recession in which customers cut back our head count
on teams developing hardware and software. As customers come out of that, we
need to develop with smaller teams; we're not going back to the head count we
used to have. By adopting TLM design and verification, our customers can
develop a lot more products more efficiently using fewer people than were required
Q: The OSCI [Open
SystemC Initiative] TLM 2.0 standard, high-level synthesis tools, and virtual
prototypes are already in place...why is it necessary to define a methodology?
more complex the domain is, the more need there is for a proscribed experience
of how to do things optimally. There are pieces of things that are valuable,
but up to now there has not been a fully described methodology that shows how
to go through this complicated process of designing and verifying through multiple
levels of abstraction. The problem is complex and the answer requires a
thorough tryout and well-written guidance and examples.
Bailey: One of
the important aspects of ESL is that it's no longer single-domain, it's
multi-domain. It's about design and verification, it's about hardware and
software. We're going to see many domains coming together. As the solution gets
larger and larger, we need methodologies to go across these domains to make
sure everyone understands the process and the flow.
Q: The book advocates
SystemC and talks about high-level synthesis. But there is no standard SystemC
TLM synthesizable subset at present. How does the methodology get around that?
is a draft standard working its way through the [OSCI] committees. Already many
companies, including Cadence, support more than what's in the draft standard.
So when the draft standard becomes an actual standard, there will be a later
version that expands the standard and covers additional areas. By necessity the
standard is the least common denominator of what everybody supports.
Bailey: The OSCI
TLM standard itself was defined with verification in mind. So Cadence has
defined a synthesizable subset of that standard, which hopefully at some point
will become an industry standard as well.
Q: One concept
stressed in the book is the separation of computational and communications IP
blocks. Why is this important?
of reuse, it is important to separate different concerns that are independent
from each other. By separating these two, each model that is used for
computation or communication has further chances to be used in another project.
For example, a computation model can be reused in another project that uses
other communications mechanisms.
We also separate behavior and timing. When we capture the
model we try not to include timing unless it's absolutely necessary to define
the behavior itself. You can see in chapters 4 and 5 how we make this
separating the computational and communications parts of design IP, we can
verify them separately. You can verify TLM communications IP once, and then use
it in a number of different blocks that are computational in nature.
Q: Do you envision
the availability of third-party TLM IP?
Some IP today is already available as TLM models, although we don't think of it
as such. If you think about processors, instruction set simulators are TLM
models. There will be a growing demand for other kinds of models. Over time, it
will become a mandatory requirement for an IP developer to provide a TLM model.
McNamara: In the
early days of RTL, there wasn't a rich IP market. But in today's market, if
you're going to have a new abstraction, you need to have an ecosystem where
people can exchange models and have confidence that they're going to run in
various tools and be usable in in-house systems as well.
Q: As some of you
have noted, the book emphasizes verification as well as design. What needs to
change for verification environments to support a TLM based flow?
first thing we did is to build upon some of the tried-and-true approaches that
people have been using over the past 10 years. We extended the concepts of
metric-driven verification, automated constrained-random stimulus, and functional
coverage metrics. We also extended the OVM [Open Verification Methodology], and
now UVM [Universal Verification Methodology], to operate on TLM models and
multiple-abstraction designs. That means you could start with TLM or even with
a pure C model, and eventually end up with an RTL model, with various features
of the design verified in a localized way at the level of abstraction at which
the feature is introduced.
Mosenson: I like
to look at this as an evolutionary revolution. It's evolutionary because we
build upon what exists. And yet because of the multiple abstraction layers, in
a way it's revolutionary also. In this methodology, verification is
multi-language more than in the past, and reuse is more critical than in the
past. You are trying to reuse between different levels of abstraction. As Yoshi
said earlier, you find the best localized place to verify, then you try to
verify as early as possible and not repeat the verification.
Q: This book hardly
mentions the term "ESL" and doesn't refer to an "ESL flow." Why is that?
Bailey: This book
is a very down-to-earth, practical set of tools and methodologies to solve a
particular part of an ESL flow. By calling it a TLM-driven design and
verification methodology, we're a lot more specific about what the methodology
does and what it doesn't do.
Q: Finally, what's
the connection between this methodology and System Realization and SoC
Realization as described in the EDA360
is a component of both System Realization and SoC Realization, and some of
Silicon Realization. There's a design and verification aspect and some notion
of getting the software in there at all these levels. [TLM] is really a
foundational technology that helps at multiple realization levels.
design and verification is an enabler for System Realization concepts such as quickly
creating an architecture, exploring architectural tradeoffs, and supporting the
development of software. It comes into SoC Realization and Silicon Realization
through its connection to synthesis, RTL functional verification, and layout
Notes: In an earlier
interview, Brian Bailey described work he's done with Cadence on the
development of a TLM design and verification flow. He also discusses the book
in his blog
Felice Balarin of Cadence (left), not part of the roundtable discussion above, was also a co-author of this book.