Functional Verification from a Manager's Perspective: When is 'Good Enough', Really Good Enough? Ira Chayut, Verification Architect (opinions are my own and do not necessarily represent the opinion of my employer)

2 Topics Achieving Balance Functional Verification Overview Optimizing Functional Verification Living with Functional (and Architectural) Errors Conclusions Questions

3 Achieving Balance Early Tapeout “Complete” Verification

4 Achieving Balance Early Tapeout Critical bugs missed Respin(s) Missed Market Window “Complete” Verification

5 Achieving Balance Early Tapeout “Complete” Verification Increased resources used Delayed tapeout Missed Market Window

6 Achieving Balance Early Tapeout Missed Market Window “Complete” Verification Missed Market Window

7 Achieving Balance Early Tapeout “Complete” Verification Balance

8 Achieving Balance Early Tapeout “Complete” Verification Diversify to manage risk Employ multiple verification techniques The whole is greater than the sum of the parts Plan to succeed, even in the presence of errors Balance

9 Functional Verification Overview

10 What is Functional Verification “Functional Verification” is the task of checking that the design implements the architecture specified, such as in:

11 A Functional Verification Example Reference models and Devices Under Test (DUTs) can be C models, RTL models, FPGA prototypes, emulation implementations, or silicon.

12 Good Enough for What? During different phases of an ASIC development, the term “good enough” can change meaning, the RTL can be good enough to: Start running simple directed tests Start running all written directed tests Synthesize for size estimates Synthesize for FPGA prototype Take to emulation Tape out Ship silicon to customers Focus of this talk

13 Why Do We Do Functional Verification The cost of extra spins is enormous (the lost opportunity costs can dwarf the cost of new masks, etc.) Even one critical bug can cause a respin We wish to greatly improve the odds that the manufactured silicon is good enough to ship

14 Why Do We Do Functional Verification The cost of extra spins is enormous (the lost opportunity costs can dwarf the cost of new masks, etc.) Even one critical bug can cause a respin We wish to greatly improve the odds that the manufactured silicon is good enough to ship

15 Common Types of Functional Verif Test stimulus applied to inputs obtained via: Manually generated Generated by a directed test program (either open- loop or closed-loop) Pseudo-Random generator Captured from a live system Output captured and checked against reference design (usually a C model, or earlier silicon) Assertions (in both RTL and C models) Formal and Semi-formal techniques Real applications in a testbench that mimics a full system

16 Cost of Incomplete Verification Extra spins Lost time-to-market Scrapped inventory Good will of customers who were promised delivery dates Company reputation Impact on next projects, as engineers are brought back to fix problems found post-silicon

17 Why Don’t We Just Do More Functional Verification? It is possible to have “too much of a good thing?”

18 It is possible to have “too much of a good thing”? Why Don’t We Just Do More Functional Verification?

19 It is possible to have “too much of a good thing?” Analogy, courtesy of my colleague, David Wyland Why Don’t We Just Do More Functional Verification?

20 Costs of Thorough Verification Time-to-market Time-edge over competition If it were (technically and financially) possible to completely test a complex ASIC we would probably miss the market window Staffing Computer time Software licenses Emulation time Opportunity costs

21 Optimizing Functional Verification

22 Functional Verification Efforts Must Be Optimized We need to find ways of maximizing the Functional Verification we can afford to do No single technique is a total answer, multiple techniques will yield the best approach Thorough unit-level verification testing and loose coupling of units

23 Loose Coupling Reduces verification time Small simulations run faster Avoids combinatorial explosion of interactions Well defined interfaces between blocks with assertions and formal verification techniques to reduce inter-block problems

24 Functional Verification Metrics

25 Functional Verification Metrics Exhaustive Testing Error Injection New Bug Rate Number/Size of Check-Ins Number of Assertions Firing Line Coverage Expression Coverage State Machine Coverage Coverage Assertions

26 Exhaustive Testing 100% coverage All possible inputs presented when the Device Under Test (DUT) is in each of its possible states Consider a two-input AND gate No internal state Four vectors fully test Consider a two-input 32-bit adder No internal state 2 64 vectors needed to fully test At 10 Billion tests per second, test will take 58 years

27 Exhaustive Testing Not Practical

28 Error Injection It is theoretically possible to inject random errors into the design code and see what percentage are caught by the regression test suite For the size of today’s designs, this is impractical due to the time it takes to run a regression test suite, even with expensive emulation platforms, and the number of runs that are needed to get statistically meaningful results The errors that are not caught are difficult to analyze to determine if new tests are needed, and difficult to determine what those tests are

29 Error Injection Not Practical

30 New Bug Rate Common metric used as a gate to tape-out Easy to measure and graph

31 New Bug Rate, Page 2 Doesn’t distinguish severity of bug (but can)

32 New Bug Rate, Page 3 “Past performance is no guarantee of future results” Doesn’t predict number of bugs not yet seen

33 New Bug Rate, Page 4 No major new bugs for at least one week after 100% test plan and randoms running

34 Number/Size of Check-Ins Easy to measure and graph Doesn’t distinguish scope of check-in or the severity of the problem being addressed (but can) Doesn’t accurately predict the size of check-ins that will be needed to address future problems (especially missing functionality that is found late in the design process)

35 Number/Size of Check-Ins None only required fixes allowed

36 Number of Assertions Firing If the RTL design code is fully instrumented with functional assertions, then the fewer assertions that fire, the better BUT, the functional assertions will only fire when an error condition is seen – they do NOT provide any measure of test coverage

37 Number of Assertions Firing None

38 Line or Block Coverage Using third-party or built-in coverage tools, monitor a full regression to find lines (or blocks) of design code is being run at least once Slight, though some, impact on run-time Does NOT tell us: Which values of registers were exercised How much of “short-circuit and/or” ( && / || ) or ?: lines are exercised Which combinations of conditions and DUT state are exercised Anything about missing lines of code Visiting a line is necessary, but not sufficient, for complete testing

39 Line or Block Coverage > 98% of reachable code

40 Expression Coverage Overcomes the limitations of line (or block) coverage w.r.t. “short-circuit and/or” ( && / || ) or ?: lines are exercised Significant impact on simulation run-time Does NOT tell us: Which values of registers were exercised Which combinations of conditions and DUT state are exercised Anything about missing expressions or code

41 Expression Coverage > 95% of reachable code

42 State Machine Coverage Can measure if all state machine states and state machine state transitions have been exercised For most designs, can be labor-intensive, as illegal (or legal) state machine states and state transitions have to be declared Does NOT tell us: Which states, transitions, or entire state machines are missing

43 State Machine Coverage 100% of functional code

44 Coverage Assertions Also know as “Functional Coverage” Designers and/or verification engineers can declare “interesting” events to be covered by test cases Events can be simple: All inputs between 0 and 10 must be seen or can be complex: All combinations of FSM1 and FSM2 state machines must be seen Quality of metric depends upon the size of the effort to declare interesting events and the quality of those declarations

45 Coverage Assertions 100%

46 Functional vs. Architectural Errors To the end-user – no difference Functional Verif. is not trying to find architectural errors, but the user doesn’t care which end of the boat is sinking – only that their feet are starting to get wet

47 Living with Functional (and Architectural) Errors

48 Living With Errors Successful companies have learned how to ship chips with functional and architectural – time to market pressures and chip complexity force the delivery of chips that are not perfect (even if that were possible). How can this be done better? For a long while, DRAMs have been made with extra components to allow a less-than-perfect chip to provide full device function and to ship How to do the same with architectural redundancy? How can a less-than-perfect architecture or hardware implementation provide full device function?

49 Architectural Redundancy A programmable abstraction layer between the real hardware and user’s API can hide functional warts Upper-layer protocols can recover from some functional or architectural errors; though there can be performance penalties when this is used Soft hardware can allow chip redesign after silicon is frozen (and shipped!)

50 Make the Silicon Testable Provide access to interior blocks for live testing Inclusion of on-chip logic analyzer capability

51 Conclusions

52 Conclusions Ever increasing chip complexity prevents total testing before tape-out (or even before shipping) We need to optimize (and re-use) verification infrastructure and tests We need a growing Functional Verification toolkit, including simulation, emulation, formal and semiformal verification We have to accept that there will be architectural and functional failures in every advanced chip that is built Architectural redundancy is needed to allow failures to be worked around or fixed after post-silicon

53 Questions?

54 Background Slides

55 Targets of Functional Verification Functional Verificaiotn tests can be run on one or more of: RTL (Verilog or VHDL) simulation Gate-level simulation C model(s) of varying levels of detail and accuracy FPGA prototypes of parts of a chip or entire chip Emulation of parts of a chip or entire chip

56 Complexity Increases Exponentially Chip component count increases exponentially over time (Moore’s law) Interactions increase super-exponentially IP reuse and parallel design teams means more functions per chip Verification effort gets combinatorially more difficult

57 When Is a Test Considered Passing? Manual review of waveforms or output (not suited for automatic regressions!) Output matches “golden model” that was previously verified, possibly by manual review Self-checking tests (test includes expected response) Comparison with reference model No assertion failures

58 Problems with Reference Models Architectural errors are not caught, as the RTL model’s behavior matches the reference model Reference model may not closely mirror the RTL Outputs can be close, but differences are acceptable Outputs can be close, but not bit-accurate Outputs can be transaction-accurate, but not order accurate Outputs can be transaction and order-accurate, but not cycle-accurate

59 Why Not a Cycle-Accurate Reference? To make a transaction-accurate, order-accurate, bit-accurate, and cycle-accurate reference model will take as much resources as building the RTL model and much more effort to keep synchronized with RTL To make a cycle-accurate reference model, often the reference model coders need to look at the RTL model code – this weakens the reference model as an independent implementation

60 Why Verification is Unable to Keep Up Verification effort gets combinatorially more difficult as functions are added BUT Verification staffing/time cannot be made combinatorially larger to compensate AND Chip lifetimes are too short to allow for complete testing THUS Chips will continue to have ever-increasing functional errors as chips get more complex

61 Architect for Verification (AFV) Analogous to Design for Test (DFT) and Design for Verification (DFV), we should start thinking about Architect for Verification (AFV) [Thanks to Dave Whipp for the phrase and acronym] Avoid combinatorial test explosion as new features added (by loosely coupling units and features) Provide access to interior blocks for live testing Inclusion of on-chip logic analyzer capability