Use of "extern" methods in Verification IPs - a PySlint case study

As we prepare for Verification Futures 2023, Austin event, an enthusiastic user ran PySlint on a small SPI VIP and found the following violation (among others).


As expected, first reaction from the user was - what's the big deal?


Below is our attempt in rationalizing why this rule is important especially for VIP developers and users. 

A quick recap - The extern keyword in SystemVerilog is an optional qualifier to class methods. As in C++, Java and other languages, this is a highly recommended coding style, some benefits include:

  • Improves readability - By declaring the method prototype in one place and the implementation in another, it can be easier to "read" the code for end users (not only the original developers) - to quickly glance the list of APIs/methods without scrolling 1000s of lines of code (if the implementation is mixed with declaration). 
  • To hide implementation details. If the method implementation is defined in a separate file, it can be hidden from users of the class. This can be useful for security reasons or to prevent users from modifying the implementation. Many-a-times VIP code is shipped as encrypted, yet users need to know the API definitions/signatures. SystemVerilog extern aids here perfectly.
  • Enhances maintainability - Developers will know what is changing - the API signature or the "body". 
  • Distributed development - managers can distribute complex VIP code such as PCIe driver across few team members, as extern methods can be implemented in separate files. 
  • Allows for separate compilation. If the method is defined in a separate file, it can be compiled independently of the rest of the code. This can be very handy for large projects. 
Learn more about how you can code your own rules, customize existing ones with PySlint - https://github.com/asfigo/pyslint 

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