Sunday, October 30, 2022

Don't copy that IP

Making copies of other people's IP is a terrible thing! And it's a really big problem in the EDA world.


Oh, I'm not talking about the legal aspects. That's something I happily leave to the lawyers. No, I'm talking about making copies of source code instead of referencing the original code. It's a big problem because suddenly you have two different versions, and the chances that a fix that was done in one version will reach the other version are slim to none. When I was entering the world of open source silicon around 2010, this was how things were normally done. And my first reaction was to try and upstream all fixes to IP cores that I found in various projects that were using those cores. My second reaction was to try and find a more sustainable way to avoid this problem in the first place. And solving this problem was one of the initial driving forces behind FuseSoC, and it still is. So it makes me very very sad when, twelve years later, I still find random copies of IP cores, all with various subsets of fixes applied to them.

When we started working on the OpenLANE Edalize backend, one important thing was to have as many example designs as possible, both to make sure the backend was flexible enough to cover all use-cases and also to have a good set of examples for anyone interested in adding FuseSoC support for their own cores. We found a number of example designs in the OpenLANE repository which were used to test OpenLANE itself. Great! ...except... it turned out most of the examples were cores or parts of cores copied from various places. So we did what every sensible person (not really) would do. We decided to upstream all those example designs, so that the OpenLANE support and any other fixes would benefit all users of that IP core.

Bringing' it all back home

At the time we started looking at this, there were 32 different example designs. Our first job was to find out where on earth all these came from. That took a fair amount of detective work, but in the end we found what we believe is the proper upstream for all, except perhaps for one where we are a bit unsure. In that case we chose to file a PR against OpenLANE since that was the closest we could get to an upstream. A few designs were dropped from the OpenLANE examples while we were working on this, in which case we also chose to ignore them.

Once we had identified the origin, we tried to figure out how they worked and then set off to add FuseSoC support for each and every core. At the minimum we added a target for linting and a target for building with OpenLANE through the Edalize backend. In the cases where we found testbenches, we also added support for running those, with a few exceptions where that required tools we didn't have access to.

In addition to adding FuseSoC support, we also added support for running the targets as GitHub CI actions, so that every new commit to the project would automatically lint, build a GDS with OpenLANE and potentially run some testbenches.

And finally we packaged it all up and sent a humongous number of pull requests to different projects with detailed instructions how they could use this. Many of these pull requests have been accepted, but not all. There's not much more we can do about that however. If you are curious, it's possible to check the progress here

So, what does this really mean? Did we make the world a better place for you and for me and the entire human race? I like to think so. At least we didn't make it worse. There are a couple of very real benefits to this work.

  • Adding FuseSoC support makes it eaiser for other users to use these cores in their own projects
  • Adding testbench targets makes it easy for other people to check the cores work as expected
  • Adding a lint target makes it easy to check code quality. Far from all of the designs we encountered pass the lint check
  • On a few occasions, fixes had been made in the copies. These were fed upstream to benefit all users of those cores
  • The CI actions makes it easy to check nothing breaks on future updates of the cores
  • We could test the Edalize OpenLANE backend on a number of different designs to ensure it was flexible enough to handle them all
  • We now have a large pool of example designs for anyone interested in doing the same for their cores
  • And finally, we have seen that some of the maintainers whose cores we added support for, have started doing the same on their other cores, which is fantastic to see.

Now, our hope is of course that you too will be bitten by the FuseSoC fever and add support for your cores too so that we can keep growing the ecosystem of FuseSoC-compatible cores, which in turn will help the EDA tool developers improve their tools.

This work was sponsored by a grant from NLNet Foundation

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