Are there GPL'd designs for PCIe, USB, etc, that could be used to incorporate this into a SoC design? If not, how much work is that compared to this?
Also, what other kind of technical considerations would be involved to make this into a "real" chip on something like 28nm?
So far I haven't needed USB/ether/PCIe/etc I've sort of sketched out a place for those to live - I think that for a high end system like this one you can't just plug something in - real performance needs some consideration of how:
- cache coherency works - VM and virtual memory works (essentially page tables for IO devices) - PMAP protections from I/O space (so that devices can't bypass the CPU PMAPs that are used to man age secure enclaves in machine mode)
So in general I'm after something uniquer, or at least slightly bespoke.
I also think there's a bit of a grand convergence going on in this area around serdes's which are sort of becoming a new generic interface PCIe, high speed ether, new USBs, disk drivers etc are all essentially bunches of serdes with different protocol engines behind them - a smart SoC is going to split things this way for maximum flexibility
So GPL'd IO blocks - This is a great question, and something I have definitely been asking myself! One thing to keep in mind is that IO interfaces like PCIe, USB, and whatnot have a Physical interface ("Phy" for short.) Those contain quite a bit of analog circuitry, which is tied to the transistor architecture that's used for the design.
That being said, A lot of interfaces that aren't DRAM protocols use what's known as a SerDes Phy (short for Serializer De-serializer Physical interface.) More or less, they have an analog front end and a digital back end, and that digital back end that connects to everything else is somewhat standardized way. So it wouldn't be unreasonable to try to build something like an open PCIe controller that only has the Transaction Layer and Data Link Layer. While there are various timing concerns/constraints when not including a Phy layer (lowest layer,) I don't think it's impossible.
The other big challenge is that anyone wanting to use an open source design will definitely want the test benches and test cases included in the repo (you can think of them like unit tests.) Unfortunately, most of the software to actually compile and run those simulations is cost prohibitive for an individual, because it's licensed software. Also, the companies that develop this software make a ton of money selling things like USB and PCIe controllers, so I'll let you draw your own conclusions about the incentives of that industry.
Even if you were able to get your hands on the software, the simulations are very computationally intensive, and contribution by individuals would be challenging ...though not impossible!
Despite those barriers, it's a direction that I desperately want to see the industry move towards, and I think it's becoming more and more inevitable as companies like Google get involved with hardware, and try to make the ecosystem more open. Chiplet architectures are also all the rage these days, so it would be less of a risk for a company to attempt to use an open source design.
I'd really be curious to hear Paul Campbell's take on this question though. He definitely knows a lot more than I do!
Here's a SerDes from Purdue. I don't think this particular design has been validated in silicon yet though.