LUT cells get ignored/deleted by Vivado

[codefreak472]

Hello,

I'm trying to get a feel for the basic functions of rapidwright and I'm already having some issues. If I use the design.createAndPlaceCell method to create a LUT cell, I can see that it's been created just fine in my design object. However, when I write that design to a file, and open it in Vivado, the LUT cell is gone. I have tried LUTs with and without init strings, as well as with the location locked, and the DONT_TOUCH property set. None of them have helped. My design is simply empty when I open it in Vivado.

[eddieh-xlnx]

Welcome @codefreak472!

Nothing comes to mind as to why this wouldn't work. A really simple example is here: https://github.com/Xilinx/RapidWright/blob/f5930b27f0c48918581274ef42f71f2fd161a25c/src/com/xilinx/rapidwright/examples/Lesson1.java

You can also find some other examples here: https://github.com/Xilinx/RapidWright/search?q=createAndPlaceCell

Failing those, can you share your code so that we may reproduce it on our end?

[eddieh-xlnx]

Sorry I didn't notice that the LUT was unplaced. I've looked at it some more, I'm not quite sure what the reason for this behaviour is. Since it's very unconventional to want to have a completely unconnected LUT in your design, it's something we haven't seen before. In fact, even after manually placing the LUT using Vivado, writing out the DCP and then loading it back, Vivado itself raises the following warning:

CRITICAL WARNING: [Designutils 20-756] Invalid physical equation for the A6LUT bel in site SLICE_X100Y100. The original INIT is '8'. The logical cell is 'and2'. Reason: The bit width of the INIT value does not match the number of used input pins '0'. Please verify that all required logical pins are used for this cell.
     Bel 14: element name: 'A6LUT'
      Attr: 'LUT_OR_MEM' Value: LUT
      Attr: 'RAMMODE' Value: NOT CONFIGURED
      Attr: 'EQN' Value: 4'h8
      Inst: 'and2'
      CellType: 'LUT2'
      Pin Swappable:
        Not assigned -> Bel pin 'A1'
        Not assigned -> Bel pin 'A2'
        Not assigned -> Bel pin 'A3'
        Not assigned -> Bel pin 'A4'
        Not assigned -> Bel pin 'A5'
        Not assigned -> Bel pin 'A6'

Please leave it with us to investigate.

Can you share the reason you need this floating LUT inside Vivado? If this isn't just a stepping stone to something bigger, perhaps there's another way of achieving what you need.

[codefreak472]

Since I am very new at this, I thought I could save my work at multiple points throughout the process to verify I was doing everything correctly. Since the LUT is disappearing, I was assuming that meant I was doing something wrong during placement. The goal is not to have a floating LUT in the design, it is meant purely as a stepping stone. It still strikes me as odd that the placement is lost, even if the DONT_TOUCH property is set though. I thought I could create the LUT with RapidWright, then open the placed design in Vivado and do routing in Vivado, but this doesn't seem possible.

[codefreak472]

After some additional toying with the HelloWorld design, if I save the design with design.writeCheckpoint() after connecting net0 to the LUT, the LUT appears in the design and is placed. So it looks like connecting a net to a cell is sufficient to convince Vivado not to get rid of it/unplace it.

I have had a lot of trouble with the net.connect() method as well though. I've gotten a lot of NullPointerException errors from it. I think it happens if a SitePinInst already exists for the pin you want to connect, but I'm not sure.

[eddieh-xlnx]

Since I am very new at this, I thought I could save my work at multiple points throughout the process to verify I was doing everything correctly. Since the LUT is disappearing, I was assuming that meant I was doing something wrong during placement. The goal is not to have a floating LUT in the design, it is meant purely as a stepping stone.

That is absolutely the right idea, and something that RapidWright does support, please don't be dissuaded from this approach!

I thought I could create the LUT with RapidWright, then open the placed design in Vivado and do routing in Vivado, but this doesn't seem possible.

That is absolutely possible (as you can see in the HelloWorld example) for the much more common use case where the LUT is actually connected to something. In your case, you're inserting a floating LUT and in doing so stumbled on a bug we've yet to track down.

The stepping stone you're actually trying to achieve is actually missing two steps before you can get to a meaningful circuit:

1. Connecting the LUT and I/Os to each other
2. Routing those connections (which you've mentioned)

Both of which you'll need to do with Vivado if you just place a floating LUT in the design. If you were to perform 1 with RapidWright, then that should bring you back onto the well-tested path for now.

[eddieh-xlnx]

I have had a lot of trouble with the net.connect() method as well though. I've gotten a lot of NullPointerException errors from it. I think it happens if a SitePinInst already exists for the pin you want to connect, but I'm not sure.

Please share these issues. We absolutely want to make RapidWright easier to use, either by enhancing the codebase or through better documentation!