NISP - Nextpnr-fpga_Interchange Site Preprocessor

What's NISP

The tool is a simple pin-to-pin site router which gathers information about routability between pairs of pins within site. The most basic information is whether a possible route between two given pins exists, but NISP can also gather constraints required for the routes and can account for alternative routes between pins.

The goal of this tool is to create a sort-of cache with routability data for a given fpga-interchange device, which can be then used to improve performace of nextpnr-fpga_interchange's SA placer, which currently suffers from a big performance hit due to long routability checks that happen during cell placement.

CAUTION!

This is an in-development tool. As such, some features might be missing, incomplete or contain various bugs. Do not take any results generated by it as granted.

Currently the tool does not output BBA/binary, but it can output JSONs with routability information. It's easier to develop it like this way at the moment.

Features

NISP has the following features at the moment

• Generate site-routing graph and export it into graphviz .dot files (--dot, --dot-prefix options)
• Generate routability lookup and constraints and export it into JSON (--json, --json-prefix options)
• Optimize constraint formulas (use --no-formula-opt to skip that step)

Building NISP

Prerequisites

1. Install Rust
2. Clone fpga-interchange-schema (NOTE: To use that schema you need a schema header from Java cap'n proto bindings. Either install the bindings or, just copy the header located at compiler/src/main/schema/capnp/java.capnp to your capn' proto include directory, which is presumably /usr/include/capnp/java.capnp if you are running Linux.)

Compiling

1. Set FPGA_INTERCHANGE_SCHEMA_DIR environmental variable to point to directory where you cloned fpga-interchange-schema.
2. Run cargo build for debug build, cargo build --release for release build.

Running NISP

nisp [OPTIONS] <DEVICE> <BBA> <COMMAND>

• <DEVICE> - Path to fpga-interchaneg device file
• <BBA> - BBA output path. Currently ignored.

Descriptions for currently available options are available when running the program with --help flag.

If an option requires you to specify a list of site type names, you can use :all as a replacement for listing all site types in the architecture. To list select entries you can repeat the flag multiple times (eq. --json CLEM --json CLEL). Keep in mind that NISP won't create any prefix directories you specify on its own.

preprocess subcommand

Gathers information for routability between BEL pins in a site. Use --with-debug-hints to add hints to the outputted JSON that make it easier to read by a human.

route-pair subcommand

For given pair of BEL pins, print all routes the site-router found as viable. This option is intended for debugging purposes.

test script

This script can be used to simplify compiling, running and debugging NISP. It's short, so the best way to understand what it does is just to read it.

JSON output (preprocess subcommand)

The JSON output is created by serializing internal structures of NISP. Its structure might change over time. As such, I won't be documenting its structure in detail, instead I'm going to provide a general overview of the content of generated JSONs.

Currently the JSON files which are outputted per-site-type contain the following information:

• Routable connections. The top-level dictionary contains keys consisting of two strings joined by -> symbol. Those two strings identify BEL pins. The format of a BEL pin identifier follows this pattern: BEL_NAME.PIN_NAME

• Each routable connection has two lists associated with it.

  1. requires list, which lists states which are required for given routing.
  2. implies list, which lists the states which are activated when a given routing is used.

  Those lists contain alternative sets, of which any can be used.

• The current constraint information is limited to constraints that would prevent invalid pseudo-pip states and reusage of already claimed wires. However, support for cell placements constraints is yet to be added, thus nextpnr's site placer can't fully rely on this feature yet.

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