Chisel Nix

Getting started

Here we provide nix templates for setting up a Chisel project.

mkdir my-shining-new-chip
cd my-shining-new-chip
git init
nix flake init -t github:chipsalliance/chisel-nix#chisel

Use the above commands to setup a chisel project skeleton. It will provide you the below code structure:

• elaborator/: source code to the chisel elaborator
• gcd/: source code for the GCD example
• gcdemu/: source code for the DPI library
• configs/: default configurations for GCD and the testbench, which can be generated by elaborator
• nix/: nix build script for the whole lowering process
• build.sc & common.sc: Scala build script
• flake.nix: the root for nix to search scripts

Usage

Our packaging strategy is using the overlay.nix to "overlay" the nixpkgs. Every thing that developers want to add or modify should go into the overlay.nix file.

This skeleton provides a simple GCD example. It's build script is in nix/gcd folder, providing the below attributes:

• {gcd,tb,formal}-compiled: JVM bytecode for the GCD/GCDTestbench and elaborator
• {gcd,tb,formal}-compiled.elaborator: A bash wrapper for running the elaborator with JDK
• [{tb,formal}-]elaborate: Unlowered MLIR bytecode output from firrtl elaborated by elaborator
• [{tb,formal}-]mlirbc: MLIR bytecode lowered by circt framework
• [{tb,formal}-]rtl: SystemVerilog generated from the lowered MLIR bytecode
• tb-dpi-lib: DPI library written in Rust for both Verilator and VCS
• verilated[-trace]: C++ simulation executable and libaray generated by Verilator with/without fst waveform trace
• vcs[-trace]: C simulation executable compiled by VCS with/without fsdb waveform trace
• jg-fpv: Formal Property Verification report generated by JasperGold

To get the corresponding output, developers can use:

nix build '.#gcd.<attr>'

For instance, if developers wish to obtain the final lowered SystemVerilog, they can execute:

nix build '.#gcd.rtl'

The build result will be a symlink to nix store placed under the ./result.

To have same environment as the build script for developing purpose, developer can use:

nix develop '.#gcd.<attr>'

For example, to modify the GCD sources, developer might run:

nix develop '.#gcd.gcd-compiled'

The above command will provide a new bash shell with mill, circt, chisel... dependencies set up.

Certain attributes support direct execution via Nix, allowing arguments to be passed using --:

nix run '.#gcd.<attr>'

For example, we use elaborator to generate configs for the design. To generate the config for GCDTestbench, developer can run:

nix run '.#gcd.gcd-compiled.elaborator' -- config --width 16 --useAsyncReset false

A JSON file named GCDMain.json will be generated in the working directory.

As another example, we can run a VCS simulation with waveform trace by:

nix run '.#gcd.vcs-trace' --impure -- +dump-start=0 +dump-end=10000 +wave-path=trace +fsdb+sva_success

The DPI lib can automatically match the arguments and does not interact with VCS. In this case, the first three parameters will be passed to the DPI lib to control waveform generation, and the last parameter will be passed to the VCS to dump the results of all sva statements.

• Note that in order to use VCS for simulation, you need to set the environment variables VC_STATIC_HOME and SNPSLMD_LICENSE_FILE and add the--impure flag.

To run the formal property verification. Then you can run:

nix build '.#gcd.jg-fpv' --impure

and the report will be generated in the result/

• Note that in order to use jasper gold for formal verification, you need to set the environment variables JASPER_HOME and CDS_LIC_FILE and add the--impure flag.

References

Format the source code

To format the Nix code, developers can run:

nix fmt

To format the Rust code, developers can run following command in gcdemu/:

nix develop -c cargo fmt

To format the Scala code, developers can run:

nix develop -c bash -c 'mill -i gcd.reformat && mill -i elaborator.reformat'

Bump dependencies

To bump nixpkgs, run:

nix flake update 

To bump Chisel and other dependencies fetched by nvfetcher, run:

cd nix/pkgs/dependencies
nix run '.#nvfetcher'

To bump mill dependencies, run:

nix build '.#gcd.gcd-compiled.millDeps' --rebuild

and Then update millDepsHash in nix/gcd/gcd.nix

Use the fetchMillDeps function

Fetch project dependencies for later offline usage.

The fetchMillDeps function accept three args: name, src, millDepsHash:

• name: name of the mill dependencies derivation, suggest using <module>-mill-deps as suffix.
• src: path to a directory that contains at least build.sc file for mill to obtain dependencies.
• millDepsHash: same functionality as the sha256, hash attr in stdenv.mkDerivation. To obtain new hash for new dependencies, replace the old hash with empty string, and let nix figure the new hash.

This derivation will read $JAVA_OPTS environment varialble, to set http proxy, you can export:

export JAVA_OPTS="-Dhttps.proxyHost=127.0.0.1 -Dhttps.proxyPort=1234"

The returning derivation have setupHook attribute to automatically setup dependencies path for mill. Add the attribute into nativeBuildInputs, and let nix run the hook.

Example:

stdenv.mkDerivation rec {
    # ...
    millDeps = fetchMillDeps {
      inherit name;
      src = with lib.fileset; toSource {
        root = ./.;
        fileset = unions [
          ./build.sc
        ];
      };
      millDepsHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
    };
    # ...
    nativeBuildInputs = [
        # ...
        millDeps.setupHook
    ];
}

Use the nvfetcherSource attribute

projectDependencies attribute is an nix setup hook that will obtain nvfetcher generated sources and place them under dependencies directory in build root.

Read nvfetcher document for nvfetcher usage. By default the nvfetcherSource attribute will read nix/pkgs/dependencies/_sources/generated.nix, so developer should place nvfetcher config and run nix run .#nvfetcher under nix/pkgs/dependencies.

Usage:

stdenv.mkDerivation {
    nativeBuildInputs = [
        projectDependencies.setupHook
    ]
}

License

The build system is released under the Apache-2.0 license, including all Nix and mill build system, All rights reserved by Jiuyang Liu liu@Jiuyang.me