# lowRISC Hardware Primitives

## Concepts

This directory contains basic building blocks to create a hardware design,
called primitives. A primitive is described by its name, and has a well-defined
list of ports and parameters.

Under the hood, primitives are slightly special, as they can have multiple
implementations. In contrast to many other modules in a hardware design,
primitives must often be implemented in technology-dependent ways. For example,
a clock multiplexer for a Xilinx FPGA is implemented differently than one for
a specific ASIC technology.

Not all primitives need to have multiple implementations.

* Primitives with a single, generic, implementation are normal SystemVerilog
  modules inside the `hw/ip/prim/rtl` directory. We call these primitives
  "technology-independent primitives".
* Primitives with multiple implementations have only a FuseSoC core file in the
  `hw/ip/prim` directory. The actual implementations are in "technology
  libraries". We call these primitives "technology-dependent primitives".

### Abstract primitives

Abstract primitives are wrappers around technology-dependent implementations of
primitives, with the ability to select a specific implementation if needed.

In more technical terms, abstract primitives are SystemVerilog modules. The
example below shows one.

```systemverilog
`ifndef PRIM_DEFAULT_IMPL
  `define PRIM_DEFAULT_IMPL prim_pkg::ImplGeneric
`endif

module prim_pad_wrapper
#(
  parameter int unsigned AttrDw = 6
) (
  inout wire         inout_io, // bidirectional pad
  output logic       in_o,     // input data
  input              out_i,    // output data
  input              oe_i,     // output enable
  // additional attributes {drive strength, keeper, pull-up, pull-down, open-drain, invert}
  input [AttrDw-1:0] attr_i
);
  parameter prim_pkg::impl_e Impl = `PRIM_DEFAULT_IMPL;

  if (Impl == prim_pkg::ImplGeneric) begin : gen_generic
    prim_generic_pad_wrapper u_impl_generic (
      .*
    );
  end else if (Impl == prim_pkg::ImplXilinx) begin : gen_xilinx
    prim_xilinx_pad_wrapper u_impl_xilinx (
      .*
    );
  end else begin : gen_failure
    // TODO: Find code that works across tools and causes a compile failure
  end

endmodule
```

As seen from the source code snippet, abstract primitives have the following
properties:

- They have an `Impl` parameter which can be set to choose a specific
  implementation of the primitive.
- The `Impl` parameter is set to a system-wide default determined by the
  `PRIM_DEFAULT_IMPL` define.
- All ports and parameters of the abstract primitive are forwarded to the
  implementations.

### Technology libraries

Technology libraries collect implementations of primitives.

At least one technology library must exist: the `generic` technology library,
which contains a pure-SystemVerilog implementation of the functionality. This
library is commonly used for simulations and as functional reference. The
`generic` technology library is contained in the `hw/ip/prim_generic` directory.

In addition to the implementation in the `generic` library, primitives may be
implemented by as many other libraries as needed.

Technology libraries are referenced by their name.

### Technology library discovery

In many cases, technology libraries contain vendor-specific code which cannot be
shared widely or openly. Therefore, a FuseSoC looks for available technology
libraries at build time, and makes all libraries it finds available.

The discovery is performed based on the agreed-on naming scheme for primitives.

- FuseSoC scans all libraries (e.g. as specified by its `--cores-root` command
  line argument) for cores.
- All cores with a name matching `lowrisc:prim_TECHLIBNAME:PRIMNAME`
  are considered. `TECHLIBNAME` is then added to the list of technology
  libraries.

After the discovery process has completed, a script (`primgen`) creates
- an abstract primitive (see above), and
- an entry in the `prim_pkg` package in the form of `prim_pkg::ImplTechlibname`
  to identify the technology library by its name.

## User Guide

### Use primitives

Primitives are normal SystemVerilog modules, and can be used as usual:
* instantiate it like a normal SystemVerilog module, and
* add a dependency in the FuseSoC core file.

Technology-dependent primitives have an additional parameter called `Impl`.
Set this parameter to use a specific implementation of the primitive for this
specific instance. For example:

```systemverilog
prim_ram_2p #(
  .Width (TotalWidth),
  .Depth (Depth),
  // Force the use of the tsmc40lp technology library for this instance, instead
  // of using the build-time default.
  .Impl(prim_pkg::ImplTsmc40lp)
) u_mem (
  .clk_a_i    (clk_i),
  ...
)
```


### Set the default technology library

If no specific technology library is chosen for an instantiated primitive the
default library is used. The SystemVerilog define `PRIM_DEFAULT_IMPL` can be
used to set the default for the whole design. Set this define to one of the enum
values in `prim_pkg.sv` in the form `prim_pkg::ImplTechlibname`. `Techlibname`
is the capitalized name of the technology library.

In the top-level FuseSoC core file the default technology library can be chosen
like this:

```yaml
# my_toplevel.core

# Declare filesets and other things (omitted)

parameters:
  # Make the parameter known to FuseSoC to enable overrides from the
  # command line. If not overwritten, use the generic technology library.
  PRIM_DEFAULT_IMPL:
    datatype: str
    paramtype: vlogdefine
    description: Primitives implementation to use, e.g. "prim_pkg::ImplGeneric".
    default: prim_pkg::ImplGeneric

targets:
  fpga_synthesis:
    filesets:
      - my_rtl_files
    parameters:
      # Use the xilinx technology library for this target by default.
      - PRIM_DEFAULT_IMPL=prim_pkg::ImplXilinx
    toplevel: my_toplevel
```


### Create a technology library

To create a technology library follow these steps:

- Choose a name for the new technology library. Names are all lower-case.
  To ease sharing of technology libraries it is encouraged to pick a very
  specific name, e.g. `tsmc40lp`, and not `asic`.
- Copy the `prim_generic` folder into an arbitrary location (can be outside
  of this repository). Name the folder `prim_YOURLIBRARYNAME`.
- Replace the word `generic` everywhere with the name of your technology
  library. This includes
  - file and directory names (e.g. `prim_generic_ram1p.sv` becomes
    `prim_tsmc40lp_ram1p.sv`),
  - module names (e.g. `prim_generic_ram1p` becomes `prim_tsmc40lp_ram1p`), and
  - all other references (grep for it!).
- Implement all primitives. Replace the module body of the generic
  implementation with a technology-specific implementation as needed. Do *not*
  modify the list of ports or parameters in any way!

## Implementation details

Technology-dependent primitives are implemented as a FuseSoC generator. The
core of the primitive (e.g. `lowrisc:prim:rom` in `prim/prim_rom.core`) calls
a FuseSoC generator. This generator is the script `util/primgen.py`. As input,
the script receives a list of all cores found by FuseSoC anywhere in its search
path. The script then looks through the cores FuseSoC discovered and extracts
a list of technology libraries out of it. It then goes on to create the
abstract primitive (copying over the list of parameters and ports from the
generic implementation), and an associated core file, which depends on all
technology-dependent libraries that were found.