path: root/fpga/CODING.md

# UHD FPGA Coding Standards

## Preamble

To quote R. W. Emerson: "A foolish consistency is the hobgoblin of little minds,
adored by little statesmen and philosophers and divines". Ignoring the little
statesmen for a minute, these coding standards are here to make our life
*easier*, not simply add additional rules. They are meant as additional guidance
for developers, and are not meant to be interpreted as law.

So, ultimately, it is up to the developer to decide how much these guidelines
should be heeded when writing code, and up to reviewers how much they are
relevant to new submissions.
That said, a consistent codebase is easier to maintain, read, understand, and
extend. Choosing personal preferences over these coding guidelines is not a
helpful move for the team and future maintainability of the UHD FPGA codebase.

## General Coding Guidelines

* Code layout: We use 2 spaces for indentation levels, and never tabs.
* Never submit code with trailing whitespace.
* Code is read more often than it's written. Code readability is thus something
  worth optimizing for.
* Try and keep line lengths to 79 characters, unless readability suffers.
* Comment your code. Especially if your code is tricky or makes unique assumptions.
* Provide a detailed description of each module or file in its header. Consider
  its purpose, how it is intended to be used, assumptions made in its design,
  limitations of the implementation, etc.
* Use the following header at the top of each file:
```verilog
//
// Copyright <YEAR> Ettus Research, a National Instruments Brand
//
// SPDX-License-Identifier: LGPL-3.0-or-later
//
// Module: <MODULE_NAME>
//
// Description:
//
//   <Add a detailed description>
//
// Parameters:
//
//   <Describe the parameters, if helpful>
//
// Signals:
//
//   <Describe the port signals, if helpful>
//
```

## Verilog Style Guidelines

### General Syntax

* Always use `begin`and `end` statements for more complex code blocks even if the enclosing code is only
one line.
* Indent begin/end as follows:
```verilog
if (foo) begin
  // Do something
end else if (bar) begin
  case(xyz)
    1'b0: begin
      // Handle 0
    end
    default: begin
      // Handle 1
    end
  endcase
end else begin
  // Do nothing
end
```
* Declare and instantiate modules as shown below. Aligning the ports into
  columns is not required but helps with readability and edibility. A good text
  editor can make this easy.
```verilog
module top_level #(
  parameter       PARAM1 = 0,
  parameter [7:0] PARAM2 = 0
) (
  input  wire       clk,
  input  wire       rst,
  input  wire [7:0] signal1,
  output wire [7:0] signal2
);

  example_one #(
    .PARAM1(0), .PARAM2(1)
  ) example_one_i (
    .clk(clk), .rst(rst),
    .port1(signal1), .port2(signal2)
  );

  example_two #(
    .PARAM1(0),
    .PARAM2(1)
  ) example_two_i (
    .clk(clk),
    .rst(rst),
    .port1(signal1),
    .port2(signal2)
  );

  example_three #(
    .PARAM1 (0),
    .PARAM2 (1)
  ) example_three_i (
    .clk   (clk),
    .rst   (rst),
    .port1 (signal1),
    .port2 (signal2)
  );

endmodule

```

### Assignments

* Sequential blocks should only have non-blocking assignments (`<=`)
* Combinational blocks should only have blocking assignments (`=`)
* Don't mix blocking and non-blocking assignments

### Modules

* Each module should be defined in a separate file
* The file name should be the same as the module name
* File and module names should be unique. Do not create a new module with the
  same name as an existing module.
* Every module requires a description in the header that gives a synopsis of
  its function.
* Use Verilog 2001 ANSI-C style port declarations:
```verilog
(
  ...
  output reg  foo,
  input  wire bar
);
```
* Declare inputs and outputs one per line. This makes searching and commenting easier.
* Add comments to the header or inline to describe the behavior of input/output ports.
* Be explicit about whether an input or output is a wire or reg.
* Group signals logically instead of by direction. If a single AXI-Stream bus has multiple inputs and
outputs, keep them together.
* Instantiate all ports for a module even if they are tied off or unconnected. Don't let the compiler
insert values for any signals automatically.
```verilog
dummy_module dummy_module_i (
  .clk    (clk),
  .rst    (1'b0),
  .status (/* unused */)
);
```
* Don't instantiate modules using positional arguments. Use the dot form illustrated above.
* Use default_nettype at the beginning and end of each module file to avoid
  problems with undeclared signals:
```verilog
`default_nettype none

module example #(
  ...
endmodule

`default_nettype wire
```

### Clocking and Resets

* Name clocks as `clk`. If there are multiple clocks then use a prefix like `bus_clk` and `radio_clk`.
* If a module has signals or input/outputs whose clock domain is not obvious, use a clock suffix
to be explicit about the domain, for example `axi_tdata_bclk`, `axi_tdata_rclk`.
* Try not to encode the frequency of the clock in the name unless the particular clock can
*never* take on any other frequency.
* Name resets as `rst`. If there are multiple clocks then use a prefix like `bus_rst` and `radio_rst`.
* If a reset is asynchronous, call it `arst`.
* Try to avoid asynchronous resets as much as possible.
* Don't use active low resets unless it is used to drive top-level IO or IP
  that requires it.

### Generate Statements

* Use the ``generate``/``endgenerate`` keywords for backwards compatibility
  with earlier versions of the Verilog language.
* Add labels to the `begin`/`end` blocks of generate statements, so that the
  labels are used when displaying the hierarchical names in the simulator and
  other tools, and so that logic can be referenced hierarchically by name in
  testbenches if needed. For example:
```verilog
genvar i;
generate
  for (i = 0; i < NUM_PORTS; i++) begin : gen_loop_example
    ...
  end
endgenerate
```

### Parameters, Defines and Constants

* Parameterize modules wherever possible, especially if they are designed for reuse. Bus widths, addresses,
buffer sizes, etc are good candidates for parametrization.
* For modules with parameters, add comments in the header or inline to describe the behavior of each parameter.
* Propagate parameters as far up the hierarchy as possible as long as it makes sense.
* Place `` `define`` statements in Verilog header files (.vh) and `` `include`` them in modules.
* Avoid placing `` `define`` statements in modules.
* For local constants, use `localparam` instead on hard-coding things like widths, etc.
* Use all uppercase names for constants (``parameter``, ``localparam``, or
  `` `define``) to distinguish them from signal names.

### AXI Guidelines

* Keep the components of an AXI-Stream or AXI-MM bus together in port/wire instantiations.
* For module ports, use the master/slave naming convention as shown below. It makes connecting modules
easier because a master always connects to a slave.
```verilog
  input  wire [63:0] s_axis_tdata,
  input  wire        s_axis_tlast,
  input  wire        s_axis_tvalid,
  output wire        s_axis_tready,
  output reg  [63:0] m_axis_tdata,
  output reg         m_axis_tlast,
  output reg         m_axis_tvalid,
  input  wire        m_axis_tready,
```
* For connections between a master and slave, *do not* use the master/slave convention. Name the bus based
on its function or underlying data.
```verilog
  wire [63:0] axis_eth2xbar_tdata,
  wire        axis_eth2xbar_tlast,
  wire        axis_eth2xbar_tvalid,
  wire        axis_eth2xbar_tready,

  // If "axis" is obvious, drop the prefix
  wire [63:0] samp_tdata,
  wire        samp_tlast,
  wire        samp_tvalid,
  wire        samp_tready,
```