Workflow using Connectal

In this section, we give an overview of the Connectal workflow and toolchain. The complete toolchain, libraries, and many running examples may be obtained at textit{www.connectal.org} or by emailing textit{connectal@googlegroups.com}.

Top level structure of Connectal applications

The simplest Connectal application consists of 4 files:

Makefile

The top-level Makefile defines parameters for the entire application build process. In its simplest form, it specifies which Bluespec interfaces to use as portals, the hardware and software source files, and the libraries to use for the hardware and software compilation.

Application Hardware

Connectal applications typically have at least one BSV file containing declarations of the interfaces being exposed as portals, along with the implementation of the application hardware itself.

Top.bsv

In this file, the developer instantiates the application hardware modules, connecting them to the generated wrappers and proxies for the portals exported to software. To connect to the host processor bus, a parameterized standard interface is used, making it easy to synthesize the application for different CPUs or for simulation. If CPU specific interface signals are needed by the design (for example, extra clocks that are generated by the PCIe core), then an optional CPU-specific interface can also be used.

If the design uses multiple clock domains or additional pins on the FPGA, those connections are also made here by exporting a ‘Pins’ interface. The Bluespec compiler generates a Verilog module from the top level BSV module, in which the methods of exposed interfaces are implemented as Verilog ports. Those ports are associated to physical pins on the FPGA using a physical constraints file.

Application CPP

The software portion of a Connectal application generally consists of at least one C++ file, which instantiates the generated software portal wrapper and proxies. The application software is also responsible for implementing main.

Development cycle

After creating or editing the source code for the application, the development cycle consists of four steps: generating makefiles, compiling the interface, building the application, and running the application.

Generating Makefiles

Given the parameters specified in the application Makefile and a platform target specified at the command line, Connectal generates a target-specific Makefile to control the build process. This Makefile contains the complete dependency information for the generation of wrappers/proxies, the use of these wrappers/proxies in compiling both the software and hardware, and the collection of build artifacts into a package that can be either run locally or over a network to a remote ‘device under test’ machine.

Compiling the Interface

The Connectal interface compiler generates the C++ and BSV files to implement wrappers and proxies for all interfaces specified in the application Makefile. Human readable textbf{JSON} is used as an intermediate representation of portal interfaces, exposing a useful debugging window as well as a path for future support of additional languages and IDLs.

Building the Application

A target in the generated Makefile invokes GCC to compiler the software components of the application. The Bluespec compiler (bsc) is then invoked to compiler the hardware components to Verilog. A parameterized Tcl scripts is used to drive Vivado to build the Xilinx FPGA configuration bitstream for the design.

A Connectal utility called fpgamake supports specification of which Bluespec and Verilog modules should be compiled to separate netlists and to enable separate place and route of those netlists given a floor plan. Separate synthesis and floor planning in this manner can reduce build times, and to make it easier to meet timing constraints.

Another Connectal utility called buildcache speeds recompilation by caching previous compilation results and detecting cases where input files have not changed. Although similar to the better-known utility textit{ccache}, this program has no specific knowledge of the tools being executed, allowing it to be integrated into any workflow and any tool set. This utility uses the system call textbf{strace} to track which files are read and written by each build step, computing an ‘input signature’ of the MD5 checksum for each of these files. When the input signature matches, the output files are just refreshed from the cache, avoiding the long synthesis times for the unchanged portions of the project.

Running the Application

As part of our goal to have a fully scripted design flow, the generated Makefile includes a texttt{run} target that will program the FPGA and launch the specified application or test bench. In order to support shared target hardware resources, the developer can direct the run to a particular machines, which can be accessed over the network. For Ubuntu target machines, ssh is used to copy/run the application. For Android target machines, ‘adb’ is used.

Continuous Integration and Debug Support

Connectal provides a fully scripted flow in order to make it easy to automate the building and running of applications for continuous integration. Our development team builds and runs large collections of tests whenever the source code repository is updated.

Connectal also provides trace ring buffers in hardware and analysis software to trace and display the last transactions on the PCIe or AXI memory bus. This trace is useful when debugging performance or correctness problems, answering questions of the form:

  • What were the last memory requests and responses?
  • What was the timing of the last memory request and responses?
  • What were the last hardware method invocations or indications?