CVA6 RISC-V CPU and SoC - Vivado Project and Software Stack

Overview

This repository contains (scripts generating) a Vivado 2023.2 project built around the CVA6 RISC-V CPU and a software stack including u-boot and embedded linux. This project currently only supports the Digilent Genesys2 board.

Hardware Stack

Currently, the SoC contains the following devices:

• the cva6 CPU in 64-bit configuration with SV39 MMU @50MHz
• CLINT (core-local interrupt controller) and PLIC (platform-level interrupt controller)
• UART console, mapped to the USB UART
• Xilinx SPI, configured for booting from the connected SD card
• Xilinx AXI Ethernet Subsystem including a DMA. Please note that you need to purchase a license from Xilinx for the underlying IP core.
• boot ROM for the cva6
• I2C controller for the audio IC on the Genesys2
• a Pmod GPIO controller
• DDR3 memory controller

Software Stack

The software stack contains:

• a boot ROM which loads a bootloader from the on-board SD card
• OpenSBI, which implements the SBI interface for the operating system that we will be running in S-Mode (Linux)
• u-boot, the boot loader for Linux; configured to default to loading Linux via tftp, but with support for booting linux from SD card as well
• embedded Linux based on buildroot, with kernel 5.10.7

Running the image

Run the following steps in-order to execute the project. This assumes Xilinx Vivado to installed in /opt/Xilinx/Vivado/2023.2/ (you can create a corresponding symlink).

Compiling hardware and software stacks

First, be sure to update the project submodules:

    git submodule update --init --recursive

After that, use the "all" target of the top-level makefile to compile hardware and software:

    make all # use "fpga" to only compile the FPGA target and "images" to only compile software

Preparing the SD card for the software stack

Currently, the bootrom is configured to load a bootloader from the on-board SD card reader. To this end, you need to program an SD card with the OpenSBI and u-boot images. The SD card uses GUID partitioning, with one partition containing the OpenSBI image and u-boot and a FAT-32 partition containing Linux and its RAM file system. The top-level Makefile contains a target for creating the SD card:

    sudo -E make flash-sdcard SDDEVICE=/dev/sdxxx # double-check device!

Preparing the TFTP server for TFTP booting

There is a convenience script in the software stack for launching a TFTP server:

    cd software/include/cva6-sdk/scripts
    sudo INTERFACE=ethXXX ./start-tftp.sh

Programming the FPGA via JTAG

The top-level make file contains a target for programming the FPGA via JTAG:

    make program-device

This assumes proper installation of Vivado drivers; see the Vivado installation manual. Make sure the SD card is inserted before attempting to program.

Connecting to the Console and Booting

You can use your favorite console emulator, e.g., picocom, to connect to the board:

    picocom -b 115200 /dev/serial/by-id/usb-XXXX

If you do not do anything, the board will do a TFTP boot assuming you have started the TFTP server. Abort the boot via TFTP by pressing any button during the u-boot countdown and use the following u-boot command to boot Linux from the SD card instead:

    mmc info; fatload mmc 0:2 90000000 uImage; setenv fdt_high 0xffffffffffffffff; bootm 90000000 - $(fdtcontroladdr)

After boot is complete, use the convenience script to log into the FPGA via TFTP:

    cd software/include/cva6-sdk/scripts
    ./ssh.sh

Making Modifications - Hardware

After generating your first bitstream, you can open the Vivado project in GUI mode using the provided script:

    cd scripts/Vivado
    ./open_project.sh

This allows you to, e.g., add additional AXI peripherals from the Xilinx or Digilent IP catalogs, to add hardware debug cores (ILAs or System ILAs), navigate the synthesized and implemented design, etc. You can find the SoC in the block design ("Open Block Design" on the left). When you want to commit modifications to the Soc, select File->Project->Write TCL and store the resulting file in a temporary directory. Open the file, copy the cr_bd_SoC method in its entirety and replace the method in the scripts/Vivado/create_bd.tcl script.

Note that the Vivado project including any modifications to the SoC is deleted by the make fpga target if you do not overwrite create_bd.tcl. Source code modifications should be picked up by Vivado; if you are unsure, run make fpga to re-create the project.

Making Modifications - Software

Go through the Makefile and the top-level directories in the software project to get an understanding of what happens where. The easiest way to make small modifications is to add patches, e.g., similar to the patches for Linux in linux_patch and to the patches for u-boot and OpenSBI in xlnx_patch. Linux patches are pulled in by buildroot automatically. For new xlnx_patches, add corresponding targets in the top-level makefile.

Changes to upstream CVA6 project

Hardware

• CVA6 has support for additional boards, such as the Nexys Video.
• CVA6 does not use a Vivado project and the IP integrator, but instead provides custom scripts for synthesizing IP cores and configures the SoC in SystemVerilog as well.
• This fork of CVA6 adds support for Xilinx Ethernet in the Device Tree contained in the Boot ROM.
• This fork of CVA6 includes an option to make the non-standard data cache control register writable in S-Mode. This is used by u-boot and Linux to flush the cache after writing and before reading DMA descriptors to ensure consistency.

Software

• The TFTP boot for u-boot is unique to this fork.
• The build generates an SSH key during each compile and embeds it into the authorized_keys file for the root user in the embedded Linux FS. The convenience script automatically loads the key with SSH.
• This Linux configuration uses the SBI console during early boot and loads the full 16550 UART driver for interfacing with the console later on.