cvw/linux

Linux for core-v-wally

Table of Contents

1. Setting up Buildroot
2. Generating Device Tree Binaries
3. Disassembling the Binaries for Debugging
4. Creating a Bootable SD Card

Setting up Buildroot

In order to generate the Linux and boot stage binaries compatible with Wally, Buildroot is used for cross-compilation.

To set up a Buildroot directory, configuration files for Buildroot, Linux, and Busybox must be copied into the correct locations inside the main Buildroot directory. Buildroot and device tree binaries must be generated as well. This can all be done automatically using the Makefile inside Wally's Linux subdirectory (this one). To install a new buildroot directory, build the Buildroot binaries, generate the device tree binaries, generate test-vectors for simulation, and install the buildroot package needed to build the SD card driver for Linux, run:

$ make

This installs to the $RISCV directory.

Optionally, you can override the BUILDROOT variable to use a different buildroot source directory.

$ make install BUILDROOT=path/to/buildroot

Generating Device Tree Binaries

The device tree files for the various FPGA's Wally supports, as well as QEMU's device tree for the virt machine, are located in the ./devicetree subdirectory. These device tree files are necessary for the boot process.

They are built automatically using the main make command. To build the device tree binaries (.dtb) from the device tree sources (.dts) separately, we can build all of them at once using:

$ make generate #optionally override BUILDROOT

The .dts files will end up in the <BUILDROOT>/output/images folder of your chosen buildroot directory.

Disassembling the Binaries for Debugging

By using the riscv64-unknown-elf-objdump utility, we can disassemble the binaries in <BUILDROOT>/output/images so that we can explore the resulting machine code instructions and see what assembly or C code the instructions came from, along with the corresponding addresses. This is useful during debugging in order to trace how code is being executed.

The disassembled binaries are built automatically using the main make command. To create the disassembled binaries separately, run:

$ make disassemble #optionally override BUILDROOT

You'll find the resulting disassembled files in <BUILDROOT>/output/images/disassembly.

Creating a Bootable SD Card

To flash a bootable sd card for Wally's bootloader, use the flash-sd.sh script located in <WALLY>/linux/sdcard. The script allows you to specify which buildroot directory you would like to use and to specify the device tree. By default it is set up for the default location of buildroot in $RISCV and uses the vcu108 device tree. To use the script with your own buildroot directory and device tree, type:

$ cd sdcard
$ ./flash-sd.sh -b <path/to/buildroot> -d <device tree name> <DEVICE>

for example

$ ./flash-sd.sh -b ~/repos/buildroot -d wally-vcu118.dtb /dev/sdb