It is possible to boot LowRISC in the absence of an SD-Card. This is useful if a large number of files are shared with a Host PC. However there are many variables to be considered which would be inconvenient to hardwire into the binary builds.

We will use the boot selection switches to choose a customised boot option. Prepare a file as follows:

    mount -t proc none /proc
    udhcpc -s /usr/share/udhcpc/default.script
    rdate -s # This should be replaced by a local server
    echo Mounting NFS root
    mount.nfs -o nolock /nfs # This should be updated appropriately
    echo Mounting proc
    mkdir -p /nfs/proc
    mount -t proc none /nfs/proc
    echo Mounting sysfs
    mkdir -p /nfs/sys
    mount -t sysfs none /nfs/sys
    echo Mounting devtmpfs
    mkdir -p /nfs/dev
    mount -t devtmpfs udev /nfs/dev
    mkdir -p /nfs/dev/pts
    echo Mounting devpts
    mount -t devpts devpts /nfs/dev/pts
    echo Mounting tmpfs
    mkdir -p /nfs/tmp
    mount -t tmpfs tmpfs /nfs/tmp
    mkdir -p /run
    mount -t tmpfs tmpfs /run
    umount /proc
    echo Executing switch_root for nfs
    exec switch_root /nfs /sbin/init

This file should be saved as on the DOS partition of the SD-Card. The FPGA board switches should be set to match this value (in hex). Referring to the table in Build the Linux Kernel, it is clear that this means SD-boot, Custom-root, MAC ending in 5.

If it is desired to boot the NFS Client kernel over the Ethernet (a common use case), the file should be called, corresponding to DHCP boot, Custom-root, MAC ending in 5.

Once the settings are working, it may be hardwired into an eventual rebuild of the kernel inside debian-riscv64/work/ (option 1). This option would then not require the SD-Card and it could be reinitialised with a different O/S or any other action requiring full unrestricted access to the card.

On your PC-workstation/server, create a suitable NFSroot mount point (replace the IP address of your FPGA board as necessary):

sudo mkdir -p /local/scratch/debian-riscv64
sudo vi /etc/xinetd.d/time
#Change the first occurence of "disable=yes" to "disable=no". This enables the time service.
sudo /etc/init.d/xinetd restart
sudo /etc/init.d/nfs-kernel-server stop
sudo vi /etc/exports
#Add the following to the end of the file.
cd  /local/scratch/debian-riscv64
sudo tar xJf $TOP/rootfs.tar.xz
#Restart the NFS server
sudo /etc/init.d/nfs-kernel-server start

The dangerous option no_root_squash which allows the NFS client to run as root on the server, may be mitigated by the confinement of the mount to a loopback mount which we can easily set up (not shown, refer to the ethernet-v0.5 release for a suitable example). If access to host files is desired this may be done by copying on the host, or with a second mount point without no_root_squash inside the server’s filing system

Turbo super-user

It is certainly inconvenient requiring slow card operations during normal use. The use of a turbo super-user on the workstation

sudo chroot /local/scratch/debian-riscv64

will rapidly allow the NFSroot to be customised for any possible application. For example if ocaml is to be installed, it can be done in a chroot on the PC, and it will immediately be available on the LowRISC without having to reboot or swap any cards.

This is in essence the best of both worlds, the security of knowing that the software runs on the FPGA, at the same time the convenience and speed of installing on the PC.