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build/docs/boot.md

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Pygos Boot Process

The boot loader of the board loads the Linux kernel (and possibly device tree files) into memory and jumps into the kernel.

The Linux kernel of the Pygos system has an initial ram disk built in with a small setup script that mounts the actual root file system, an overlay filesystem and does a switch_root.

Filesystem Setup

The init script processes the following parameters from the kernel command line to set up the VFS root:

  • root the path to a device file containing the root filesystem image. This parameter is mandatory.
  • root_sfs the path to a Squashfs image on the root device. If this parameter is missing, the root partition is mounted to the VFS root instead of the Squashfs image.
  • overlay_dev the path to a device file containing the overlay filesystem. The device is mounted to /cfg/overlay and an overlay mount to /etc is created with the lower directory in /cfg/preserve/etc and the upper in /cfg/overlay/etc. If the overlay_dev parameter is omitted, a bind mount to /cfg/preserve/etc is created instead.

The additional parameters root_type and overlay_type can be used to specify additional mount options for the boot and overlay partitions respectively.

The following type options are currently supported:

  • hwdevice (default if the option is missing). Wait in a loop until the specified device special file exists, then attempt to mount it and let the kernel auto-detect the filesystem.
  • qemu specifies that the device is actually a mount tag for a virtio connected 9p network filesystem.
  • Every other option is passed to the mount command as filesystem type without waiting for the device file to be present.

After mounting the overlay_dev device, the script populates the mount point with expected default directories, so even a freshly formatted filesystem partition can be used without further preparation.

Examples

The following kernel command line mounts a the Squashfs image rootfs.img stored on /dev/sda1 to the filesystem root with a tmpfs overlay:

root=/dev/sda1 root_sfs=rootfs.img overlay_dev=none overlay_type=tmpfs

For a Qemu virtual machine, the following command line mounts a virtio attached disk to the filesystem root and attaches as overlay device a directory on the host system via a virtio attached 9p network mount:

root=/dev/vda1 overlay_dev=ovdef overlay_type=qemu

Switchroot Transition

Once the setup script completes, it switches to the newly mounted and configured filesystem root.

If the option singleuser is present on the kernel command line, it executes a shell, otherwise it launches the init system.