mirror of
https://github.com/pygos/build.git
synced 2024-11-16 16:37:10 +01:00
68 lines
2.6 KiB
Markdown
68 lines
2.6 KiB
Markdown
# 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.
|