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69 lines
2.6 KiB
Markdown
69 lines
2.6 KiB
Markdown
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# Pygos Boot Process
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The boot loader of the board loads the Linux kernel (and possibly device tree
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files) into memory and jumps into the kernel.
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The Linux kernel of the Pygos system has an initial ram disk built in with a
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small setup script that mounts the actual root file system, an overlay
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filesystem and does a switch_root.
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## Filesystem Setup
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The init script processes the following parameters from the kernel
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command line to set up the VFS root:
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* `root` the path to a device file containing the root filesystem image.
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This parameter is mandatory.
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* `root_sfs` the path to a Squashfs image on the `root` device. If this
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parameter is missing, the `root` partition is mounted to the VFS root
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instead of the Squashfs image.
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* `overlay_dev` the path to a device file containing the overlay filesystem.
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The device is mounted to `/cfg/overlay` and an overlay mount to `/etc` is
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created with the lower directory in `/cfg/preserve/etc` and the upper
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in `/cfg/overlay/etc`. If the `overlay_dev` parameter is omitted, a bind
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mount to `/cfg/preserve/etc` is created instead.
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The additional parameters `root_type` and `overlay_type` can be used to specify
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additional mount options for the boot and overlay partitions respectively.
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The following type options are currently supported:
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* `hwdevice` (default if the option is missing). Wait in a loop until the
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specified device special file exists, then attempt to mount it and let
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the kernel auto-detect the filesystem.
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* `qemu` specifies that the device is actually a mount tag for a virtio
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connected 9p network filesystem.
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* Every other option is passed to the `mount` command as filesystem type
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without waiting for the device file to be present.
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After mounting the `overlay_dev` device, the script populates the mount point
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with expected default directories, so even a freshly formatted filesystem
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partition can be used without further preparation.
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## Examples
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The following kernel command line mounts a the Squashfs image `rootfs.img`
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stored on `/dev/sda1` to the filesystem root with a `tmpfs` overlay:
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root=/dev/sda1 root_sfs=rootfs.img overlay_dev=none overlay_type=tmpfs
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For a Qemu virtual machine, the following command line mounts a virtio attached
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disk to the filesystem root and attaches as overlay device a directory on the
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host system via a virtio attached 9p network mount:
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root=/dev/vda1 overlay_dev=ovdef overlay_type=qemu
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## Switchroot Transition
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Once the setup script completes, it switches to the newly mounted and
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configured filesystem root.
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If the option `singleuser` is present on the kernel command line, it executes
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a shell, otherwise it launches the init system.
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