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Merge pull request ostreedev#3232 from ericcurtin/aboot-bootloader
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docs: More accurate diagram in bootloaders documentation for aboot
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@cgwalters
cgwalters committed May 2, 2024
2 parents fa720d1 + 9e51fb1 commit cb13977
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24 changes: 12 additions & 12 deletions docs/bootloaders.md
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Expand Up @@ -30,9 +30,9 @@ For historical reasons, OSTree defaults to detecting the bootloader; if some GRU

## OSTree and aboot

The Android bootloader is another bootloader than may be used with ostree. It still uses the files in `/boot/loader/entries` as metadata, but the boootloader does not read these files. Android bootloaders package their kernel+initramfs+cmdline+dtb in a signed binary blob called an [Android Boot Image](https://source.android.com/docs/core/architecture/bootloader/boot-image-header). This binary blob then is written to either partition boot_a or boot_b depending on which slot is suitable.
The Android bootloader is another bootloader that may be used with ostree. It still uses the files in `/boot/loader/entries` as metadata, but the boootloader does not read these files. Android bootloaders package their kernel+initramfs+cmdline+dtb in a signed binary blob called an [Android Boot Image](https://source.android.com/docs/core/architecture/bootloader/boot-image-header). This binary blob then is written to either partition boot_a or boot_b depending on which slot is suitable.

Android bootloaders by design inject kargs into the cmdline, some patches may be required in the Android bootloader implementation to ensure that the firmware does not switch between system_a and system_b partitions by populating a `root=` karg, or that a `ro` karg that is incompatible with ostree is not inserted. Conversly leaving the `androidboot.slot_suffix=` karg injecting functionality is required but this is commonplace for any Android Bootloader that does AB updates.
Android bootloaders by design inject kargs into the cmdline, some patches may be required in the Android bootloader implementation to ensure that the firmware does not switch between system_a and system_b partitions by populating a `root=` karg, or that a `ro` karg is not inserted (this karg is incompatible with ostree).

We have two accompanying scripts that work with this type of environment:

Expand All @@ -41,19 +41,19 @@ We have two accompanying scripts that work with this type of environment:
[aboot-deploy](https://gitlab.com/CentOS/automotive/rpms/aboot-deploy) reads what the current slot is according to the `androidboot.slot_suffix=` karg, writes to the alternate boot_a or boot_b slot and sets a symlink either /ostree/root.a or /ostree/root.b so that it is known which userspace directory to boot into based on the `androidboot.slot_suffix=` karg, on subsequent boots.

```
+-----------------------------+ +---------------------------------+
| | +------------------+ | |
| firmware appends: | | | | |
+---------------------------------+
+-----------------------------+ +------------------+ | |
| bootloader_a appends karg: | | | | |
| +--->+ boot_a partition +--->+ |
| androidboot.slot_suffix=_a | | | | /ostree/root.a |
| | +------------------+ | |
| or | | system_a partition |
| | +------------------+ | |
| androidboot.slot_suffix=_b | | | | /ostree/root.b |
+-----------------------------+ +------------------+ | |
| system partition |
+-----------------------------+ +------------------+ | |
| bootloader_b appends karg: | | | | /ostree/root.b |
| +--->+ boot_b partition +--->+ |
| to cmdline | | | | |
| | +------------------+ | |
+-----------------------------+ +---------------------------------+
| androidboot.slot_suffix=_b | | | | |
+-----------------------------+ +------------------+ | |
+---------------------------------+
```

## GRUB and os-prober
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