docs: Add OS topic on partition table

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

docs/index.rst

@@ -13,6 +13,7 @@ Build System Guide
    repo-workspace
    build-flavours/index
    ohos-build/index
+   os/index
    ci/index
    blueprints/index
    supported-images

docs/os/index.rst

+.. SPDX-FileCopyrightText: Huawei Inc.
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+Operating System
+################
+
+All Scenarios OS provides support for a set of kernels. This documentation
+details various aspects for each kernel type.
+
+.. toctree::
+   :maxdepth: 2
+
+   linux/index

docs/os/linux/index.rst

+.. SPDX-FileCopyrightText: Huawei Inc.
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+All Scenarios OS - Linux
+########################
+
+.. toctree::
+   :maxdepth: 1
+
+   partition-table

docs/os/linux/partition-table.rst

+.. SPDX-FileCopyrightText: Huawei Inc.
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+Disk Partition Table
+####################
+
+.. contents::
+   :depth: 2
+
+Overview
+********
+
+The OS defines the partitions included as part of the Linux-based distro as it
+follows:
+
+* boot
+
+  * filesystem label: x-boot (partition name when relevant)
+
+  * It provides boot artefacts required by the lower bootloader assumptions. It
+    is device-specific both in terms of filesystem and content.
+
+* sys-a
+
+  * filesystem label: x-sys-a (partition name when relevant)
+
+  * It provides the root filesystem hierarchy.
+
+  * Filesystem type, configuration and structure are device-independent.
+
+  * This partition is the only one provided with a redundant counterpart (see
+    below).
+
+* sys-b
+
+  * filesystem label: x-sys-b (partition name when relevant)
+
+  * It provides a redundant root filesystem hierarchy used as part of the
+    system update strategies.
+
+  * Filesystem type, configuration and structure are device-independent.
+
+* dev-data
+
+  * filesystem label: x-dev-data (partition name when relevant)
+
+  * Device-specific data meant to be preserved over system reset (factory
+    reset).
+
+  * The runtime will completely treat this data read-only.
+
+* sys-data
+
+  * filesystem label: x-sys-data (partition name when relevant)
+
+  * This partition holds the system state to deal with the root filesystem as a
+    read-only asset.
+
+  * It ties closely into the system update strategies.
+
+  * Data is kept over system updates (subject to state transition hooks) but
+    discarded over factory reset.
+
+* app-data
+
+  * filesystem label: x-app-data (partition name when relevant)
+
+  * This partition provides application data storage.
+
+  * Data is kept over system updates (subject to state transition hooks) but
+    discarded over factory reset.
+
+The build system tries to unify the partition as much as possible, leaving
+upper layers (for example, the system update layers) with as few deviations to
+deal with as feasible. This means that filesystem labels and partition names
+are to be assumed by the OS components.
+
+Partition Table
+***************
+
+The OS will support both MBR and GPT as partition table type. In this way, the
+OS can achieve more extensive device support.
+
+The OS assumes a GPT disk layout as it follows:
+
+* 4MiB is left untouched at the start of the disk (to accommodate for
+  hardware-specific requirements).
+
+* All partitions are aligned to 4MiB.
+
+* The filesystem labels and partition names are as described above.
+
+On the MBR side, the disk layout is similar to GPT. The design mainly
+workarounds the four physical partitions limitation:
+
+* 4MiB is left untouched at the start of the disk (to accommodate for
+  hardware-specific requirements).
+
+* All partitions are aligned to 4MiB.
+
+* The filesystem labels and partition names are as described above.
+
+* The 4th partition is defined as extended and contains all the data partitions
+  (dev-data, sys-data and app-data).