Introduction: Navigating the Dynamic Partition Landscape

The Android ecosystem is constantly evolving, and one of the most significant architectural shifts in recent years has been the introduction of Dynamic Partitions, spearheaded by Project Treble. Initially appearing in Android 10, dynamic partitions revolutionize how storage is managed, allowing over-the-air (OTA) updates to resize or even add partitions without re-flashing the entire device. While this offers immense flexibility for OEMs, it introduces new complexities for custom ROM developers and enthusiasts looking to port AOSP or LineageOS to newer devices. This article delves into the advanced techniques required to successfully port custom ROMs to dynamic partition-enabled devices, focusing on critical device tree adjustments, build considerations, and common troubleshooting.

Understanding Android’s Dynamic Partitions

Traditionally, Android devices used a fixed partition layout where partitions like system, vendor, product, odm, and oem had pre-defined sizes. Dynamic partitions consolidate these into a single logical entity called the super partition. Within this super partition, individual partitions are managed by the Logical Partition Manager (LPM). This system facilitates seamless A/B updates, as Android can now resize or reconfigure logical partitions on the fly to accommodate new images for the inactive slot.

Key Concepts:

• Super Partition: A single physical partition that hosts all dynamic partitions (e.g., system, vendor, product, odm).
• Logical Partitions: Partitions like system, vendor, etc., that reside within the super partition and can be resized or added dynamically.
• Virtual A/B: A system where A/B updates are managed within the super partition, often without requiring dedicated physical A/B slots for all partitions, saving storage.
• LPM Tools: Utilities like lpmake (for creating super images), lpunpack (for extracting), and lp_dump (for inspecting) are crucial for working with dynamic partitions.

Prerequisites and Initial Device Analysis

Before attempting a port, ensure you have a robust AOSP or LineageOS build environment set up and access to your device’s kernel source, device tree, and vendor blobs. Identifying if your target device uses dynamic partitions is the first step. You can usually confirm this by:

1. Checking build.prop: Look for ro.boot.dynamic_partitions_enabled=true in /vendor/build.prop or a similar property.
2. Inspecting fstab: Examine fstab files (e.g., fstab.qcom, fstab.enforcemode) within your device’s vendor image or extracted device tree. You’ll typically find entries for a /super partition and no explicit block device paths for /system, /vendor, etc., but rather references to /dev/mapper/xxx or /dev/block/by-name/super.
3. Analyzing BoardConfig.mk: In the device tree, look for variables like BOARD_SUPER_PARTITION_SIZE, BOARD_SUPER_PARTITION_GROUPS, and BOARD_${GROUP_NAME}_PARTITION_LIST.

Adapting Your Device Tree for Dynamic Partitions

The core of porting to dynamic partition devices lies in correctly configuring your device’s BoardConfig.mk and `fstab` files.

1. BoardConfig.mk Modifications:

The BoardConfig.mk file in your device tree (e.g., device/manufacturer/codename/BoardConfig.mk) needs specific definitions to tell the build system how to construct the super image. Here’s a typical example:

# Dynamic Partitions
BOARD_SUPER_PARTITION_SIZE := 9126805504 # Total size of super partition (e.g., 8.5GB)
BOARD_SUPER_PARTITION_GROUPS := qti_dynamic_partitions # Or any name

# Define the properties for the dynamic partition group
BOARD_QTI_DYNAMIC_PARTITIONS_SIZE := 9122611200 # Must be less than BOARD_SUPER_PARTITION_SIZE
BOARD_QTI_DYNAMIC_PARTITIONS_PARTITION_LIST := system system_ext product vendor odm # List of dynamic partitions

# Optional: If you have a separate product partition
BOARD_USES_PRODUCT_PARTITION := true

# Recovery considerations for A/B devices
TARGET_NO_RECOVERY := true # Often true for dynamic partition devices with A/B updates
TARGET_NO_RECOVERY_RAMDISK := true
TARGET_SUPPORTS_VIRTUAL_AB := true # If your device supports Virtual A/B
TARGET_BOARD_USES_VNDK_SET := true

# Mount points must be defined in fstab, not here
# For devices with logical partitions, ensure these are not hardcoded as block devices
# TARGET_COPY_OUT_SYSTEM := system
# TARGET_COPY_OUT_VENDOR := vendor
# ... and so on for other dynamic partitions

Explanation:

• BOARD_SUPER_PARTITION_SIZE: This is the raw size of the physical super partition on your device. You can often find this by dumping partition information from your stock ROM (e.g., cat /proc/partitions, sgdisk -p /dev/block/by-name/super, or examining `fastboot getvar all` output).
• BOARD_SUPER_PARTITION_GROUPS: Defines the name of your logical partition group.
• BOARD_${GROUP_NAME}_PARTITION_LIST: Lists all the logical partitions that reside within this group and will be part of the super image.
• BOARD_${GROUP_NAME}_SIZE: The *total allocated space* for all logical partitions within the group. This should be slightly less than BOARD_SUPER_PARTITION_SIZE to allow for metadata overhead.

2. fstab Adjustments:

Your fstab file (e.g., fstab.qcom or fstab.hardware) needs to reflect the dynamic nature of partitions. Instead of direct block device paths, you’ll see references to a super partition and mount points for logical partitions.

Example fstab snippet for dynamic partitions:

# Super partition entry
/dev/block/by-name/super        /super                  ext4    ro,barrier=1,discard                              wait,slotselect

# Logical partitions - these will be mounted from the super partition
/dev/block/mapper/system        /system                 ext4    ro,barrier=1                                      wait
/dev/block/mapper/vendor        /vendor                 ext4    ro,barrier=1                                      wait
/dev/block/mapper/product       /product                ext4    ro,barrier=1                                      wait
/dev/block/mapper/odm           /odm                    ext4    ro,barrier=1                                      wait

# If using system-as-root (common on newer devices)
/dev/block/mapper/system        /                       ext4    ro,barrier=1                                      wait

Crucially, ensure that partitions are mounted as `ro` (read-only) for system-level partitions, as modifications are handled via `overlayfs` in Android. The `wait` and `slotselect` flags are also important for correct boot behavior.

3. Vendor Blobs and `Android.mk`/`Android.bp`:

Your device’s vendor tree typically contains Android.mk or Android.bp files that define how vendor-specific libraries and binaries are included. For dynamic partition devices, ensure these build files correctly specify the target partition for their components (e.g., `proprietary/vendor/bin/hw/[email protected]` goes to `/vendor/bin/hw/`). Incorrect placements can lead to SELinux denials or missing services.

Building and Flashing Your Custom ROM

Once your device tree is properly configured, building a custom ROM for dynamic partition devices involves similar steps to traditional builds:

source build/envsetup.sh
lunch lineage_codename-userdebug # Or aosp_codename-userdebug
m make -j$(nproc)

The build system will now use the BoardConfig.mk definitions to create the `super.img` containing all dynamic partitions. Flashing is where the process differs significantly.

Flashing Steps:

For dynamic partition devices, you typically flash the super.img directly using `fastboot`. If your device supports A/B updates and uses `payload.bin`, you might use `fastboot update –skip-reboot <rom_zip>` which handles the `super` partition updates internally.

# Example flashing procedure (varies by device)
fastboot flash abl_a abl.img
fastboot flash abl_b abl.img # If A/B
fastboot flash xbl_a xbl.img
fastboot flash xbl_b xbl.img
# ... flash other critical bootloader/firmware partitions

fastboot erase userdata

# Flash super image (contains system, vendor, product, etc.)
fastboot flash super super.img

# Flash boot.img (kernel + ramdisk)
fastboot flash boot boot.img

# Flash recovery.img (if not using A/B recovery or if flashing dedicated recovery)
# fastboot flash recovery recovery.img

fastboot reboot

Common Errors:

• Failed to allocate space or Unable to load super partition metadata: Indicates an issue with your BOARD_SUPER_PARTITION_SIZE, BOARD_QTI_DYNAMIC_PARTITIONS_SIZE, or BOARD_QTI_DYNAMIC_PARTITIONS_PARTITION_LIST in BoardConfig.mk. Ensure sizes are correct and the partition list matches the dynamic partitions on your device.
• Bootloops: Often related to incorrect fstab entries, missing vendor blobs, or SELinux issues. Verify all partition paths and permissions.

Post-Porting Verification and Troubleshooting

After flashing, boot your device and perform critical checks:

• Check Mount Points: Use adb shell and then mount or df -h to verify that /system, /vendor, /product, etc., are mounted correctly and have appropriate sizes.
• Logcat Analysis: Pay close attention to logcat output during boot for any errors related to mounting, services, or SELinux denials.
• Test Functionality: Verify basic functionalities like Wi-Fi, RIL, camera, and sensors. Missing vendor blobs or incorrect contexts often manifest here.

For SELinux issues, examine dmesg or logcat | grep 'avc:' for denials. These often require adding specific rules to your device’s sepolicy files. Use tools like audit2allow to generate initial rules, but always review and refine them for security.

Conclusion

Porting custom ROMs to dynamic partition devices is undoubtedly more challenging than traditional methods, primarily due to the intricate interplay of the super partition and its logical constituents. However, by meticulously configuring your device tree’s BoardConfig.mk, adapting your fstab, and carefully managing vendor blobs, you can successfully bring the latest AOSP and LineageOS experiences to these advanced devices. The key is a thorough understanding of the dynamic partition architecture and methodical troubleshooting of build and boot issues. With these advanced techniques, the custom ROM community can continue to thrive on the latest Android hardware.