Pixel 9a Boot Chain Notes

These notes document the Pixel 9a tegu boot-chain behavior found during the first OpenPhone smoke install.

Relevant Partitions

The Pixel 9a uses A/B slots. The first OpenPhone smoke OTA installed to slot _b during bringup.

Partitions involved in the current boot path:

boot
init_boot
vendor_boot
vendor_kernel_boot
dtbo
vbmeta
vbmeta_system
vbmeta_vendor
dynamic partitions: system, system_ext, product, vendor, system_dlkm, vendor_dlkm

OpenPhone userspace was valid enough to boot, but the generated vendor_kernel_boot.img was not. With the OpenPhone-generated vendor_kernel_boot_b, the phone fell back to fastboot before Android started.

The failure did not produce useful Android userspace logs because the kernel never reached normal boot.

Isolation Result

Starting from a working slot _b with OpenPhone dynamic partitions and the official Lineage boot chain:

OpenPhone boot_b: booted
OpenPhone init_boot_b: booted
OpenPhone vendor_boot_b: booted
OpenPhone vendor_kernel_boot_b: fell back to fastboot

This isolated the failure to vendor_kernel_boot.

DTB Packaging Issue

device/google/zumapro/BoardConfig-common.mk configures:

BOARD_PREBUILT_DTBIMAGE_DIR := $(TARGET_KERNEL_DIR)

Android's target-files packaging creates dtb.img by concatenating:

$(wildcard $(BOARD_PREBUILT_DTBIMAGE_DIR)/*.dtb)

For the tegu prebuilts observed during bringup, device/google/tegu-kernels/6.1/ did not contain any standalone *.dtb files. It did contain a prebuilt vendor_kernel_boot.img with the DTB embedded.

Result: target-files generated VENDOR_KERNEL_BOOT/dtb as a zero-byte file, then built a vendor_kernel_boot.img with DTB size 0.

Build Fix

scripts/build.sh now handles this automatically for openphone_tegu and openphone_tegu_smoke when the build goal produces target-files or an OTA. It builds unpack_bootimg if needed, extracts the DTB from the prebuilt image, and verifies the generated vendor_kernel_boot.img after the Android build.

The manual equivalent is:

cd /path/to/OpenPhone
OPENPHONE_ANDROID_DIR=/path/to/android/tree ./scripts/prepare-tegu-dtb.sh

Expected DTB SHA-256:

f1aed2bc4c07d3cb1e610f5227a566f22e995dfe05341ca6bf14805be6928688

Then build target-files and the OTA.

Validation

After rebuilding, inspect the generated image manually with:

cd /path/to/OpenPhone
OPENPHONE_ANDROID_DIR=/path/to/android/tree ./scripts/verify-tegu-bootchain.sh openphone_tegu

Expected result:

DTB size: 1546258
DTB SHA-256: f1aed2bc4c07d3cb1e610f5227a566f22e995dfe05341ca6bf14805be6928688

Open Question

The product variable PRODUCT_BUILD_VENDOR_KERNEL_BOOT_IMAGE := false was tried in the OpenPhone product overlay but did not prevent target-files from rebuilding vendor_kernel_boot.img. Treat that variable as insufficient for this branch until proven otherwise.