I've found a couple of different solutions for anyone else who stumbles on 
this issue.

If you follow the directions and use TARGET_PREBUILT_KERNEL, all of the 
kernel modules in vendor.img and vendor_boot.img are out of sync. Not only 
do you need to get the updated kernel in the aosp build, but all of the 
updated kernel modules.

After building the kernel, I would suggest ignoring the 
TARGET_PREBUILT_KERNEL option and instead, just copy everything from 
aosp-kernel/out/android-msms-pixel-4.19/dist to 
aosp/device/google/redbull-kernel.

A little background, the vendor_boot.img has a ramdisk with kernel modules 
pulled from this directory.

Next is the problem with the vendor.img, which is cumbersome to 
unpack/repack with the updated kernel modules.

For the vendor.img, I suggest using product-based vendor overlay files. 
Vendor overlay files populate the product.img with the updated kernel 
modules that get overlaid on the vendor image on boot.

https://source.android.com/devices/bootloader/partitions/system-as-root#using-vendor-overlay
 
<http://I've found a couple of different solutions for anyone else who 
stumbles on this issue. If you follow the directions and use 
TARGET_PREBUILT_KERNEL, all of the kernel modules in vendor.img and 
vendor_boot.img are out of sync. Not only do you need to get the updated 
kernel in the aosp build, but all of the updated kernel modules. After 
building the kernel, I would suggest ignoring the TARGET_PREBUILT_KERNEL 
option and instead, just copy everything from 
aosp-kernel/out/android-msms-pixel-4.19/dist to 
aosp/device/google/redbull-kernel. A little background, the vendor_boot.img 
has a ramdisk with kernel modules pulled from this directory. Next is the 
problem with the vendor.img, which is not trivial to unpack/repack with the 
updated kernel modules. For the vendor.img, I suggest using product-based 
vendor overlay files. Vendor overlay files populate the product.img with 
the updated kernel modules that get overlaid on the vendor image on boot. 
https://source.android.com/devices/bootloader/partitions/system-as-root#using-vendor-overlay>

After doing all of the above and rebuilding asop, your vendor_boot.img and 
product.img (with vendor_overlay) has the updated kernel modules that will 
work with the new kernel.

On Tuesday, June 15, 2021 at 6:35:08 PM UTC-4 Andrew Fernandes wrote:

> I'm following the instructions on source.android.com 
> <https://source.android.com/setup/build/building-kernels> for building 
> kernels and unable to get it working on the Pixel 5.
>
> There are a couple of paths I've taken with differing results.
>
> 1. Check out the aosp-kernel android-msm-redbull-4.19-android11-qpr2 branch
> 2. Run the build_redbull.sh (or build_rebull-gki.sh) script.
> 3. In the aosp directory, export TARGET_PREBUILT_KERNEL to the built 
> Image.lz4-dtb from aosp-kernel
> 4. flash aosp using `fastboot flashall -w`
> 5. Observe bootloop, the devices never reaches a point where adbd starts 
> and I can shell in
>
> Then I saw from other posts that when recompiling the kernel the prebuilt 
> kernel modules that are in the aosp repository are out of date and are 
> being packaged into the vendor_boot image and failing to load.
>
> So... next I tried copying all of the compiled aosp-kernel modules and 
> images into the aosp/device/google/redbull-kernel directory and rebuilding.
>
> This gets farther and I'm now able to adb shell into the device, but the 
> display stays on the Android boot logo and various other services/sensors 
> don't work.
>
> So, my question is, what's the process for a successful build/flash of the 
> aosp kernel on the Pixel 5?
>
> For reference I'm using the aosp android-11.0.0_r37 branch and aosp kernel 
> android-msm-redbull-4.19-android11-qpr2 branch.
>
> Any help/instruction would be greatly appreciated!
>

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