The problem...

OpenMandriva Mobile is working, but while we're waiting for our PinePhone, there is no device in the correct form factor to run it on. This is because phone handsets, unlike PCs or ARM server hardware, don't support installing alternative operating systems, and even if a bootloader can be unlocked, typically only Android drivers are available.

Even devices with open bootloaders and halfway recent kernels (e.g. Sony Xperia 1) come with a combination that isn't supportable without considerable work (e.g. Freedreno in kernel 4.14 doesn't support the variant of Adreno found in the SM8150 chipset).

Pixel 3 is another phone worth looking at, but it uses a display that requires Display Stream Compression, which is not yet supported by open drivers. Others (with a lot more manpower) are facing the same problems even when trying to run a custom Android build on top of a current kernel. https://www.linaro.org/blog/aosp-on-pixel3-pocof1-running-aosp-with-mainline-kernel-on-form-factor-devices/

Most devices not having a serial port or any other way to see kernel bootup messages doesn't make it easier.

There's libhybris, but that would force us into building a 32-bit OS for 64-bit hardware, so we've opted not to do this.

The workaround...

Plasma Mobile reference builds have gone the 32-bit/libhybris route and use the same user interface and a largely comparable system structure (systemd, simplelogin, plasma-mobile, ...).

So for previews, we're taking the Plasma Mobile reference build, adding 64-bit libraries and binaries where UI access (and therefore libhybris) isn't required, and patching some settings. This includes adding rpm for installation of extra packages.

The result isn't as good as what we're running on devboards, but gives a good enough preview, with the user visible (without opening a terminal) differences being mostly the lower speed of a 32-bit OS and the older versions of some applications in the Plasma Mobile reference build.

Building the image

Run ./modify-plasma-image. It will:

Download the TWRP recovery image (necessary for installation)
Download the Plasma Mobile reference build
Download relevant OpenMandriva packages and create an OpenMandriva rootfs
Repackage the root filesystem containing parts of both rootfs images
Ignore errors about files already existing during decompression, they're harmless and caused by conflicts between the PM reference build and OMV packages. DNF does the right thing, so there is no need to preprocess the rootfs to get rid of those warnings.

Installing on a Nexus 5X

Unlock the bootloader as described here: https://forum.xda-developers.com/nexus-5x/general/guides-how-to-guides-beginners-t3206930
Boot the device into fastboot mode (Power off, then press Volume Down and hold it while turning the phone back on)
Run: sudo ./pm-flash
Follow the prompts of pm-flash
If it doesn't work, you may have to reinstall the vendor files (this may be necessary depending on which version of Android was installed on the device before) by running "sudo ./flash-vendor"
The PIN for the screen lock (and password for ssh access) is 1234