Chuwi Hi9 Air

Chuwi Hi9 Air (Mediatek Helio X20 chipset)

Pretty complete hardware support and good stability with 8.1 Treble GSI (Oreo).

Baseline: Hi9 Air with Chuwi 20181122 stock ROM.

enable "OEM unlock" in Android developer options (type on build number seven times to unlock developer mode in Android system seetings)
use factory settings reset in Android system settings and power off
start device with power and vol+ simultaneously for bootloader menu, select "Fastboot Mode" via 'vol+', then confirm OK via 'vol-'
unlock bootloader (this will persist unless a Chuwi stock ROM is restored using the MTK SP Flash toolkit for Format All + Download)
```
$ fastboot oem unlock
```
Confirm OK on screen.

Caveat: When relocking the bootloader again via fastboot oem lock the device won't boot any custom image...
flash image with the fastboot utility (after uncompressing downloaded xz archive) -> this may be repeated for later image releases with fixes and Android security patches:
```
$ fastboot erase system
$ fastboot erase cache
$ fastboot -u flash system system-arm64-aonly-gapps-su.img
$ fastboot reboot
```
Alternatively, you could flash via TWRP as "System Image" and format data (but this has not worked for me): Link
wait patiently for initial boot to complete (this will take several minutes, you may monitor progress via adb shell top for instance)

Component	Comment
Camera	yes (both work, front cam orientation upside down)
Speaker / Mic	yes / yes
Bluetooth	yes
WiFi	yes
SIM / Mobile Data	yes (SIM1 only, may not reactivate from standby occasionally)
Voice	yes
SMS	yes
VoLTE	no (MTK 6797)
Fingerprint	n/a
NFC	n/a
SD Card	yes
Casting	no, only via Google Home (as for Chuwi stock ROM)

Tested by bofh2k - phh-treble 8.1 v31 (arm64-aonly-gapps-su) - 2019-05-31