Confirmed working on HP DL360p G8 with P420i on firmware 8.32 (latest).
A few notes that might help someone:
1. The controller automatically creates a temporary logical volume when it detects a disk. You MUST delete all logical volumes before enabling HBA mode or you'll get "operation not supported" error, example:
ssacli ctrl slot=0 ld 1 delete forced
2. When you have you logical volumes clean, you can activate HBA:
ssacli ctrl slot=0 modify hbamode=on
3. I didn't know about this, but in a Arch liveiso, you can expand space right into the liveiso remounting
mount -o remount,size=2G /run/archiso/cowspace
4. Also,
ssacli-bin (AUR) its broken, the comments post a fix, you need to download PKGBUILD, and change the version to "v6.45_8.0" and the b2sum and install with "makepkg -si", so, i can also confirm its work with ssacli 6.45.
After reboot, POST shows "Hardware RAID is disabled via NVRAM config" or something like that. So, still working ^^
Thanks for the guide! 10/10