I assume this is in case P420i doesn't quite work right in Freenas?
The P420i is an Adaptec/PMC/Microsemi/Microchip Tech (the company changed hands a lot in the last few years) controller. As
@BeTeP noted, its software switchable between HBA and RAID modes. But generally, the BSD backend of Free/TrueNAS hasn't supported the Adaptec (PMC, etc.) controllers as extensively as LSI, so some folks have had problems. There's been a lot of work over the years to bake support for them into BSD (if you google it, you'll find a lot of forum threads with "support was improved in version xxx, try it now..." comments) But, the general wisdom is to just use an LSI based controller and not have to worry about if its working right or not.
Some models of some generations of HP servers (I think including Gen 8 DL380) run fans at 100% if a non-HP card is detected. So, I'd recommend sticking to the HP branded ones if you go the add-in card route just to be safe.