The P421i is a fairly simple SAS RAID controller. I picked mine up at the end of 11-2019 as part of a set of servers. I've found its option ROM does not like my z420, it may have issues with other systems but I haven't tested them. On the z420 the solution was to disable downloading the option ROM for the slot that the card is installed in.
In linux, the card can be managed. Here's how.
curl http://downloads.linux.hpe.com/SDR/hpPublicKey1024.pub | apt-key add - curl http://downloads.linux.hpe.com/SDR/hpPublicKey2048.pub | apt-key add - curl http://downloads.linux.hpe.com/SDR/hpPublicKey2048_key1.pub | apt-key add - curl http://downloads.linux.hpe.com/SDR/hpePublicKey2048_key1.pub | apt-key add -
deb http://downloads.linux.hpe.com/SDR/repo/mcp bionic/current non-free
sudo apt update ; sudo apt install ssa ssacli ssaducli
ssacli ctrl all show config