Flashing an LSI HBA to IT mode should be easy and well-documented - it seems to be a common enough procedure - but it is far from straightforward.
First, there’s a FreeDOS 1.1 bootable USB on Joe Linoff’s website neatly packaged with the files needed to flash an LSI SAS 9211-8i to IT mode. This is a great package for compatible motherboards but it seems many new consumer-grade boards won’t work. I have flashed one 9211-8i this way but did so on an older computer at a friend’s office.
When I’ve attempted to use the DOS utility on a number of other, newer motherboards, it either returns the error “ERROR: Failed to initialize PAL. Exiting program” (on Intel boards) or hangs (on AMD motherboards).
Luckily, Bryan Vyhmeister has a solution for flashing the 9211-8i via the UEFI shell packaged in the BIOS of many modern motherboards. The process is a little obscure but Bryan’s instructions are thorough; I’ve flashed 5 HBAs to IT mode this way. The one issue I’ve encountered is that some UEFI motherboards don’t have a UEFI shell built-in (such as my Gigabyte Z77X-UD5H). The less expensive ASRock motherboards I’ve used do have an easily-accessible UEFI shell (as it seems all UEFI motherboards should).
On ASRock, hit F11 at boot to enter the boot options menu (with all hard drives removed and only the SAS you are working on inserted), then select the UEFI shell and proceed with Bryan’s instructions (near the bottom of the page).
Current Freenas builds use P20 drivers; P20 firmware is easily available on the Avago website.