All H.264 (and other CODECs) prediction are covered. As stated correctly in previous comment, userspace do the bookkeeping. Kernel is aware of the reference buffers and it's attachements (HW specific buffers). All references needed for predictive decoding (regardless if it's B or P), are programmed for each jobs.

Exception to FMO and ASO mode, which is rarely supported in HW, even FFMPEG sw decoder didn't bother implementing that