Please find below my inline answers…
how do you know what kind of processing is supported in the HiFi DSP?
The HiFi 3 DSP is a powerful VLIW, SIMD featured DSP. It runs upto 533MHz frequency on Hikey960. It has a good set of tools.
Therefore any DSP type application with realtime processing needs can be run on the HiFi DSP. As an example, we are providing Ogg-Vorbis decoder and PCM playback.
How do you know what should the driver support?
Driver handles HiFi 3 load, IPC/mailbox and it is just an interface between DSP app from user space to DSP firmware running at HiFi 3 DSP
are you able to create you own custom HiFi image and load it to HiFi? Are there any instructions for that?
Yes, we can create custom HiFi image and load it to HiFi 3 DSP. When we release software then will release associated document describing how to build image and load it.