Disclaimer: I should have done it probably with the DAC+ PRO but since I only ordered the DIGI+ I/O I had to try it anyway. Also, Elk Audio OS might be the better choice to get an even lower latency.
My goal was to have an EQ connected via optical SPDIF to even out some room modes for better intelligibility.
1) Installing Twister OS which is 32 bit. I chose it only because I read in one article about it how REAPER ran very well so I didn't think no further.
2) Activate driver for DIGI+ I/O with all the steps from here.
3) Install REAPER for Linux 32 bit armv7l with desktop integration (following the install script)
4) Open REAPER, hit Ctrl+P and choose ALSA with hifiberry as IN and OUT. Samplerate: 48 kHz, bit-depth: 24 bit, buffer: 128 samples. This way you should have enough buffer for the EQ and still a rather low latency (2.6/5.3ms). Also, make sure to turn up the RT priority to 40. Apply settings.
5) Create a new track, enable recording and activate input monitoring. As an effect add ReaEQ. I applied 6 bands and still didn't have drop-outs or anything. The input channel should be stereo, not mono.
6) I measured the room with Room EQ Wizard and applied the suggested EQ filter curves via text file export. Converter between Q and bandwidth using this site.
7) Make REAPER autostart with Twister OS, so you don't have to use display, keyboard or mouse. I followed the steps from here, but you can probably also just go to Session and Startup and add REAPER to your startup programmes.
8) Disable WiFi and Bluetooth by editing /boot/config.txt with root access. Add the two following lines:
Now you simply need to leave the HDMI output connected to the TV or another display. Otherwise REAPER or the audio engine won't start properly. I don't know why, but connecting it to a display helps. Then you can easily leave it on the whole time.
Have fun creating your own room adjuster without the need of a DSP and tell me what how we could even make it more efficient.