DC offset on microphones and workaround.

When recording from the on-board DMICs and the microphone on my headset, I have noticed DC offset.
On my headset microphone, not so much, but on the DMICs there is a lot.
When recording silence, the signal rests at around -30dB on the positive side.

The easy way to remove DC offset is to apply a fairly steep high-pass filter with a low cutoff frequency, in post-production.
E.g. in Audacity you can go to Effects... -> Equalization... and apply the 100Hz Rumble EQ curve.

But perusing the WM5102 data sheet revealed that it has filters on-chip, so why not use them?

Here's my modified use case script for recording from the DMICs:

#!/bin/bash
# Record from onboard DMICs to RPi.
# Route through High Pass Filter to get rid of DC offset.
#
# IN2 --> LHPF --> AIF1TX
# May want to tune the gain here
# Need at least -6dB
amixer -Dhw:0 cset name='IN2L Digital Volume' 128
amixer -Dhw:0 cset name='IN2R Digital Volume' 128
# DMICs are incorrectly labeled on the board.
# DMICL is actually the right channel and vice versa,
# so we'll swap them here.
# i.e. IN2R --> LHPF1
#      IN2L --> LHPF2
# Route DMICs (IN2) to LHPFs.
amixer -Dhw:0 cset name='LHPF1 Input 1' IN2R
amixer -Dhw:0 cset name='LHPF2 Input 1' IN2L
amixer -Dhw:0 cset name='LHPF1 Mode' High-pass
amixer -Dhw:0 cset name='LHPF2 Mode' High-pass
amixer -Dhw:0 cset name='LHPF1 Coefficients' 240,3
amixer -Dhw:0 cset name='LHPF2 Coefficients' 240,3 
# Connect outputs of LHPFs to inputs of AIF1 (I2s to RPi)
amixer -Dhw:0 cset name='AIF1TX1 Input 1' LHPF1
amixer -Dhw:0 cset name='AIF1TX1 Input 1 Volume' 32
amixer -Dhw:0 cset name='AIF1TX2 Input 1' LHPF2
amixer -Dhw:0 cset name='AIF1TX2 Input 1 Volume' 32
amixer -Dhw:0 cset name='DMIC Switch' on

As can bee seen in the picture, the difference is obvious.
The top stereo track was recorded using the supplied Record_from_DMIC.sh use case script
and the bottom track with my modifications.

I have no idea about what the cut-off frequency of the filter is or how steep it is. The command

     amixer -Dhw:0 cget name='LHPF1 Coefficients'

only reveals two values, both of which are zero:

numid=85,iface=MIXER,name='LHPF1 Coefficients'
       ; type=BYTES,access=rw------,values=2
       : values=0x00,0x00

I'm guessing they represent frequency and steepness in some way, but the datasheet only has this to say about them:
"These coefficients are derived using tools provided in Wolfson’s WISCE evaluation board control software; please contact your local Wolfson representative for more details."