http://www.wolfsonmicro.com/products/codecs/WM8758/We thought previously that it only has 2 shelving filters but according to released datasheet
M128EN in PWRMGMT2 controls low power mode in which only two shelving filters work, set by default.
Some sections has differences compared to based-on WM8983 : BEEPCTRL, OUTCTRL, removed speaker driver,
improved DAC SNR THD, lowered ADC performance, common ground settings rejecting ground noise
and proper powering sequences.
I hope it helps developing more complete driver.