did you test dac+ (or amp2) with Linux 5.10 from raspberry ?
I build an image with BuildRoot 2021-02 which use linux-custom 5.10.
The overlay (from /proc/device-tree) is corresponding to the one in the kernel linux. The pcm512x driver contains the patch about SCLK, and it uses the I2C address 0x4d. The hifiberry-dacplus is loaded with dependencies, and it uses the pcm512x driver.
Alsamixer gives some entries (Analogue, Analogue Playback Boost,...) and all looks fines.
aplay -L shows the card and speaker-test request 2 channels. BUT no sound from the speaker, just a "pok" when the test is stopping.
I tested with hifiberryOS and the hardware runs correctly.
Do you have any idea ?
I saw a patch from hifiberryOS about pcm512x to switch off the PM. Is it mandatory ?