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amixer/ALSA mixer API weirdness

Hi, on an RPi 3B+ with AMP2 running 4.14.93-v7+, the ALSA mixer behaves weirdly. If I set mixer values for the "Digital" control programmatically, I get "Input/output error" from snd_mixer_selem_set_playback_dB(elem, chlist[i], chval[i], val). With "amixer sset Digital 60%" on the command line, the first time I set a new value it claims it fails ("Invalid command!", the second time it apparently succeeds.
In fact, each attempt actually succeeds the first time (I'm running "watch amixer sget Digital" on another console and the value changes immediately), but the error message is irritating. Can other HifiBerry users reproduce this? It might hint at some corner-case bug in the driver, or in libasound...

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