how to change volume for a2dp headset?

hi,

I just wonder if it is possible to change a2dp headset volume by D-bus
command?

I notice there is a D-bus API SetSpeakerGain in bluez 3.36. I've try it
with my sco headset and it works. But it doesn't work when I try it with
a a2dp headset. Dbus returns an error that is unconnected. Then I
download bluez 4.13 which is the latest build. I notice that API
SetSpeakerGain becomes deprecated. Meanwhile I find control-api.txt
which seems haven't been implemented yet. 

Could anybody give me a hint how to control a2dp headset from host side
by D-bus command?

regards,
Galaha


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