Comment 22 for bug 1201528

you have to ask the author since he assume no gain in vmaster but slaves can have gain in your case

https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/commit/?id=3b0a5f22d4649433a5842ffc7313803292e95718

+/*
+ * Add a slave control to the group with the given master control
+ *
+ * All slaves must be the same type (returning the same information
+ * via info callback). The fucntion doesn't check it, so it's your
+ * responsibility.
+ *
+ * Also, some additional limitations:
+ * - at most two channels
+ * - logarithmic volume control (dB level), no linear volume
+ * - master can only attenuate the volume, no gain