wm8975: document audiohw settings

Change-Id: I0456f85e4ebc821c23e25026fbee3d8cf7526ee7

2 files changed, 7 insertions, 0 deletions

diff --git a/firmware/drivers/audio/wm8975.c b/firmware/drivers/audio/wm8975.c
index 88be2cf5ce..a1af1331f2 100644
--- a/firmware/drivers/audio/wm8975.c
+++ b/firmware/drivers/audio/wm8975.c
@@ -296,6 +296,8 @@ void audiohw_disable_recording(void)
     wm8975_write(PWRMGMT1, wm8975_regs[PWRMGMT1]);
 }
 
+/* volume in 0 .. 63, corresponds to -17.25dB .. 30dB in steps of 0.75dB
+ * microphone has an extra 20dB boost so 0 .. 63 corresponds to 2.75dB .. 50dB */
 void audiohw_set_recvol(int left, int right, int type)
 {
     switch (type)
diff --git a/firmware/export/wm8975.h b/firmware/export/wm8975.h
index cdae7af865..b8ef17910c 100644
--- a/firmware/export/wm8975.h
+++ b/firmware/export/wm8975.h
@@ -29,8 +29,13 @@ AUDIOHW_SETTING(VOLUME,     "dB", 0, 1, -74,  6, -25)
 AUDIOHW_SETTING(BASS,       "dB", 0, 1,  -6,  9,   0)
 AUDIOHW_SETTING(TREBLE,     "dB", 0, 1,  -6,  9,   0)
 #ifdef HAVE_RECORDING
+/* The input PGAs have a gain range from -17.25dB .. 30dB in steps of 0.75dB
+ * Values: 0, 1, ..., 63
+ *      => -17.25, -16.5dB, ..., 30.0dB */
 AUDIOHW_SETTING(LEFT_GAIN,  "dB", 1, 1,   0, 63,  23, ((val - 23) * 15) / 2)
 AUDIOHW_SETTING(RIGHT_GAIN, "dB", 1, 1,   0, 63,  23, ((val - 23) * 15) / 2)
+/* The microphone has an extra 20dB boost before the input PGA so 0 .. 63
+ * corresponds to 2.75dB .. 50dB */
 AUDIOHW_SETTING(MIC_GAIN,   "dB", 1, 1,   0, 63,   0, ((val - 23) * 15) / 2 + 200)
 #endif /* HAVE_RECORDING */