Fix race condition in initializing downmix

The code that initializes downmix GUID searches for an effect of
the right type.  If two or more AudioMixers are created, there is a
possibility that the GUID will temporarily have the wrong value.

Now the initialization is only done once, regardless of how many
AudioMixers there are.

Other minor changes:
 - use naming convention of leading 's' for static member variables
 - log message for no downmix effect is now a warning instead of an error,
   since it is not an error until there is actual content to downmix
   without a downmix effect

Change-Id: I33ab0e51cf9d24ecaf8ae868883736b0772178c4

1 files changed, 2 insertions, 2 deletions

diff --git a/services/audioflinger/AudioMixer.h b/services/audioflinger/AudioMixer.h
index 43aeb86..a15bdca 100644
--- a/services/audioflinger/AudioMixer.h
+++ b/services/audioflinger/AudioMixer.h
@@ -256,9 +256,9 @@ private:
     state_t         mState __attribute__((aligned(32)));
 
     // effect descriptor for the downmixer used by the mixer
-    static effect_descriptor_t dwnmFxDesc;
+    static effect_descriptor_t sDwnmFxDesc;
     // indicates whether a downmix effect has been found and is usable by this mixer
-    static bool                isMultichannelCapable;
+    static bool                sIsMultichannelCapable;
 
     // Call after changing either the enabled status of a track, or parameters of an enabled track.
     // OK to call more often than that, but unnecessary.