Mute ALSA audio completely

Mute ALSA audio completely

The PCM control is needed to mute audio completely, but it is not available until at least one pcm stream is opened.  This fix just opens and closes a pcm stream before gathering information about which mixer controls are available.

BUG=chromium-os:12151
TEST=Manual, play audio then mute.  No audio should be heard through headphones.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=76943

[davejcool, 2011-03-02 02:12:10]

This should fix http://crosbug.com/12151 if the problem is that some audio at a
low volume is still being heard with headphones when muted.

[Daniel Erat, 2011-03-02 17:45:07]

LGTM

I noticed some warnings getting printed when I pressed the mute key on a Mario
device in a build that I did a day or two ago.  Does this fix that?  (Sorry, I'm
unable to repro it on a different machine right now -- I'll let you know the
exact text if I see it again.)

http://codereview.chromium.org/6591095/diff/1/chrome/browser/chromeos/audio_m...
File chrome/browser/chromeos/audio_mixer_alsa.h (right):

http://codereview.chromium.org/6591095/diff/1/chrome/browser/chromeos/audio_m...
chrome/browser/chromeos/audio_mixer_alsa.h:86: void PingPCM();
worthwhile to return a bool here?

[scherkus (not reviewing), 2011-03-02 19:07:11]

LGTM w/ recommendation

http://codereview.chromium.org/6591095/diff/1/chrome/browser/chromeos/audio_m...
File chrome/browser/chromeos/audio_mixer_alsa.cc (right):

http://codereview.chromium.org/6591095/diff/1/chrome/browser/chromeos/audio_m...
chrome/browser/chromeos/audio_mixer_alsa.cc:249: PingPCM();
maybe inline this code considering it's not caleld anywhere else and you already
have a bunch of snd_XXX_YYY() functions + error checking + logging

[davejcool, 2011-03-03 20:05:02]

I think it's ready to go now.

> I noticed some warnings getting printed when I pressed the mute key on a Mario
> device in a build that I did a day or two ago.  Does this fix that?  (Sorry,
I'm
> unable to repro it on a different machine right now -- I'll let you know the
> exact text if I see it again.)

Before this change there was an error, "ALSA unable to find simple control PCM"
which should not happen anymore.  I noticed a new warning after this CL when
hitting Mute (snd_mixer_selem_set_playback_switch_all() failed:) but that is
fixed now as well since there is no need to set mute on the PCM control.

http://codereview.chromium.org/6591095/diff/1/chrome/browser/chromeos/audio_m...
File chrome/browser/chromeos/audio_mixer_alsa.cc (right):

http://codereview.chromium.org/6591095/diff/1/chrome/browser/chromeos/audio_m...
chrome/browser/chromeos/audio_mixer_alsa.cc:249: PingPCM();
It is remarkably close to the other calls in InitializeAlsaMixer(), so I'm now
inlining instead.

http://codereview.chromium.org/6591095/diff/1/chrome/browser/chromeos/audio_m...
File chrome/browser/chromeos/audio_mixer_alsa.h (right):

http://codereview.chromium.org/6591095/diff/1/chrome/browser/chromeos/audio_m...
chrome/browser/chromeos/audio_mixer_alsa.h:86: void PingPCM();
I thought about returning bool here as well, but Andrew suggested just in-lining
it... which I'm also fine with since it's not called from anywhere else, and is
very similar to the other code in that area.

http://codereview.chromium.org/6591095/diff/7001/chrome/browser/chromeos/audi...
File chrome/browser/chromeos/audio_mixer_alsa.cc (left):

http://codereview.chromium.org/6591095/diff/7001/chrome/browser/chromeos/audi...
chrome/browser/chromeos/audio_mixer_alsa.cc:179:
SetElementMuted_Locked(elem_pcm_, mute);
It turns out the PCM mixer element does have a mute control and does not need to
be muted as well.  Before this change CL, the PCM element was not even being
found, so this was not noticed.