[gstreamer-bugs] [Bug 518082] New: alsamixer: playback volumes overwritten by capture volumes if track has both playback and capture caps

[GStreamer (bugzilla.gnome.org)]

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  GStreamer | gst-plugins-base | Ver: 0.10.17
           Summary: alsamixer: playback volumes overwritten by capture
                    volumes if track has both playback and capture caps
           Product: GStreamer
           Version: 0.10.17
          Platform: Other
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gst-plugins-base
        AssignedTo: gstreamer-bugs at lists.sourceforge.net
        ReportedBy: koslund at sympatico.ca
         QAContact: gstreamer-bugs at lists.sourceforge.net
     GNOME version: 2.21/2.22
   GNOME milestone: Unspecified


The gst alsamixer implementation supports alsa element tracks that have both
playback and capture capabilities by creating two separate tracks for the
element. See gst-plugins-base/ext/alsa/gstalsamixer.c function
gst_alsa_mixer_ensure_track_list(). When the track is created in
gstalsamixertrack.c gst_alsa_mixer_track_new() it is passed a flag field which
indicates whether the track is _INPUT or _OUTPUT and the function initializes a
separate field alsa_flags in function
gst_alsa_mixer_track_update_alsa_capabilities using alsa functions
snd_mixer_selem_has_playback_volume and snd_mixer_selem_has_capture_volume. As
a consequence a dual capability track will have both
GST_ALSA_MIXER_TRACK_PVOLUME and GST_ALSA_MIXER_TRACK_CVOLUME flags set on
*both* the _INPUT and _OUTPUT tracks created for the alsa element.

This leads to a bug when gst_alsa_mixer_track_update() is called to update a
tracks volumes. This function checks only the alsa_flags (via
alsa_track_has_cap). If a track has both _PVOLUME AND _CVOLUME set, the
function will first retrieve the playback volumes and store them in
GstMixerTrack->volumes, then it will retrieve the capture volumes and overwrite
the playback volumes in the structure, since GstMixerTrack supports only one
set of volumes.

This bug showed up in a recent version of gnome-applets/applet_mixer2 on
Mandriva Cooker, which includes a patch not yet accepted in the gnome-applets
trunck, but which has been implement in Redhat and Ubuntu distributions and
which implements mixer notifications. When used with an Audigy2 card, which
alsa shows as having a dual capability Master element, changes to the Master
volume result in notifications in which the Master playback volume is always
set to zero when in fact it is non-zero.

This bug was located and verified by tracing with gdb and inspection of the
code.

Output from amixer:
$ amixer -c 0 info
Card hw:0 'Audigy2'/'Audigy 2 [SB0240] (rev.4, serial:0x10071102) at 0xa000,
irq 20'
  Mixer name    : 'SigmaTel STAC9721,23'
  Components    : 'AC97a:83847609'
  Controls      : 213
  Simple ctrls  : 48

$ amixer -c 0 | head
Simple mixer control 'Master',0
  Capabilities: pvolume pvolume-joined cvolume cswitch cswitch-joined
  Playback channels: Mono
  Capture channels: Front Left - Front Right
  Limits: Playback 0 - 100 Capture 0 - 31
  Mono: Playback 9 [9%] [-36.40dB]
  Front Left: Capture 0 [0%] [-46.50dB] [on]
  Front Right: Capture 0 [0%] [-46.50dB] [on]
Simple mixer control 'Tone',0
  Capabilities: pswitch


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