In case of sound devices which are created by default (some of them even with
-nodefaults) it would not be nice to users if qemu required explicit:

  -audiodev driver=none,id=audio0 -machine default-audiodev=audio0

when they just want to run a VM and not care about any audio output.  Instead
this little helper makes it easy to create a dummy audiodev (driver=none) in
case there is no default-audiodev specified for the machine.  To make sure users
are not surprised by no sound output a helping message is also printed out.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
---
 audio/audio.c | 34 ++++++++++++++++++++++++++++++++++
 audio/audio.h |  2 ++
 2 files changed, 36 insertions(+)

diff --git a/audio/audio.c b/audio/audio.c
index 9e91a5a4f2b8..671845c65d18 100644
--- a/audio/audio.c
+++ b/audio/audio.c
@@ -37,6 +37,7 @@
 #include "sysemu/runstate.h"
 #include "ui/qemu-spice.h"
 #include "trace.h"
+#include "hw/boards.h"
 
 #define AUDIO_CAP "audio"
 #include "audio_int.h"
@@ -2122,6 +2123,39 @@ void audio_init_audiodevs(void)
     }
 }
 
+static void audio_init_dummy(const char *id)
+{
+    Audiodev *dev = g_new0(Audiodev, 1);
+    AudiodevListEntry *e = g_new0(AudiodevListEntry, 1);
+
+    dev->driver = AUDIODEV_DRIVER_NONE;
+    dev->id = g_strdup(id);
+
+    audio_validate_opts(dev, &error_abort);
+    audio_init(dev, NULL);
+
+    e->dev = dev;
+    QSIMPLEQ_INSERT_TAIL(&audiodevs, e, next);
+}
+
+const char *audio_maybe_init_dummy(const char *default_id)
+{
+    MachineState *ms = MACHINE(qdev_get_machine());
+
+    if (ms->default_audiodev) {
+        return ms->default_audiodev;
+    }
+
+    dolog("warning: No audiodev specified for implicit machine devices, "
+          "no audio output will be available for these. "
+          "For setting a backend audiodev for such devices try using "
+          "the default-audiodev machine option.\n");
+
+    audio_init_dummy(default_id);
+
+    return default_id;
+}
+
 audsettings audiodev_to_audsettings(AudiodevPerDirectionOptions *pdo)
 {
     return (audsettings) {
diff --git a/audio/audio.h b/audio/audio.h
index 3d5ecdecd5c1..335704a4ddb1 100644
--- a/audio/audio.h
+++ b/audio/audio.h
@@ -175,6 +175,8 @@ void audio_legacy_help(void);
 AudioState *audio_state_by_name(const char *name);
 const char *audio_get_id(QEMUSoundCard *card);
 
+const char *audio_maybe_init_dummy(const char *default_id);
+
 #define DEFINE_AUDIO_PROPERTIES(_s, _f)         \
     DEFINE_PROP_AUDIODEV("audiodev", _s, _f)
 
-- 
2.35.1