sdlaudio: add -audiodev sdl,out.buffer-count option

Currently there is a crackling noise with SDL2 audio playback.
Commit bcf19777df: "audio/sdlaudio: Allow audio playback with
SDL2" already mentioned the crackling noise.

Add an out.buffer-count option to give users a chance to select
sane settings for glitch free audio playback. The idea was taken
from the coreaudio backend.

The in.buffer-count option will be used with one of the next
patches.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-id: 9315afe5-5958-c0b4-ea1e-14769511a9d5@t-online.de
Message-Id: <20210110100239.27588-3-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/audio.c

@@ -2003,7 +2003,7 @@ void audio_create_pdos(Audiodev *dev)
         CASE(JACK, jack, Jack);
         CASE(OSS, oss, Oss);
         CASE(PA, pa, Pa);
-        CASE(SDL, sdl, );
+        CASE(SDL, sdl, Sdl);
         CASE(SPICE, spice, );
         CASE(WAV, wav, );
 

audio/audio_legacy.c

@@ -286,7 +286,8 @@ static void handle_sdl(Audiodev *dev)
 {
     /* SDL is output only */
     get_samples_to_usecs("QEMU_SDL_SAMPLES", &dev->u.sdl.out->buffer_length,
-                         &dev->u.sdl.out->has_buffer_length, dev->u.sdl.out);
+        &dev->u.sdl.out->has_buffer_length,
+        qapi_AudiodevSdlPerDirectionOptions_base(dev->u.sdl.out));
 }
 
 /* wav */

audio/audio_template.h

@@ -337,7 +337,7 @@ AudiodevPerDirectionOptions *glue(audio_get_pdo_, TYPE)(Audiodev *dev)
     case AUDIODEV_DRIVER_PA:
         return qapi_AudiodevPaPerDirectionOptions_base(dev->u.pa.TYPE);
     case AUDIODEV_DRIVER_SDL:
-        return dev->u.sdl.TYPE;
+        return qapi_AudiodevSdlPerDirectionOptions_base(dev->u.sdl.TYPE);
     case AUDIODEV_DRIVER_SPICE:
         return dev->u.spice.TYPE;
     case AUDIODEV_DRIVER_WAV:

audio/sdlaudio.c

@@ -276,12 +276,18 @@ static int sdl_init_out(HWVoiceOut *hw, struct audsettings *as,
     int endianness;
     int err;
     AudioFormat effective_fmt;
+    AudiodevSdlPerDirectionOptions *spdo = s->dev->u.sdl.out;
     struct audsettings obt_as;
 
     req.freq = as->freq;
     req.format = aud_to_sdlfmt (as->fmt);
     req.channels = as->nchannels;
-    req.samples = audio_buffer_samples(s->dev->u.sdl.out, as, 11610);
+    /*
+     * This is wrong. SDL samples are QEMU frames. The buffer size will be
+     * the requested buffer size multiplied by the number of channels.
+     */
+    req.samples = audio_buffer_samples(
+        qapi_AudiodevSdlPerDirectionOptions_base(spdo), as, 11610);
     req.callback = sdl_callback;
     req.userdata = sdl;
 
@@ -301,7 +307,8 @@ static int sdl_init_out(HWVoiceOut *hw, struct audsettings *as,
     obt_as.endianness = endianness;
 
     audio_pcm_init_info (&hw->info, &obt_as);
-    hw->samples = obt.samples;
+    hw->samples = (spdo->has_buffer_count ? spdo->buffer_count : 4) *
+        obt.samples;
 
     s->initialized = 1;
     s->exit = 0;

qapi/audio.json

@@ -301,6 +301,37 @@
     '*out':    'AudiodevPaPerDirectionOptions',
     '*server': 'str' } }
 
+##
+# @AudiodevSdlPerDirectionOptions:
+#
+# Options of the SDL audio backend that are used for both playback and
+# recording.
+#
+# @buffer-count: number of buffers (default 4)
+#
+# Since: 6.0
+##
+{ 'struct': 'AudiodevSdlPerDirectionOptions',
+  'base': 'AudiodevPerDirectionOptions',
+  'data': {
+    '*buffer-count': 'uint32' } }
+
+##
+# @AudiodevSdlOptions:
+#
+# Options of the SDL audio backend.
+#
+# @in: options of the recording stream
+#
+# @out: options of the playback stream
+#
+# Since: 6.0
+##
+{ 'struct': 'AudiodevSdlOptions',
+  'data': {
+    '*in':  'AudiodevSdlPerDirectionOptions',
+    '*out': 'AudiodevSdlPerDirectionOptions' } }
+
 ##
 # @AudiodevWavOptions:
 #
@@ -385,6 +416,6 @@
     'jack':      'AudiodevJackOptions',
     'oss':       'AudiodevOssOptions',
     'pa':        'AudiodevPaOptions',
-    'sdl':       'AudiodevGenericOptions',
+    'sdl':       'AudiodevSdlOptions',
     'spice':     'AudiodevGenericOptions',
     'wav':       'AudiodevWavOptions' } }

qemu-options.hx

@@ -588,6 +588,7 @@ DEF("audiodev", HAS_ARG, QEMU_OPTION_audiodev,
 #endif
 #ifdef CONFIG_AUDIO_SDL
     "-audiodev sdl,id=id[,prop[=value][,...]]\n"
+    "                in|out.buffer-count= number of buffers\n"
 #endif
 #ifdef CONFIG_SPICE
     "-audiodev spice,id=id[,prop[=value][,...]]\n"
@@ -745,7 +746,12 @@ SRST
 ``-audiodev sdl,id=id[,prop[=value][,...]]``
     Creates a backend using SDL. This backend is available on most
     systems, but you should use your platform's native backend if
-    possible. This backend has no backend specific properties.
+    possible.
+
+    SDL specific options are:
+
+    ``in|out.buffer-count=count``
+        Sets the count of the buffers.
 
 ``-audiodev spice,id=id[,prop[=value][,...]]``
     Creates a backend that sends audio through SPICE. This backend