From f85d4d28d1e9c6e6079424d5c599102348b7d9f6 Mon Sep 17 00:00:00 2001
From: Max Kellermann <max@musicpd.org>
Date: Wed, 29 Mar 2017 20:12:14 +0200
Subject: [PATCH] output/alsa: work around dmix non-blocking snd_pcm_drain()
bug
See code comment. Bug was reported against MPD, but it's really an
alsa-lib bug.
https://bugs.musicpd.org/view.php?id=4662
---
src/output/plugins/AlsaOutputPlugin.cxx | 36 +++++++++++++++++++++++++
1 file changed, 36 insertions(+)
diff --git a/src/output/plugins/AlsaOutputPlugin.cxx b/src/output/plugins/AlsaOutputPlugin.cxx
index b93a5362b..6b7f9a7f3 100644
--- a/src/output/plugins/AlsaOutputPlugin.cxx
+++ b/src/output/plugins/AlsaOutputPlugin.cxx
@@ -20,6 +20,7 @@
#include "config.h"
#include "AlsaOutputPlugin.hxx"
#include "lib/alsa/NonBlock.hxx"
+#include "lib/alsa/Version.hxx"
#include "../OutputAPI.hxx"
#include "../Wrapper.hxx"
#include "mixer/MixerList.hxx"
@@ -107,6 +108,17 @@ class AlsaOutput final
*/
snd_pcm_uframes_t period_frames;
+ /**
+ * Is this a buggy alsa-lib version, which needs a workaround
+ * for the snd_pcm_drain() bug always returning -EAGAIN? See
+ * alsa-lib commits fdc898d41135 and e4377b16454f for details.
+ * This bug was fixed in alsa-lib version 1.1.4.
+ *
+ * The workaround is to re-enable blocking mode for the
+ * snd_pcm_drain() call.
+ */
+ bool work_around_drain_bug;
+
/**
* After Open(), has this output been activated by a Play()
* command?
@@ -988,6 +1000,19 @@ AlsaOutput::SetupOrDop(AudioFormat &audio_format, PcmExport::Params ¶ms)
#endif
}
+static constexpr bool
+MaybeDmix(snd_pcm_type_t type)
+{
+ return type == SND_PCM_TYPE_DMIX || type == SND_PCM_TYPE_PLUG;
+}
+
+gcc_pure
+static bool
+MaybeDmix(snd_pcm_t *pcm)
+{
+ return MaybeDmix(snd_pcm_type(pcm));
+}
+
inline void
AlsaOutput::Open(AudioFormat &audio_format)
{
@@ -1012,6 +1037,9 @@ AlsaOutput::Open(AudioFormat &audio_format)
GetDevice()));
}
+ work_around_drain_bug = MaybeDmix(pcm) &&
+ GetRuntimeAlsaVersion() < MakeAlsaVersion(1, 1, 4);
+
snd_pcm_nonblock(pcm, 1);
#ifdef ENABLE_DSD
@@ -1117,6 +1145,14 @@ AlsaOutput::DrainInternal()
}
/* .. and finally drain the ALSA hardware buffer */
+
+ if (work_around_drain_bug) {
+ snd_pcm_nonblock(pcm, 0);
+ bool result = snd_pcm_drain(pcm) != -EAGAIN;
+ snd_pcm_nonblock(pcm, 1);
+ return result;
+ }
+
return snd_pcm_drain(pcm) != -EAGAIN;
}