output/alsa: support native DSD playback

Translate SampleFormat::DSD to SND_PCM_FORMAT_DSD_U8, which was added
to alsa-lib 1.0.27.1.

NEWS

@@ -52,6 +52,8 @@ ver 0.19 (not yet released)
   - mp4v2: support playback of MP4 files.
 * encoder:
   - shine: new encoder plugin
+* output
+  - alsa: support native DSD playback
 * threads:
   - the update thread runs at "idle" priority
   - the output thread runs at "real-time" priority

src/output/plugins/AlsaOutputPlugin.cxx

@@ -22,6 +22,7 @@
 #include "../OutputAPI.hxx"
 #include "mixer/MixerList.hxx"
 #include "pcm/PcmExport.hxx"
+#include "config/ConfigError.hxx"
 #include "util/Manual.hxx"
 #include "util/Error.hxx"
 #include "util/Domain.hxx"
@@ -32,6 +33,11 @@
 
 #include <string>
 
+#if SND_LIB_VERSION >= 0x1001c
+/* alsa-lib supports DSD since version 1.0.27.1 */
+#define HAVE_ALSA_DSD
+#endif
+
 static const char default_device[] = "default";
 
 static constexpr unsigned MPD_ALSA_BUFFER_TIME_US = 500000;
@@ -236,9 +242,15 @@ get_bitformat(SampleFormat sample_format)
 {
 	switch (sample_format) {
 	case SampleFormat::UNDEFINED:
-	case SampleFormat::DSD:
 		return SND_PCM_FORMAT_UNKNOWN;
 
+	case SampleFormat::DSD:
+#ifdef HAVE_ALSA_DSD
+		return SND_PCM_FORMAT_DSD_U8;
+#else
+		return SND_PCM_FORMAT_UNKNOWN;
+#endif
+
 	case SampleFormat::S8:
 		return SND_PCM_FORMAT_S8;