output/alsa: add option to enable DSD over USB

NEWS

@@ -17,6 +17,7 @@ ver 0.17 (2011/??/??)
   - oggflac: delete this obsolete plugin
   - dsdiff: new decoder plugin
 * output:
+  - alsa: support DSD-over-USB (dCS suggested standard)
   - httpd: support for streaming to a DLNA client
   - openal: improve buffer cancellation
   - osx: allow user to specify other audio devices

doc/user.xml

@@ -1119,6 +1119,23 @@ systemctl start mpd.socket</programlisting>
                   bit, floating point, ...).
                 </entry>
               </row>
+              <row>
+                <entry>
+                  <varname>dsd_usb</varname>
+                  <parameter>yes|no</parameter>
+                </entry>
+                <entry>
+                  If set to <parameter>yes</parameter>, then DSD over
+                  USB according to the <ulink
+                  url="http://www.dcsltd.co.uk/page/assets/DSDoverUSB.pdf">dCS
+                  suggested standard</ulink> is enabled.  This wrapsa
+                  DSD samples in fake 24 bit PCM, and is understood by
+                  some DSD capable products, but may be harmful to
+                  other hardware.  Therefore, the default is
+                  <parameter>no</parameter> and you can enable the
+                  option at your own risk.
+                </entry>
+              </row>
             </tbody>
           </tgroup>
         </informaltable>

src/output/alsa_output_plugin.c

@@ -55,6 +55,14 @@ struct alsa_data {
 	/** use memory mapped I/O? */
 	bool use_mmap;
 
+	/**
+	 * Enable DSD over USB according to the dCS suggested
+	 * standard?
+	 *
+	 * @see http://www.dcsltd.co.uk/page/assets/DSDoverUSB.pdf
+	 */
+	bool dsd_usb;
+
 	/** libasound's buffer_time setting (in microseconds) */
 	unsigned int buffer_time;
 
@@ -128,6 +136,8 @@ alsa_configure(struct alsa_data *ad, const struct config_param *param)
 
 	ad->use_mmap = config_get_block_bool(param, "use_mmap", false);
 
+	ad->dsd_usb = config_get_block_bool(param, "dsd_usb", false);
+
 	ad->buffer_time = config_get_block_unsigned(param, "buffer_time",
 			MPD_ALSA_BUFFER_TIME_US);
 	ad->period_time = config_get_block_unsigned(param, "period_time", 0);
@@ -575,6 +585,49 @@ error:
 	return false;
 }
 
+static bool
+alsa_setup_dsd(struct alsa_data *ad, struct audio_format *audio_format,
+	       GError **error_r)
+{
+	assert(ad->dsd_usb);
+	assert(audio_format->format == SAMPLE_FORMAT_DSD);
+
+	/* pass 24 bit to alsa_setup() */
+
+	audio_format->format = SAMPLE_FORMAT_S24_P32;
+	audio_format->sample_rate /= 2;
+
+	const struct audio_format check = *audio_format;
+
+	if (!alsa_setup(ad, audio_format, error_r))
+		return false;
+
+	if (!audio_format_equals(audio_format, &check)) {
+		/* no bit-perfect playback, which is required
+		   for DSD over USB */
+		g_set_error(error_r, alsa_output_quark(), 0,
+			    "Failed to configure DSD-over-USB on ALSA device \"%s\"",
+			    alsa_device(ad));
+		return false;
+	}
+
+	/* the ALSA device has accepted 24 bit playback,
+	   return DSD_OVER_USB to the caller */
+
+	audio_format->format = SAMPLE_FORMAT_DSD_OVER_USB;
+	return true;
+}
+
+static bool
+alsa_setup_or_dsd(struct alsa_data *ad, struct audio_format *audio_format,
+		  GError **error_r)
+{
+	if (ad->dsd_usb && audio_format->format == SAMPLE_FORMAT_DSD)
+		return alsa_setup_dsd(ad, audio_format, error_r);
+
+	return alsa_setup(ad, audio_format, error_r);
+}
+
 static bool
 alsa_open(struct audio_output *ao, struct audio_format *audio_format, GError **error)
 {
@@ -591,7 +644,7 @@ alsa_open(struct audio_output *ao, struct audio_format *audio_format, GError **e
 		return false;
 	}
 
-	success = alsa_setup(ad, audio_format, error);
+	success = alsa_setup_or_dsd(ad, audio_format, error);
 	if (!success) {
 		snd_pcm_close(ad->pcm);
 		return false;