output/alsa: add attributes "dop" and "allowed_formats"

src/lib/alsa/AllowedFormat.cxx

@@ -21,6 +21,7 @@
 #include "AllowedFormat.hxx"
 #include "AudioParser.hxx"
 #include "util/IterableSplitString.hxx"
+#include "util/StringBuffer.hxx"
 
 #include <stdexcept>
 
@@ -65,4 +66,24 @@ AllowedFormat::ParseList(StringView s)
 	return list;
 }
 
+std::string
+ToString(const std::forward_list<AllowedFormat> &allowed_formats) noexcept
+{
+	std::string result;
+
+	for (const auto &i : allowed_formats) {
+		if (!result.empty())
+			result.push_back(' ');
+
+		result += ::ToString(i.format);
+
+#ifdef ENABLE_DSD
+		if (i.dop)
+			result += "=dop";
+#endif
+	}
+
+	return result;
+}
+
 } // namespace Alsa

src/lib/alsa/AllowedFormat.hxx

@@ -24,6 +24,7 @@
 #include "AudioFormat.hxx"
 
 #include <forward_list>
+#include <string>
 
 struct StringView;
 
@@ -54,6 +55,9 @@ struct AllowedFormat {
 	static std::forward_list<AllowedFormat> ParseList(StringView s);
 };
 
+std::string
+ToString(const std::forward_list<AllowedFormat> &allowed_formats) noexcept;
+
 } // namespace Alsa
 
 #endif