output/alsa: set up the ALSA channel map

This is necessary for proper multi-channel support because many ALSA
drivers do not use the channel maps from surround*.conf.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/2063

NEWS

@@ -30,6 +30,7 @@ ver 0.24 (not yet released)
   - nfs: support libnfs URL arguments
 * input
   - alsa: limit ALSA buffer time to 2 seconds
+  - alsa: set up a channel map
   - curl: add "connect_timeout" configuration
 * decoder
   - ffmpeg: require FFmpeg 4.0 or later

src/lib/alsa/ChannelMap.cxx

@@ -0,0 +1,263 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Copyright The Music Player Daemon Project
+
+#include "ChannelMap.hxx"
+#include "Error.hxx"
+#include "util/Domain.hxx"
+#include "util/ScopeExit.hxx"
+#include "Log.hxx"
+
+#include <algorithm> // for std::is_permutation()
+
+static constexpr Domain alsa_output_domain{"alsa_output"};
+
+namespace Alsa {
+
+static constexpr unsigned chmap_flac_50[]{
+	SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_FC, SND_CHMAP_RL, SND_CHMAP_RR
+};
+
+static constexpr unsigned chmap_alsa_50[]{
+	SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_RL, SND_CHMAP_RR, SND_CHMAP_FC
+};
+
+static constexpr unsigned chmap_flac_51[]{
+	SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_FC, SND_CHMAP_LFE, SND_CHMAP_RL, SND_CHMAP_RR
+};
+
+static constexpr unsigned chmap_alsa_51[]{
+	SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_RL, SND_CHMAP_RR, SND_CHMAP_FC, SND_CHMAP_LFE
+};
+
+static constexpr unsigned chmap_flac_7[]{
+	SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_FC, SND_CHMAP_LFE, SND_CHMAP_RC, SND_CHMAP_SL, SND_CHMAP_SR
+};
+
+static constexpr unsigned chmap_flac_8[]{
+	SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_FC, SND_CHMAP_LFE, SND_CHMAP_RL, SND_CHMAP_RR, SND_CHMAP_SL, SND_CHMAP_SR
+};
+
+/**
+ * Same as #chmap_flac_8, but with "rear R/L center" instead of "side
+ * R/L".
+ */
+static constexpr unsigned chmap_flac_8b[]{
+	SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_FC, SND_CHMAP_LFE, SND_CHMAP_RL, SND_CHMAP_RR, SND_CHMAP_RLC, SND_CHMAP_RRC
+};
+
+static constexpr unsigned chmap_alsa_71[]{
+	SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_RL, SND_CHMAP_RR, SND_CHMAP_FC, SND_CHMAP_LFE, SND_CHMAP_SL, SND_CHMAP_SR
+};
+
+[[gnu::pure]]
+static std::string
+ChannelPositionArrayToString(const snd_pcm_chmap &chmap) noexcept
+{
+	std::string s;
+	for (unsigned c = 0, n = chmap.channels; c < n; ++c) {
+		if (!s.empty())
+			s.push_back(',');
+		s.append(snd_pcm_chmap_name(static_cast<enum snd_pcm_chmap_position>(chmap.pos[c])));
+	}
+
+	return s;
+}
+
+[[gnu::pure]]
+static constexpr bool
+ChannelMapsEqual(const unsigned *a, const unsigned *b, unsigned n) noexcept
+{
+	return std::equal(a, a + n, b);
+}
+
+[[gnu::pure]]
+static constexpr bool
+ChannelMapsEqual(const snd_pcm_chmap_query_t &a, const unsigned *b, unsigned n) noexcept
+{
+	if (a.map.channels != n)
+		return false;
+
+	switch (a.type) {
+	case SND_CHMAP_TYPE_NONE:
+		return false;
+
+	case SND_CHMAP_TYPE_FIXED:
+	case SND_CHMAP_TYPE_VAR:
+	case SND_CHMAP_TYPE_PAIRED:
+		return ChannelMapsEqual(a.map.pos, b, n);
+	}
+
+	return false;
+}
+
+[[gnu::pure]]
+static const snd_pcm_chmap_t *
+FindExactChannelMap(const snd_pcm_chmap_query_t *const*maps,
+		    const unsigned *other, unsigned channels) noexcept
+{
+	for (; *maps != nullptr; ++maps) {
+		const auto &map = **maps;
+		if (ChannelMapsEqual(map, other, channels))
+			return &map.map;
+	}
+
+	return nullptr;
+}
+
+[[gnu::pure]]
+static constexpr bool
+IsChannelMapPermutation(const unsigned *a, const unsigned *b, unsigned n) noexcept
+{
+	return std::is_permutation(a, a + n, b);
+}
+
+[[gnu::pure]]
+static constexpr bool
+IsChannelMapPermutation(const snd_pcm_chmap_query_t &a, const unsigned *b, unsigned n) noexcept
+{
+	if (a.map.channels != n)
+		return false;
+
+	switch (a.type) {
+	case SND_CHMAP_TYPE_NONE:
+	case SND_CHMAP_TYPE_FIXED:
+		return false;
+
+	case SND_CHMAP_TYPE_VAR:
+		return IsChannelMapPermutation(a.map.pos, b, n);
+
+	case SND_CHMAP_TYPE_PAIRED:
+		// TODO implement check for this
+		return false;
+	}
+
+	return false;
+}
+
+[[gnu::pure]]
+static snd_pcm_chmap_t *
+FindVarChannelMap(snd_pcm_chmap_query_t *const*maps,
+		  const unsigned *other, unsigned channels) noexcept
+{
+	for (; *maps != nullptr; ++maps) {
+		auto &map = **maps;
+
+		if (IsChannelMapPermutation(map, other, channels))
+			return &map.map;
+	}
+
+	return nullptr;
+}
+
+static bool
+TrySetupChannelMap(snd_pcm_t *pcm, snd_pcm_chmap_query_t **maps,
+		   unsigned channels,
+		   const unsigned *want_map)
+{
+	assert(want_map != nullptr);
+
+	/* find an exact channel map for MPD's map (= FLAC) */
+	if (const auto *map = FindExactChannelMap(maps, want_map, channels)) {
+		FmtDebug(alsa_output_domain, "Selected exact channel map {:?}",
+			 ChannelPositionArrayToString(*map));
+
+		if (int err = snd_pcm_set_chmap(pcm, map); err < 0)
+			throw MakeError(err, "snd_pcm_set_chmap() failed");
+
+		return true;
+	}
+
+	/* find a variable channel map which is a permutation of MPD's
+	   and ask ALSA to swap channels */
+	if (auto *map = FindVarChannelMap(maps, want_map, channels)) {
+		FmtDebug(alsa_output_domain, "Selected variable channel map {:?}",
+			 ChannelPositionArrayToString(*map));
+
+		std::copy_n(want_map, channels, map->pos);
+
+		if (int err = snd_pcm_set_chmap(pcm, map); err < 0)
+			throw MakeError(err, "snd_pcm_set_chmap() failed");
+
+		FmtDebug(alsa_output_domain, "Configured custom channel map {:?}",
+			 ChannelPositionArrayToString(*map));
+
+		return true;
+	}
+
+	return false;
+}
+
+static void
+SetupChannelMap(snd_pcm_t *pcm,
+		unsigned channels,
+		const unsigned *flac1,
+		const unsigned *flac2,
+		const unsigned *alsa,
+		PcmExport::Params &params)
+{
+	const auto maps = snd_pcm_query_chmaps(pcm);
+	if (maps == nullptr) {
+		LogWarning(alsa_output_domain, "No channel maps available");
+
+		/* assume defaults and hope for the best */
+		params.alsa_channel_order = true;
+		return;
+	}
+
+	AtScopeExit(maps) { snd_pcm_free_chmaps(maps); };
+
+	/* dump the available channel maps */
+	for (const auto *const*m = maps; *m != nullptr; ++m) {
+		const auto &map = **m;
+
+		if (map.map.channels == channels)
+			FmtDebug(alsa_output_domain, "Channel map: type={:?} {:?}",
+				 snd_pcm_chmap_type_name(map.type), ChannelPositionArrayToString(map.map));
+	}
+
+	if (flac1 != nullptr && TrySetupChannelMap(pcm, maps, channels, flac1))
+		return;
+
+	if (flac2 != nullptr && TrySetupChannelMap(pcm, maps, channels, flac2))
+		return;
+
+	if (alsa != nullptr) {
+		/* find an exact channel map for the (obsolete) ALSA default
+		   map; this is a special case implemented by class
+		   PcmExport */
+		if (const auto *map = FindExactChannelMap(maps, alsa, channels)) {
+			FmtDebug(alsa_output_domain, "Selected ALSA channel map {:?}",
+				 ChannelPositionArrayToString(*map));
+
+			if (int err = snd_pcm_set_chmap(pcm, map); err < 0)
+				throw MakeError(err, "snd_pcm_set_chmap() failed");
+
+			params.alsa_channel_order = true;
+			return;
+		}
+	}
+
+	FmtWarning(alsa_output_domain, "No matching channel map found");
+}
+
+void
+SetupChannelMap(snd_pcm_t *pcm,
+		unsigned channels,
+		PcmExport::Params &params)
+{
+	switch (channels) {
+	case 5:
+		return SetupChannelMap(pcm, channels, chmap_flac_50, nullptr, chmap_alsa_50, params);
+
+	case 6:
+		return SetupChannelMap(pcm, channels, chmap_flac_51, nullptr, chmap_alsa_51, params);
+
+	case 7:
+		return SetupChannelMap(pcm, channels, chmap_flac_7, nullptr, nullptr, params);
+
+	case 8:
+		return SetupChannelMap(pcm, channels, chmap_flac_8, chmap_flac_8b, chmap_alsa_71, params);
+	}
+}
+
+} // namespace Alsa

src/lib/alsa/ChannelMap.hxx

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Copyright The Music Player Daemon Project
+
+#pragma once
+
+#include "pcm/Export.hxx"
+
+#include <alsa/asoundlib.h>
+
+namespace Alsa {
+
+/**
+ * Choose and set an ALSA channel map using snd_pcm_set_chmap().
+ *
+ * Throws on error.  Logs a warning for non-fatal errors.
+ */
+void
+SetupChannelMap(snd_pcm_t *pcm,
+		unsigned channels,
+		PcmExport::Params &params);
+
+} // namespace Alsa

src/lib/alsa/meson.build

@@ -16,6 +16,7 @@ alsa = static_library(
   'Version.cxx',
   'Error.cxx',
   'AllowedFormat.cxx',
+  'ChannelMap.cxx',
   'HwSetup.cxx',
   'NonBlock.cxx',
   include_directories: inc,

src/output/plugins/AlsaOutputPlugin.cxx

@@ -4,6 +4,7 @@
 #include "config.h"
 #include "AlsaOutputPlugin.hxx"
 #include "lib/alsa/AllowedFormat.hxx"
+#include "lib/alsa/ChannelMap.hxx"
 #include "lib/alsa/Error.hxx"
 #include "lib/alsa/HwSetup.hxx"
 #include "lib/alsa/NonBlock.hxx"
@@ -551,6 +552,8 @@ AlsaOutput::Setup(AudioFormat &audio_format,
 		 hw_result.buffer_size,
 		 hw_result.period_size);
 
+	Alsa::SetupChannelMap(pcm, audio_format.channels, params);
+
 	AlsaSetupSw(pcm, hw_result.buffer_size - hw_result.period_size,
 		    hw_result.period_size);
 
@@ -820,7 +823,6 @@ AlsaOutput::Open(AudioFormat &audio_format)
 #endif
 
 	PcmExport::Params params;
-	params.alsa_channel_order = true;
 
 	try {
 		SetupOrDop(audio_format, params