Initial Commit

linux-postmarketos-qcom-msm8916 now supports voice call audio.
However, at the moment the audio stream needs to be explicitly started
by opening some PCM audio devices when the call is started
(even though no audio data is written to the PCM device).

Eventually this should go away by implementing codec2codec support
for q6voice in the kernel. In that case, the stream would be started
once the audio routes are set up using ALSA UCM. However, this is a bit
complicated actually so for now it's nice to have a workaround:

q6voiced listens on dbus for signals from oFono, and opens/closes the
PCM device when a call is initiated/ended in oFono. This essentially
makes voice call audio work out of the box (provided that the audio
routing, e.g. Earpiece and a microphone is set up appropriately).

Co-Authored-By: Nikita Travkin <nikitos.tr@gmail.com>

LICENSE

+The MIT License (MIT)
+
+Copyright (c) 2020 postmarketOS
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

README.md

+# q6voiced
+q6voiced is a userspace daemon for the QDSP6 voice call audio driver.
+Voice call audio is directly routed from the modem to the input/output audio
+devices, but something needs to start the audio streams for that to happen.
+
+q6voiced listens on dbus for signals from oFono, and opens/closes the
+PCM device when a call is initiated/ended in oFono. This essentially
+makes voice call audio work out of the box (provided that the audio
+routing, e.g. Earpiece and a microphone is set up appropriately).
+
+The q6voiced patches can be currently found in the [msm8916-mainline/linux]
+repository (look for `ASoC: qdsp6:`) commits). It may also work for downstream
+since the currently implemented approach is very similar.
+
+## Note
+It is expected that this daemon will be obsolete eventually.
+Different approaches exists for activating such audio streams that are not
+processed directly by Linux. (See [Hostless PCM streams]) At the moment the
+kernel driver implements the "Hostless FE" approach (as on downstream), but
+eventually this should be replaced by a Codec <-> Codec link.
+
+In that case the audio streams would be activated by setting some ALSA mixers
+(e.g. through ALSA UCM) and this daemon could be removed. However, there is
+quite some work involved to make that work properly. Until then, this daemon
+allows voice call audio to work without activating audio manually.
+
+[msm8916-mainline/linux]: https://github.com/msm8916-mainline/linux
+[Hostless PCM streams]: https://www.kernel.org/doc/html/latest/sound/soc/dpcm.html#hostless-pcm-streams

q6voiced.c

+// SPDX-License-Identifier: MIT
+#include <stdio.h>
+#include <dbus/dbus.h>
+#include <tinyalsa/asoundlib.h>
+
+/* Note: These parameters have little relevance (no audio data written) */
+struct pcm_config pcm_config_voice_call = {
+	.channels	= 1,
+	.rate		= 8000,
+	.period_size	= 160,
+	.period_count	= 2,
+	.format		= PCM_FORMAT_S16_LE,
+};
+
+int main(int argc, char **argv)
+{
+	struct pcm *tx = NULL, *rx = NULL;
+	unsigned int card, device;
+
+	DBusMessage *msg;
+	DBusConnection *conn;
+	DBusError err;
+
+	if (argc != 2 || sscanf(argv[1], "hw:%u,%u", &card, &device) != 2) {
+		fprintf(stderr, "Usage: q6voiced hw:<card>,<device>\n");
+		return 1;
+	}
+
+	// See: http://web.archive.org/web/20100309103206/http://dbus.freedesktop.org/doc/dbus/libdbus-tutorial.html
+	// "Receiving a Signal"
+
+	dbus_error_init(&err);
+
+	conn = dbus_bus_get(DBUS_BUS_SYSTEM, &err);
+	if (dbus_error_is_set(&err)) {
+		fprintf(stderr, "Connection error: %s\n", err.message);
+		dbus_error_free(&err);
+		return 1;
+	}
+	if (!conn)
+		return 1;
+
+	dbus_bus_add_match(conn, "type='signal',interface='org.ofono.VoiceCallManager'", &err);
+	dbus_connection_flush(conn);
+	if (dbus_error_is_set(&err)) {
+		fprintf(stderr, "Match error: %s\n", err.message);
+		dbus_error_free(&err);
+		return 1;
+	}
+
+	printf("Listening for VoiceCallManager signals.\n");
+
+	// Loop listening for signals being emmitted
+	while (dbus_connection_read_write(conn, -1)) {
+		// We need to process all received messages
+		while (msg = dbus_connection_pop_message(conn)) {
+			// Check if the message is a signal from the correct interface and with the correct name
+			if (dbus_message_is_signal(msg, "org.ofono.VoiceCallManager", "CallAdded")) {
+				if (!tx) {
+					/*
+					 * Opening the PCM devices starts the stream.
+					 * This should be replaced by a codec2codec link probably.
+					 */
+					tx = pcm_open(card, device, PCM_IN, &pcm_config_voice_call);
+					if (!pcm_is_ready(tx))
+						perror("Failed to open tx");
+
+					rx = pcm_open(card, device, PCM_OUT, &pcm_config_voice_call);
+					if (!pcm_is_ready(rx))
+						perror("Failed to open rx");
+
+					printf("PCM devices were opened.\n");
+				} else
+					printf("PCM is already opened!\n");
+
+			} else if (dbus_message_is_signal(msg, "org.ofono.VoiceCallManager", "CallRemoved")) {
+				if (rx) {
+					pcm_close(rx);
+					pcm_close(tx);
+
+					printf("PCM devices were closed.\n");
+					rx = tx = NULL;
+				} else
+					printf("PCM is already closed!\n");
+			}
+
+			dbus_message_unref(msg);
+		}
+	}
+
+	return 0;
+}