ALSA: usb-audio: qcom: Add USB offload route kcontrol

In order to allow userspace/applications know about USB offloading status,
expose a sound kcontrol that fetches information about which sound card
and PCM index the USB device is mapped to for supporting offloading.  In
the USB audio offloading framework, the ASoC BE DAI link is the entity
responsible for registering to the SOC USB layer.

It is expected for the USB SND offloading driver to add the kcontrol to the
sound card associated with the USB audio device.  An example output would
look like:

tinymix -D 1 get 'USB Offload Playback Route PCM#0'
-1, -1 (range -1->255)

This example signifies that there is no mapped ASoC path available for the
USB SND device.

tinymix -D 1 get 'USB Offload Playback Route PCM#0'
0, 0 (range -1->255)

This example signifies that the offload path is available over ASoC sound
card index#0 and PCM device#0.

The USB offload kcontrol will be added in addition to the existing
kcontrols identified by the USB SND mixer.  The kcontrols used to modify
the USB audio device specific parameters are still valid and expected to be
used.  These parameters are not mirrored to the ASoC subsystem.

Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409194804.3773260-31-quic_wcheng@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

sound/usb/qcom/Makefile

@@ -1,2 +1,4 @@
 snd-usb-audio-qmi-y := usb_audio_qmi_v01.o qc_audio_offload.o
+snd-usb-audio-qmi-y += mixer_usb_offload.o
 obj-$(CONFIG_SND_USB_AUDIO_QMI) += snd-usb-audio-qmi.o
+

sound/usb/qcom/mixer_usb_offload.c

@@ -0,0 +1,155 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2022-2025 Qualcomm Innovation Center, Inc. All rights reserved.
+ */
+
+#include <linux/usb.h>
+
+#include <sound/core.h>
+#include <sound/control.h>
+#include <sound/soc-usb.h>
+
+#include "../usbaudio.h"
+#include "../card.h"
+#include "../helper.h"
+#include "../mixer.h"
+
+#include "mixer_usb_offload.h"
+
+#define PCM_IDX(n)  ((n) & 0xffff)
+#define CARD_IDX(n) ((n) >> 16)
+
+static int
+snd_usb_offload_card_route_get(struct snd_kcontrol *kcontrol,
+			       struct snd_ctl_elem_value *ucontrol)
+{
+	struct device *sysdev = snd_kcontrol_chip(kcontrol);
+	int ret;
+
+	ret = snd_soc_usb_update_offload_route(sysdev,
+					       CARD_IDX(kcontrol->private_value),
+					       PCM_IDX(kcontrol->private_value),
+					       SNDRV_PCM_STREAM_PLAYBACK,
+					       SND_SOC_USB_KCTL_CARD_ROUTE,
+					       ucontrol->value.integer.value);
+	if (ret < 0) {
+		ucontrol->value.integer.value[0] = -1;
+		ucontrol->value.integer.value[1] = -1;
+	}
+
+	return 0;
+}
+
+static int snd_usb_offload_card_route_info(struct snd_kcontrol *kcontrol,
+					   struct snd_ctl_elem_info *uinfo)
+{
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = -1;
+	uinfo->value.integer.max = SNDRV_CARDS;
+
+	return 0;
+}
+
+static struct snd_kcontrol_new snd_usb_offload_mapped_card_ctl = {
+	.iface = SNDRV_CTL_ELEM_IFACE_CARD,
+	.access = SNDRV_CTL_ELEM_ACCESS_READ,
+	.info = snd_usb_offload_card_route_info,
+	.get = snd_usb_offload_card_route_get,
+};
+
+static int
+snd_usb_offload_pcm_route_get(struct snd_kcontrol *kcontrol,
+			      struct snd_ctl_elem_value *ucontrol)
+{
+	struct device *sysdev = snd_kcontrol_chip(kcontrol);
+	int ret;
+
+	ret = snd_soc_usb_update_offload_route(sysdev,
+					       CARD_IDX(kcontrol->private_value),
+					       PCM_IDX(kcontrol->private_value),
+					       SNDRV_PCM_STREAM_PLAYBACK,
+					       SND_SOC_USB_KCTL_PCM_ROUTE,
+					       ucontrol->value.integer.value);
+	if (ret < 0) {
+		ucontrol->value.integer.value[0] = -1;
+		ucontrol->value.integer.value[1] = -1;
+	}
+
+	return 0;
+}
+
+static int snd_usb_offload_pcm_route_info(struct snd_kcontrol *kcontrol,
+					  struct snd_ctl_elem_info *uinfo)
+{
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = -1;
+	/* Arbitrary max value, as there is no 'limit' on number of PCM devices */
+	uinfo->value.integer.max = 0xff;
+
+	return 0;
+}
+
+static struct snd_kcontrol_new snd_usb_offload_mapped_pcm_ctl = {
+	.iface = SNDRV_CTL_ELEM_IFACE_CARD,
+	.access = SNDRV_CTL_ELEM_ACCESS_READ,
+	.info = snd_usb_offload_pcm_route_info,
+	.get = snd_usb_offload_pcm_route_get,
+};
+
+/**
+ * snd_usb_offload_create_ctl() - Add USB offload bounded mixer
+ * @chip: USB SND chip device
+ * @bedev: Reference to USB backend DAI device
+ *
+ * Creates a sound control for a USB audio device, so that applications can
+ * query for if there is an available USB audio offload path, and which
+ * card is managing it.
+ */
+int snd_usb_offload_create_ctl(struct snd_usb_audio *chip, struct device *bedev)
+{
+	struct snd_kcontrol_new *chip_kctl;
+	struct snd_usb_substream *subs;
+	struct snd_usb_stream *as;
+	char ctl_name[48];
+	int ret;
+
+	list_for_each_entry(as, &chip->pcm_list, list) {
+		subs = &as->substream[SNDRV_PCM_STREAM_PLAYBACK];
+		if (!subs->ep_num || as->pcm_index > 0xff)
+			continue;
+
+		chip_kctl = &snd_usb_offload_mapped_card_ctl;
+		chip_kctl->count = 1;
+		/*
+		 * Store the associated USB SND card number and PCM index for
+		 * the kctl.
+		 */
+		chip_kctl->private_value = as->pcm_index |
+					  chip->card->number << 16;
+		sprintf(ctl_name, "USB Offload Playback Card Route PCM#%d",
+			as->pcm_index);
+		chip_kctl->name = ctl_name;
+		ret = snd_ctl_add(chip->card, snd_ctl_new1(chip_kctl, bedev));
+		if (ret < 0)
+			break;
+
+		chip_kctl = &snd_usb_offload_mapped_pcm_ctl;
+		chip_kctl->count = 1;
+		/*
+		 * Store the associated USB SND card number and PCM index for
+		 * the kctl.
+		 */
+		chip_kctl->private_value = as->pcm_index |
+					  chip->card->number << 16;
+		sprintf(ctl_name, "USB Offload Playback PCM Route PCM#%d",
+			as->pcm_index);
+		chip_kctl->name = ctl_name;
+		ret = snd_ctl_add(chip->card, snd_ctl_new1(chip_kctl, bedev));
+		if (ret < 0)
+			break;
+	}
+
+	return ret;
+}

sound/usb/qcom/mixer_usb_offload.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (c) 2022-2025 Qualcomm Innovation Center, Inc. All rights reserved.
+ */
+
+#ifndef __USB_OFFLOAD_MIXER_H
+#define __USB_OFFLOAD_MIXER_H
+
+int snd_usb_offload_create_ctl(struct snd_usb_audio *chip, struct device *bedev);
+
+#endif /* __USB_OFFLOAD_MIXER_H */

sound/usb/qcom/qc_audio_offload.c

@@ -38,6 +38,7 @@
 #include "../pcm.h"
 #include "../power.h"
 
+#include "mixer_usb_offload.h"
 #include "usb_audio_qmi_v01.h"
 
 /* Stream disable request timeout during USB device disconnect */
@@ -1792,6 +1793,7 @@ static void qc_usb_audio_offload_probe(struct snd_usb_audio *chip)
 		sdev->card_idx = chip->card->number;
 		sdev->chip_idx = chip->index;
 
+		snd_usb_offload_create_ctl(chip, uaudio_qdev->auxdev->dev.parent);
 		snd_soc_usb_connect(uaudio_qdev->auxdev->dev.parent, sdev);
 	}