[PATCH v24 30/34] ALSA: usb-audio: Allow for rediscovery of connected USB SND devices

[Wesley Cheng]

In case of notifying SND platform drivers of connection events, some of
these use cases, such as offloading, require an ASoC USB backend device to
be initialized before the events can be handled.  If the USB backend device
has not yet been probed, this leads to missing initial USB audio device
connection events.

Expose an API that traverses the usb_chip array for connected devices, and
to call the respective connection callback registered to the SND platform
driver.

Signed-off-by: Wesley Cheng <quic_wcheng@xxxxxxxxxxx>
---
 sound/usb/card.c                  | 21 +++++++++++++++++++++
 sound/usb/card.h                  |  2 ++
 sound/usb/qcom/qc_audio_offload.c |  2 ++
 3 files changed, 25 insertions(+)

diff --git a/sound/usb/card.c b/sound/usb/card.c
index 8f1051e1716e..70b7f22ff495 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -155,6 +155,27 @@ int snd_usb_unregister_platform_ops(void)
 }
 EXPORT_SYMBOL_GPL(snd_usb_unregister_platform_ops);
 
+/*
+ * in case the platform driver was not ready at the time of USB SND
+ * device connect, expose an API to discover all connected USB devices
+ * so it can populate any dependent resources/structures.
+ */
+void snd_usb_rediscover_devices(void)
+{
+	int i;
+
+	guard(mutex)(&register_mutex);
+
+	if (!platform_ops || !platform_ops->connect_cb)
+		return;
+
+	for (i = 0; i < SNDRV_CARDS; i++) {
+		if (usb_chip[i])
+			platform_ops->connect_cb(usb_chip[i]);
+	}
+}
+EXPORT_SYMBOL_GPL(snd_usb_rediscover_devices);
+
 /*
  * disconnect streams
  * called from usb_audio_disconnect()
diff --git a/sound/usb/card.h b/sound/usb/card.h
index e98883785301..f0794aa06cd7 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -218,4 +218,6 @@ struct snd_usb_platform_ops {
 
 int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops);
 int snd_usb_unregister_platform_ops(void);
+
+void snd_usb_rediscover_devices(void);
 #endif /* __USBAUDIO_CARD_H */
diff --git a/sound/usb/qcom/qc_audio_offload.c b/sound/usb/qcom/qc_audio_offload.c
index b862aa15aa37..3a89dedfd9ac 100644
--- a/sound/usb/qcom/qc_audio_offload.c
+++ b/sound/usb/qcom/qc_audio_offload.c
@@ -1898,6 +1898,8 @@ static int __init qc_usb_audio_offload_init(void)
 	if (ret < 0)
 		goto release_qmi;
 
+	snd_usb_rediscover_devices();
+
 	return 0;
 
 release_qmi: