[PATCH v22 35/38] ALSA: usb-audio: Allow for rediscovery of connected USB SND devices

[Wesley Cheng <quic_wcheng@xxxxxxxxxxx>]

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                  |  1 +
 sound/usb/qcom/qc_audio_offload.c |  2 ++
 3 files changed, 24 insertions(+)

diff --git a/sound/usb/card.c b/sound/usb/card.c
index fb548a3abcfc..a7f1e437f67a 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -186,6 +186,27 @@ struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
 }
 EXPORT_SYMBOL_GPL(snd_usb_find_suppported_substream);
 
+/*
+ * 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 3c900e5afbce..140928dd9d63 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -221,4 +221,5 @@ int snd_usb_unregister_platform_ops(void);
 
 struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
 				struct snd_pcm_hw_params *params, int direction);
+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 9aac0985f43c..01aa0753ca83 100644
--- a/sound/usb/qcom/qc_audio_offload.c
+++ b/sound/usb/qcom/qc_audio_offload.c
@@ -1880,6 +1880,8 @@ static int __init qc_usb_audio_offload_init(void)
 	if (ret < 0)
 		goto release_qmi;
 
+	snd_usb_rediscover_devices();
+
 	return 0;
 
 release_qmi: