[PATCH v4 31/32] sound: usb: card: 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 | 19 +++++++++++++++++++
 sound/usb/card.h |  2 ++
 2 files changed, 21 insertions(+)

diff --git a/sound/usb/card.c b/sound/usb/card.c
index 365f6d978608..27a89aaa0bf3 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -170,6 +170,25 @@ 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;
+
+	mutex_lock(&register_mutex);
+	for (i = 0; i < SNDRV_CARDS; i++) {
+		if (usb_chip[i])
+			if (platform_ops && platform_ops->connect_cb)
+				platform_ops->connect_cb(usb_chip[i]);
+	}
+	mutex_unlock(&register_mutex);
+}
+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 db735680d155..dfd241334e3d 100644
--- a/sound/usb/card.h
+++ b/sound/usb/card.h
@@ -219,6 +219,7 @@ int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops);
 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);
 #else
 static int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops)
 {
@@ -235,5 +236,6 @@ static struct snd_usb_stream *snd_usb_find_suppported_substream(int card_idx,
 {
 	return NULL;
 }
+void snd_usb_rediscover_devices(void) { }
 #endif /* IS_ENABLED(CONFIG_SND_USB_AUDIO) */
 #endif /* __USBAUDIO_CARD_H */