ALSA: usb-audio: Allow for rediscovery of connected USB SND devices

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 <[e-mail]>
Acked-by: Mark Brown <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

sound/usb/card.c

···
 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);
+
+/*
  * Checks to see if requested audio profile, i.e sample rate, # of
  * channels, etc... is supported by the substream associated to the
  * USB audio device.

sound/usb/card.h

···
 
 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 */