ALSA: usb-audio: Introduce USB SND platform op callbacks

Allow for different platforms to be notified on USB SND connect/disconnect
sequences. This allows for platform USB SND modules to properly initialize
and populate internal structures with references to the USB SND chip
device.

Tested-by: Puma Hsu <[e-mail]>
Tested-by: Daehwan Jung <[e-mail]>
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

···
 static DEFINE_MUTEX(register_mutex);
 static struct snd_usb_audio *usb_chip[SNDRV_CARDS];
 static struct usb_driver usb_audio_driver;
+static struct snd_usb_platform_ops *platform_ops;
+
+/*
+ * Register platform specific operations that will be notified on events
+ * which occur in USB SND.  The platform driver can utilize this path to
+ * enable features, such as USB audio offloading, which allows for audio data
+ * to be queued by an audio DSP.
+ *
+ * Only one set of platform operations can be registered to USB SND.  The
+ * platform register operation is protected by the register_mutex.
+ */
+int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops)
+{
+	guard(mutex)(&register_mutex);
+	if (platform_ops)
+		return -EEXIST;
+
+	platform_ops = ops;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(snd_usb_register_platform_ops);
+
+/*
+ * Unregisters the current set of platform operations.  This allows for
+ * a new set to be registered if required.
+ *
+ * The platform unregister operation is protected by the register_mutex.
+ */
+int snd_usb_unregister_platform_ops(void)
+{
+	guard(mutex)(&register_mutex);
+	platform_ops = NULL;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(snd_usb_unregister_platform_ops);
 
 /*
  * Checks to see if requested audio profile, i.e sample rate, # of
···
 	chip->num_interfaces++;
 	usb_set_intfdata(intf, chip);
 	atomic_dec(&chip->active);
+
+	if (platform_ops && platform_ops->connect_cb)
+		platform_ops->connect_cb(chip);
 	mutex_unlock(&register_mutex);
+
 	return 0;
 
  __error:
···
 	card = chip->card;
 
 	mutex_lock(&register_mutex);
+	if (platform_ops && platform_ops->disconnect_cb)
+		platform_ops->disconnect_cb(chip);
+
 	if (atomic_inc_return(&chip->shutdown) == 1) {
 		struct snd_usb_stream *as;
 		struct snd_usb_endpoint *ep;
···
 		chip->system_suspend = chip->num_suspended_intf;
 	}
 
+	if (platform_ops && platform_ops->suspend_cb)
+		platform_ops->suspend_cb(intf, message);
+
 	return 0;
 }
 
···
 	}
 
 	snd_usb_midi_v2_resume_all(chip);
+
+	if (platform_ops && platform_ops->resume_cb)
+		platform_ops->resume_cb(intf);
 
  out:
 	if (chip->num_suspended_intf == chip->system_suspend) {

sound/usb/card.h

···
 	struct list_head list;
 };
 
+struct snd_usb_platform_ops {
+	void (*connect_cb)(struct snd_usb_audio *chip);
+	void (*disconnect_cb)(struct snd_usb_audio *chip);
+	void (*suspend_cb)(struct usb_interface *intf, pm_message_t message);
+	void (*resume_cb)(struct usb_interface *intf);
+};
+
 struct snd_usb_stream *
 snd_usb_find_suppported_substream(int card_idx, struct snd_pcm_hw_params *params,
 				  int direction);
+
+int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops);
+int snd_usb_unregister_platform_ops(void);
 #endif /* __USBAUDIO_CARD_H */