ALSA: usb-audio: Inform devices that need delayed registration

The USB-audio driver may call snd_card_register() multiple times as
its probe function is per USB interface while some USB-audio devices
may provide multiple interfaces to assign different streams although
they belong to the same device.  This works in most cases but the
registration is racy, hence it may miss the device recognition,
e.g. PA doesn't see certain devices when hotplugged.

The recent addition of the delayed registration quirk allows to sync
the registration at the last known interface, and the previous commit
added a new module option to allow the dynamic setup for that
purpose.

Now, this patch tries to find out and notifies for such devices that
require the delayed registration.  It shows a message like:

  Found post-registration device assignment: 1234abcd:02

If you hit this message, you can pass delayed_register module option
like:

  snd_usb_audio.delayed_register=1234abcd:02

by just copying the last shown entry.  If this works, it can be added
statically in the quirk list, registration_quirks[] found at the end
of sound/usb/quirks.c.

Link: https://lore.kernel.org/r/20200325103322.2508-4-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.c

@@ -680,6 +680,13 @@ static int usb_audio_probe(struct usb_interface *intf,
 			goto __error;
 	}
 
+	if (chip->need_delayed_register) {
+		dev_info(&dev->dev,
+			 "Found post-registration device assignment: %08x:%02x\n",
+			 chip->usb_id, ifnum);
+		chip->need_delayed_register = false; /* clear again */
+	}
+
 	/* we are allowed to call snd_card_register() many times, but first
 	 * check to see if a device needs to skip it or do anything special
 	 */

sound/usb/stream.c

@@ -502,6 +502,9 @@ static int __snd_usb_add_audio_stream(struct snd_usb_audio *chip,
 		subs = &as->substream[stream];
 		if (subs->ep_num)
 			continue;
+		if (snd_device_get_state(chip->card, as->pcm) !=
+		    SNDRV_DEV_BUILD)
+			chip->need_delayed_register = true;
 		err = snd_pcm_new_stream(as->pcm, stream, 1);
 		if (err < 0)
 			return err;

sound/usb/usbaudio.h

@@ -34,6 +34,7 @@ struct snd_usb_audio {
 	unsigned int txfr_quirk:1; /* Subframe boundaries on transfers */
 	unsigned int tx_length_quirk:1; /* Put length specifier in transfers */
 	unsigned int setup_fmt_after_resume_quirk:1; /* setup the format to interface after resume */
+	unsigned int need_delayed_register:1; /* warn for delayed registration */
 	int num_interfaces;
 	int num_suspended_intf;
 	int sample_rate_read_error;