ALSA: usb-audio: Register card at the last interface

The USB-audio driver matches per interface, and as default, it
registers the card instance at the very first instance.  This can be a
problem for the devices that have multiple interfaces to be probed, as
the udev rule isn't applied properly for the later appearing
interfaces.  Although we introduced the delayed_register option and
the quirks for covering those shortcomings, it's nothing but a
workaround for specific devices.

This patch is an another attempt to fix the problem in a more generic
way.  Now the driver checks the whole USB device descriptor at the
very first time when an interface is attached to a sound card.  It
looks at each matching interface in the descriptor and remembers the
last matching one.  The snd_card_register() is invoked only when this
last interface is probed.

After this change, the quirks for the delayed registration become
superfluous, hence they are removed along with the patch.  OTOH, the
delayed_register option is still kept, as it might be useful for some
corner cases (e.g. a special driver overtakes the interface probe from
the standard driver, and the last interface probe may miss).

Link: https://lore.kernel.org/r/20220904161247.16461-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/sound/usb/usbaudio.h b/sound/usb/usbaudio.h
index ffbb4b0d09a0..2c6575029b1c 100644
--- a/sound/usb/usbaudio.h
+++ b/sound/usb/usbaudio.h
@@ -37,6 +37,7 @@ struct snd_usb_audio {
 	unsigned int quirk_flags;
 	unsigned int need_delayed_register:1; /* warn for delayed registration */
 	int num_interfaces;
+	int last_iface;
 	int num_suspended_intf;
 	int sample_rate_read_error;