ALSA: usb-audio: Create a registration quirk for Kingston HyperX Amp (0951:16d8)

Create a quirk that allows special processing and/or
skipping the call to snd_card_register.

For HyperX AMP, which uses two interfaces, but only has
a capture stream in the second, this allows the capture
stream to merge with the first PCM.
Signed-off-by: Chris Wulff <crwulff@gmail.com>
Link: https://lore.kernel.org/r/20200314165449.4086-3-crwulff@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.c

@@ -662,10 +662,14 @@ static int usb_audio_probe(struct usb_interface *intf,
 			goto __error;
 	}
 
-	/* we are allowed to call snd_card_register() many times */
-	err = snd_card_register(chip->card);
-	if (err < 0)
-		goto __error;
+	/* 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
+	 */
+	if (snd_usb_registration_quirk(chip, ifnum) == 0) {
+		err = snd_card_register(chip->card);
+		if (err < 0)
+			goto __error;
+	}
 
 	if (quirk && quirk->shares_media_device) {
 		/* don't want to fail when snd_media_device_create() fails */

sound/usb/quirks.c

@@ -1808,3 +1808,17 @@ void snd_usb_audioformat_attributes_quirk(struct snd_usb_audio *chip,
 		break;
 	}
 }
+
+int snd_usb_registration_quirk(struct snd_usb_audio *chip,
+			       int iface)
+{
+	switch (chip->usb_id) {
+	case USB_ID(0x0951, 0x16d8): /* Kingston HyperX AMP */
+		/* Register only when we reach interface 2 so that streams can
+		 * merge correctly into PCMs from interface 0
+		 */
+		return (iface != 2);
+	}
+	/* Register as normal */
+	return 0;
+}

sound/usb/quirks.h

@@ -51,4 +51,7 @@ void snd_usb_audioformat_attributes_quirk(struct snd_usb_audio *chip,
 					  struct audioformat *fp,
 					  int stream);
 
+int snd_usb_registration_quirk(struct snd_usb_audio *chip,
+			       int iface);
+
 #endif /* __USBAUDIO_QUIRKS_H */