ALSA: usb-audio: Add native DSD support for Mytek DACs

Add new mostly generic code with Mytek VID to support native DSD mode.

This implementation should be easier to maintain when manufacturers
release new products.
Signed-off-by: NJussi Laako <jussi@sonarnerd.net>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/usb/card.h

@@ -32,6 +32,7 @@ struct audioformat {
 	struct snd_pcm_chmap_elem *chmap; /* (optional) channel map */
 	bool dsd_dop;			/* add DOP headers in case of DSD samples */
 	bool dsd_bitrev;		/* reverse the bits of each DSD sample */
+	bool dsd_raw;			/* altsetting is raw DSD */
 };
 
 struct snd_usb_substream;

sound/usb/format.c

@@ -64,8 +64,11 @@ static u64 parse_audio_format_i_type(struct snd_usb_audio *chip,
 		sample_width = fmt->bBitResolution;
 		sample_bytes = fmt->bSubslotSize;
 
-		if (format & UAC2_FORMAT_TYPE_I_RAW_DATA)
+		if (format & UAC2_FORMAT_TYPE_I_RAW_DATA) {
 			pcm_formats |= SNDRV_PCM_FMTBIT_SPECIAL;
+			/* flag potentially raw DSD capable altsettings */
+			fp->dsd_raw = true;
+		}
 
 		format <<= 1;
 		break;

sound/usb/quirks.c

@@ -1443,6 +1443,19 @@ u64 snd_usb_interface_dsd_format_quirks(struct snd_usb_audio *chip,
 			return SNDRV_PCM_FMTBIT_DSD_U32_BE;
 	}
 
+	/* Mostly generic method to detect many DSD-capable implementations -
+	 * from XMOS/Thesycon
+	 */
+	switch (USB_ID_VENDOR(chip->usb_id)) {
+	case 0x25ce:  /* Mytek devices */
+		if (fp->dsd_raw)
+			return SNDRV_PCM_FMTBIT_DSD_U32_BE;
+		break;
+	default:
+		break;
+
+	}
+
 	return 0;
 }