[PATCH] ALSA: usb-audio: Filter out unsupported sample rates on Focusrite devices

[Alexander Tsoy]

Many Focusrite devices supports a limited set of sample rates per
altsetting. This includes interfaces with ADAT ports and interfaces with
dedicated feedback channels:
 - Scarlett 18i6, 18i8 1st gen, 18i20 1st gen;
 - Scarlett 18i8 2nd gen, 18i20 2nd gen;
 - all Scarlett 3rd gen devices (feedback channels works only up to 96
   kHz);
 - Clarett 2Pre USB, 4Pre USB, 8Pre USB.

Tested-by: Alexey Skobkin <skobkin-ru at ya.ru>
Signed-off-by: Alexander Tsoy <alexander at tsoy.me>
---
 sound/usb/format.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/sound/usb/format.c b/sound/usb/format.c
index 50e1874c847c..5ffb457cc88c 100644
--- a/sound/usb/format.c
+++ b/sound/usb/format.c
@@ -277,6 +277,52 @@ static bool s1810c_valid_sample_rate(struct audioformat *fp,
 	return false;
 }
 
+/*
+ * Many Focusrite devices supports a limited set of sampling rates per
+ * altsetting. Maximum rate is exposed in the last 4 bytes of Format Type
+ * descriptor which has a non-standard bLength = 10.
+ */
+static bool focusrite_valid_sample_rate(struct snd_usb_audio *chip,
+					struct audioformat *fp,
+					unsigned int rate)
+{
+	struct usb_interface *iface;
+	struct usb_host_interface *alts;
+	unsigned char *fmt;
+	unsigned int max_rate;
+
+	iface = usb_ifnum_to_if(chip->dev, fp->iface);
+	if (!iface)
+		return true;
+
+	alts = &iface->altsetting[fp->altset_idx];
+	fmt = snd_usb_find_csint_desc(alts->extra, alts->extralen,
+				      NULL, UAC_FORMAT_TYPE);
+	if (!fmt)
+		return true;
+
+	if (fmt[0] == 10) { /* bLength */
+		max_rate = combine_quad(&fmt[6]);
+
+		/* Validate max rate */
+		if (max_rate != 48000 &&
+		    max_rate != 96000 &&
+		    max_rate != 192000 &&
+		    max_rate != 384000) {
+
+			usb_audio_info(chip,
+				"%u:%d : unexpected max rate: %u\n",
+				fp->iface, fp->altsetting, max_rate);
+
+			return true;
+		}
+
+		return rate <= max_rate;
+	}
+
+	return true;
+}
+
 /*
  * Helper function to walk the array of sample rate triplets reported by
  * the device. The problem is that we need to parse whole array first to
@@ -319,6 +365,11 @@ static int parse_uac2_sample_rate_range(struct snd_usb_audio *chip,
 			    !s1810c_valid_sample_rate(fp, rate))
 				goto skip_rate;
 
+			/* Filter out invalid rates on Focusrite devices */
+			if (USB_ID_VENDOR(chip->usb_id) == 0x1235 &&
+			    !focusrite_valid_sample_rate(chip, fp, rate))
+				goto skip_rate;
+
 			if (fp->rate_table)
 				fp->rate_table[nr_rates] = rate;
 			if (!fp->rate_min || rate < fp->rate_min)
-- 
2.25.3