[PATCH 6.6 064/166] ALSA: usb-audio: Improve Focusrite sample rate filtering

Greg Kroah-Hartman
2026-04-24T13:36:12Z

From: Geoffrey D. Bennett <g@b4.vu>

[ Upstream commit 24d2d3c5f94007a5a0554065ab7349bb69e28bcb ]

Replace the bLength == 10 max_rate check in
focusrite_valid_sample_rate() with filtering that also examines the
bmControls VAL_ALT_SETTINGS bit.

When VAL_ALT_SETTINGS is readable, the device uses strict
per-altsetting rate filtering (only the highest rate pair for that
altsetting is valid). When it is not readable, all rates up to
max_rate are valid.

For devices without the bLength == 10 Format Type descriptor extension
but with VAL_ALT_SETTINGS readable and multiple altsettings (only seen
in Scarlett 18i8 3rd Gen playback), fall back to the Focusrite
convention: alt 1 = 48kHz, alt 2 = 96kHz, alt 3 = 192kHz.

This produces correct rate tables for all tested Focusrite devices
(all Scarlett 2nd, 3rd, and 4th Gen, Clarett+, and Vocaster) using
only USB descriptors, allowing QUIRK_FLAG_VALIDATE_RATES to be removed
for Focusrite in the next commit.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/7e18c1f393a6ecb6fc75dd867a2c4dbe135e3e22.1771594828.git.g@b4.vu
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
sound/usb/format.c | 86 +++++++++++++++++++++++++++++++++++++++-------
1 file changed, 74 insertions(+), 12 deletions(-)

diff --git a/sound/usb/format.c b/sound/usb/format.c
index f33d25a4e4cc7..682adbdf7ee79 100644
--- a/sound/usb/format.c
+++ b/sound/usb/format.c
@@ -304,9 +304,37 @@ static bool s1810c_valid_sample_rate(struct audioformat *fp,
}

/*
- * 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.
+ * Focusrite devices use rate pairs: 44100/48000, 88200/96000, and
+ * 176400/192000. Return true if rate is in the pair for max_rate.
+ */
+static bool focusrite_rate_pair(unsigned int rate,
+ unsigned int max_rate)
+{
+ switch (max_rate) {
+ case 48000: return rate == 44100 || rate == 48000;
+ case 96000: return rate == 88200 || rate == 96000;
+ case 192000: return rate == 176400 || rate == 192000;
+ default: return true;
+ }
+}
+
+/*
+ * Focusrite devices report all supported rates in a single clock
+ * source but only a subset is valid per altsetting.
+ *
+ * Detection uses two descriptor features:
+ *
+ * 1. Format Type descriptor bLength == 10: non-standard extension
+ * with max sample rate in bytes 6..9.
+ *
+ * 2. bmControls VAL_ALT_SETTINGS readable bit: when set, the device
+ * only supports the highest rate pair for that altsetting, and when
+ * clear, all rates up to max_rate are valid.
+ *
+ * For devices without the bLength == 10 extension but with
+ * VAL_ALT_SETTINGS readable and multiple altsettings (only seen in
+ * Scarlett 18i8 3rd Gen playback), fall back to the Focusrite
+ * convention: alt 1 = 48kHz, alt 2 = 96kHz, alt 3 = 192kHz.
*/
static bool focusrite_valid_sample_rate(struct snd_usb_audio *chip,
struct audioformat *fp,
@@ -314,8 +342,10 @@ static bool focusrite_valid_sample_rate(struct snd_usb_audio *chip,
{
struct usb_interface *iface;
struct usb_host_interface *alts;
+ struct uac2_as_header_descriptor *as;
unsigned char *fmt;
unsigned int max_rate;
+ bool val_alt;

iface = usb_ifnum_to_if(chip->dev, fp->iface);
if (!iface)
@@ -327,26 +357,58 @@ static bool focusrite_valid_sample_rate(struct snd_usb_audio *chip,
if (!fmt)
return true;

+ as = snd_usb_find_csint_desc(alts->extra, alts->extralen,
+ NULL, UAC_AS_GENERAL);
+ if (!as)
+ return true;
+
+ val_alt = uac_v2v3_control_is_readable(as->bmControls,
+ UAC2_AS_VAL_ALT_SETTINGS);
+
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) {
-
+ if (val_alt)
+ return focusrite_rate_pair(rate, max_rate);
+
+ /* No val_alt: rates fall through from higher */
+ switch (max_rate) {
+ case 192000:
+ if (rate == 176400 || rate == 192000)
+ return true;
+ fallthrough;
+ case 96000:
+ if (rate == 88200 || rate == 96000)
+ return true;
+ fallthrough;
+ case 48000:
+ return (rate == 44100 || rate == 48000);
+ default:
usb_audio_info(chip,
"%u:%d : unexpected max rate: %u\n",
fp->iface, fp->altsetting, max_rate);
-
return true;
}
+ }

- return rate <= max_rate;
+ if (!val_alt)
+ return true;
+
+ /* Multi-altsetting device with val_alt but no max_rate
+ * in the format descriptor. Use Focusrite convention:
+ * alt 1 = 48kHz, alt 2 = 96kHz, alt 3 = 192kHz.
+ */
+ if (iface->num_altsetting <= 2)
+ return true;
+
+ switch (fp->altsetting) {
+ case 1: max_rate = 48000; break;
+ case 2: max_rate = 96000; break;
+ case 3: max_rate = 192000; break;
+ default: return true;
}

- return true;
+ return focusrite_rate_pair(rate, max_rate);
}

/*
--
2.53.0