[PATCH v2 0/2] usb: gadget: uac2: support explicit channel counts

[PATCH v2 0/2] usb: gadget: uac2: support explicit channel counts

[Kyle Farrell]

USB Audio Class 2.0 permits up to 255 channels in an audio channel cluster
via bNrChannels. Independently, a channel-location bitmap is specified via
bmChannelConfig for up to 27 predefined spatial locations. Setting
bmChannelConfig to zero indicates that the channels have no predefined
spatial positions.

The current UAC2 gadget driver derives bNrChannels from the number of set
bits in bmChannelConfig. Since bmChannelConfig provides 27 predefined
spatial locations this limits playback and capture streams to 27 channels.

This series allows channel counts up to the UAC2 limit of 255 without
requiring predefined spatial channel locations in bmChannelConfig.
Existing ConfigFS channel-mask configuration continues to work unchanged.
New optional playback and capture count attributes allow the number of
channels to be specified separately from the channel-location bitmap.

The series is split so that the shared u_audio infrastructure is updated
first without changing behavior, followed by the UAC2 functional change.

Patch 1:
  Pass resolved playback and capture channel counts explicitly into
  the UAC1/UAC2 shared u_audio implementation instead of deriving them
  from channel masks.

Patch 2:
  Add ConfigFS playback/capture channel-count attributes to the UAC2
  function driver and use them when generating descriptors and stream
  configuration, allowing counts beyond the 27 predefined channel
  locations.

Testing

Testing was performed using a Raspberry Pi Zero 2 W configured as a USB
UAC2 gadget. Streaming and descriptor validation were performed against
both Linux and macOS hosts.  Windows hosts were not tested.

UAC1
  - Verified playback/capture operation via alsaloop.
  - No descriptor or behavioral changes observed.

UAC2
  - Verified legacy channel-mask configuration (1-27 channels).
  - Verified explicit channel-count mode with 2, 28, 61, 62, 96 channels.
  - Verified invalid channel-count/channel-mask combinations are rejected.
  - Verified Feature Unit descriptor size limits at 61/62 channel boundary.
  - Verified descriptors including:
      * bNrChannels
      * bmChannelConfig
      * Feature Unit presence and bLength
      * AudioControl wTotalLength
      * endpoint wMaxPacketSize
  - Verified ALSA-reported playback and capture channel counts.
  - Verified streaming using alsaloop.

Changes in v2:
  - Edit commit messages for clarity and brevity.
  - Add Assisted-by: LLM attribution.
  - No code changes.

v1: https://lore.kernel.org/linux-usb/20260729054115.889174-1-kaf@nwlink.com/

Kyle Farrell (2):
  usb: gadget: u_audio: pass channel counts explicitly
  usb: gadget: f_uac2: allow explicit channel count independent of
    channel mask

 .../ABI/testing/configfs-usb-gadget-uac2      | 18 ++++-
 Documentation/usb/gadget-testing.rst          | 18 ++++-
 drivers/usb/gadget/function/f_uac1.c          |  2 +
 drivers/usb/gadget/function/f_uac2.c          | 69 ++++++++++++++-----
 drivers/usb/gadget/function/u_audio.c         | 32 ++++-----
 drivers/usb/gadget/function/u_audio.h         |  2 +
 drivers/usb/gadget/function/u_uac2.h          |  4 ++
 7 files changed, 105 insertions(+), 40 deletions(-)


base-commit: d58dffe9ee2c8883193959ff4ef995ec07932874
-- 
2.34.1

[PATCH v2 1/2] usb: gadget: u_audio: pass channel counts explicitly

[Kyle Farrell]

u_audio currently derives the channel count from the channel mask in
struct uac_params in several places.  Instead, pass the resolved count
explicitly, so the shared audio implementation does not depend on how
an individual USB audio function represents its channel configuration.

Initialize the new channel count fields from the existing channel masks.
This preserves generated descriptors and makes no change to user-visible
behavior.

Assisted-by: LLM
Signed-off-by: Kyle Farrell <kaf@nwlink.com>
---
 drivers/usb/gadget/function/f_uac1.c  |  2 ++
 drivers/usb/gadget/function/f_uac2.c  |  2 ++
 drivers/usb/gadget/function/u_audio.c | 32 ++++++++++++---------------
 drivers/usb/gadget/function/u_audio.h |  2 ++
 4 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/drivers/usb/gadget/function/f_uac1.c b/drivers/usb/gadget/function/f_uac1.c
index 7a81cd176abd..17f856b96d2a 100644
--- a/drivers/usb/gadget/function/f_uac1.c
+++ b/drivers/usb/gadget/function/f_uac1.c
@@ -1448,6 +1448,7 @@ static int f_audio_bind(struct usb_configuration *c, struct usb_function *f)
 	audio->out_ep_maxpsize = le16_to_cpu(as_out_ep_desc.wMaxPacketSize);
 	audio->in_ep_maxpsize = le16_to_cpu(as_in_ep_desc.wMaxPacketSize);
 	audio->params.c_chmask = audio_opts->c_chmask;
+	audio->params.c_channels = num_channels(audio_opts->c_chmask);
 	memcpy(audio->params.c_srates, audio_opts->c_srates,
 			sizeof(audio->params.c_srates));
 	audio->params.c_ssize = audio_opts->c_ssize;
@@ -1461,6 +1462,7 @@ static int f_audio_bind(struct usb_configuration *c, struct usb_function *f)
 		audio->params.p_fu.volume_res = audio_opts->p_volume_res;
 	}
 	audio->params.p_chmask = audio_opts->p_chmask;
+	audio->params.p_channels = num_channels(audio_opts->p_chmask);
 	memcpy(audio->params.p_srates, audio_opts->p_srates,
 			sizeof(audio->params.p_srates));
 	audio->params.p_ssize = audio_opts->p_ssize;
diff --git a/drivers/usb/gadget/function/f_uac2.c b/drivers/usb/gadget/function/f_uac2.c
index d8cf710085a0..46255ad0a9cf 100644
--- a/drivers/usb/gadget/function/f_uac2.c
+++ b/drivers/usb/gadget/function/f_uac2.c
@@ -1308,6 +1308,7 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
 	agdev->gadget = gadget;
 
 	agdev->params.p_chmask = uac2_opts->p_chmask;
+	agdev->params.p_channels = num_channels(uac2_opts->p_chmask);
 	memcpy(agdev->params.p_srates, uac2_opts->p_srates,
 			sizeof(agdev->params.p_srates));
 	agdev->params.p_ssize = uac2_opts->p_ssize;
@@ -1320,6 +1321,7 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
 		agdev->params.p_fu.volume_res = uac2_opts->p_volume_res;
 	}
 	agdev->params.c_chmask = uac2_opts->c_chmask;
+	agdev->params.c_channels = num_channels(uac2_opts->c_chmask);
 	memcpy(agdev->params.c_srates, uac2_opts->c_srates,
 			sizeof(agdev->params.c_srates));
 	agdev->params.c_ssize = uac2_opts->c_ssize;
diff --git a/drivers/usb/gadget/function/u_audio.c b/drivers/usb/gadget/function/u_audio.c
index f0ee83abcebc..de0622ee72ab 100644
--- a/drivers/usb/gadget/function/u_audio.c
+++ b/drivers/usb/gadget/function/u_audio.c
@@ -390,25 +390,22 @@ static int uac_pcm_open(struct snd_pcm_substream *substream)
 	struct uac_params *params;
 	struct uac_rtd_params *prm;
 	int p_ssize, c_ssize;
-	int p_chmask, c_chmask;
 
 	audio_dev = uac->audio_dev;
 	params = &audio_dev->params;
 	p_ssize = params->p_ssize;
 	c_ssize = params->c_ssize;
-	p_chmask = params->p_chmask;
-	c_chmask = params->c_chmask;
 	uac->p_residue_mil = 0;
 
 	runtime->hw = uac_pcm_hardware;
 
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
 		runtime->hw.formats = uac_ssize_to_fmt(p_ssize);
-		runtime->hw.channels_min = num_channels(p_chmask);
+		runtime->hw.channels_min = params->p_channels;
 		prm = &uac->p_prm;
 	} else {
 		runtime->hw.formats = uac_ssize_to_fmt(c_ssize);
-		runtime->hw.channels_min = num_channels(c_chmask);
+		runtime->hw.channels_min = params->c_channels;
 		prm = &uac->c_prm;
 	}
 
@@ -736,8 +733,7 @@ int u_audio_start_playback(struct g_audio *audio_dev)
 		factor = 8000;
 
 	/* pre-compute some values for iso_complete() */
-	uac->p_framesize = params->p_ssize *
-			    num_channels(params->p_chmask);
+	uac->p_framesize = params->p_ssize * params->p_channels;
 	uac->p_interval = factor / (1 << (ep_desc->bInterval - 1));
 	p_pktsize = min_t(unsigned int,
 				uac->p_framesize *
@@ -1205,7 +1201,7 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 	struct snd_pcm *pcm;
 	struct snd_kcontrol *kctl;
 	struct uac_params *params;
-	int p_chmask, c_chmask;
+	unsigned int p_channels, c_channels;
 	int i, err;
 
 	if (!g_audio)
@@ -1218,10 +1214,10 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 	uac->audio_dev = g_audio;
 
 	params = &g_audio->params;
-	p_chmask = params->p_chmask;
-	c_chmask = params->c_chmask;
+	p_channels = params->p_channels;
+	c_channels = params->c_channels;
 
-	if (c_chmask) {
+	if (c_channels) {
 		struct uac_rtd_params *prm = &uac->c_prm;
 
 		spin_lock_init(&prm->lock);
@@ -1245,7 +1241,7 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 		}
 	}
 
-	if (p_chmask) {
+	if (p_channels) {
 		struct uac_rtd_params *prm = &uac->p_prm;
 
 		spin_lock_init(&prm->lock);
@@ -1284,7 +1280,7 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 	 * Create a substream only for non-zero channel streams
 	 */
 	err = snd_pcm_new(uac->card, pcm_name, 0,
-			       p_chmask ? 1 : 0, c_chmask ? 1 : 0, &pcm);
+			       p_channels ? 1 : 0, c_channels ? 1 : 0, &pcm);
 	if (err < 0)
 		goto snd_fail;
 
@@ -1299,12 +1295,12 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 	 * Create mixer and controls
 	 * Create only if it's required on USB side
 	 */
-	if ((c_chmask && g_audio->in_ep_fback)
-			|| (p_chmask && params->p_fu.id)
-			|| (c_chmask && params->c_fu.id))
+	if ((c_channels && g_audio->in_ep_fback)
+			|| (p_channels && params->p_fu.id)
+			|| (c_channels && params->c_fu.id))
 		strscpy(card->mixername, card_name);
 
-	if (c_chmask && g_audio->in_ep_fback) {
+	if (c_channels && g_audio->in_ep_fback) {
 		kctl = snd_ctl_new1(&u_audio_controls[UAC_FBACK_CTRL],
 				    &uac->c_prm);
 		if (!kctl) {
@@ -1320,7 +1316,7 @@ int g_audio_setup(struct g_audio *g_audio, const char *pcm_name,
 			goto snd_fail;
 	}
 
-	if (p_chmask) {
+	if (p_channels) {
 		kctl = snd_ctl_new1(&u_audio_controls[UAC_P_PITCH_CTRL],
 				    &uac->p_prm);
 		if (!kctl) {
diff --git a/drivers/usb/gadget/function/u_audio.h b/drivers/usb/gadget/function/u_audio.h
index 9512b8fccfaa..46e1d6b5ca4e 100644
--- a/drivers/usb/gadget/function/u_audio.h
+++ b/drivers/usb/gadget/function/u_audio.h
@@ -41,12 +41,14 @@ struct uac_fu_params {
 struct uac_params {
 	/* playback */
 	int p_chmask;	/* channel mask */
+	unsigned int p_channels;	/* resolved number of channels */
 	int p_srates[UAC_MAX_RATES];	/* available rates in Hz (0 terminated list) */
 	int p_ssize;	/* sample size */
 	struct uac_fu_params p_fu;	/* Feature Unit parameters */
 
 	/* capture */
 	int c_chmask;	/* channel mask */
+	unsigned int c_channels;	/* resolved number of channels */
 	int c_srates[UAC_MAX_RATES];	/* available rates in Hz (0 terminated list) */
 	int c_ssize;	/* sample size */
 	struct uac_fu_params c_fu;	/* Feature Unit parameters */
-- 
2.34.1

[PATCH v2 2/2] usb: gadget: f_uac2: allow explicit channel count independent of channel mask

[Kyle Farrell]

The UAC2 gadget driver currently derives bNrChannels from the number
of set bits in bmChannelConfig. Since the bmChannelConfig
channel-location bitmap provides only 27 predefined channel locations,
this limits playback and capture streams to 27 channels. UAC2 permits
bNrChannels values up to 255, independent of bmChannelConfig.

Add ConfigFS p_channels and c_channels attributes to configure the
playback and capture channel counts explicitly.  Preserve the existing
p_chmask and c_chmask interface and semantics. The default value of
zero for p_channels and c_channels continues to derive the channel count
from the corresponding p_chmask and c_chmask.

When an explicit channel count and a nonzero channel mask are both
specified, require the number of set bits in the mask to match the
channel count. Partial channel-location assignments permitted by UAC2
are therefore not supported. Additionally, reject Feature Unit
configurations whose descriptor length would exceed the one-byte
bLength field.

Assisted-by: LLM
Signed-off-by: Kyle Farrell <kaf@nwlink.com>
---
 .../ABI/testing/configfs-usb-gadget-uac2      | 18 ++++-
 Documentation/usb/gadget-testing.rst          | 18 ++++-
 drivers/usb/gadget/function/f_uac2.c          | 71 +++++++++++++------
 drivers/usb/gadget/function/u_uac2.h          |  4 ++
 4 files changed, 87 insertions(+), 24 deletions(-)

diff --git a/Documentation/ABI/testing/configfs-usb-gadget-uac2 b/Documentation/ABI/testing/configfs-usb-gadget-uac2
index 133e995c3e92..8e264843e492 100644
--- a/Documentation/ABI/testing/configfs-usb-gadget-uac2
+++ b/Documentation/ABI/testing/configfs-usb-gadget-uac2
@@ -5,7 +5,14 @@ Description:
 		The attributes:
 
 		=====================	=======================================
-		c_chmask		capture channel mask
+		c_chmask		capture channel-location mask. The number of
+					capture channels is derived from the number of
+					set bits unless c_channels is specified.
+		c_channels		explicit capture channel count. A value of 0
+					derives the channel count from c_chmask. When
+					nonzero, c_chmask specifies channel locations
+					and may be 0. If both c_channels and c_chmask
+					are nonzero, the values must agree.
 		c_srate			list of capture sampling rates (comma-separated)
 		c_ssize			capture sample size (bytes)
 		c_hs_bint		capture bInterval for HS/SS (1-4: fixed, 0: auto)
@@ -20,7 +27,14 @@ Description:
 		c_volume_res		capture volume control resolution
 					(in 1/256 dB)
 		fb_max			maximum extra bandwidth in async mode
-		p_chmask		playback channel mask
+		p_chmask		playback channel-location mask. The number of
+					playback channels is derived from the number of
+					set bits unless p_channels is specified.
+		p_channels		explicit playback channel count. A value of 0
+					derives the channel count from p_chmask. When
+					nonzero, p_chmask specifies channel locations
+					and may be 0. If both p_channels and p_chmask
+					are nonzero, the values must agree.
 		p_srate			list of playback sampling rates (comma-separated)
 		p_ssize			playback sample size (bytes)
 		p_hs_bint		playback bInterval for HS/SS (1-4: fixed, 0: auto)
diff --git a/Documentation/usb/gadget-testing.rst b/Documentation/usb/gadget-testing.rst
index 4921e5307d49..c050218291d1 100644
--- a/Documentation/usb/gadget-testing.rst
+++ b/Documentation/usb/gadget-testing.rst
@@ -741,7 +741,14 @@ The function name to use when creating the function directory is "uac2".
 The uac2 function provides these attributes in its function directory:
 
 	================ ====================================================
-	c_chmask         capture channel mask
+	c_chmask         capture channel-location mask. The number of
+			 capture channels is derived from the number of
+			 set bits unless c_channels is specified.
+	c_channels	 explicit capture channel count. A value of 0
+			 derives the channel count from c_chmask. When
+			 nonzero, c_chmask specifies channel locations
+			 and may be 0. If both c_channels and c_chmask
+			 are nonzero, the values must agree.
 	c_srate          list of capture sampling rates (comma-separated)
 	c_ssize          capture sample size (bytes)
 	c_sync           capture synchronization type (async/adaptive)
@@ -752,7 +759,14 @@ The uac2 function provides these attributes in its function directory:
 	c_volume_res     capture volume control resolution (in 1/256 dB)
 	c_hs_bint        capture bInterval for HS/SS (1-4: fixed, 0: auto)
 	fb_max           maximum extra bandwidth in async mode
-	p_chmask         playback channel mask
+	p_chmask         playback channel-location mask. The number of
+			 playback channels is derived from the number of
+			 set bits unless p_channels is specified.
+	p_channels	 explicit playback channel count. A value of 0
+			 derives the channel count from p_chmask. When
+			 nonzero, p_chmask specifies channel locations
+			 and may be 0. If both p_channels and p_chmask
+			 are nonzero, the values must agree.
 	p_srate          list of playback sampling rates (comma-separated)
 	p_ssize          playback sample size (bytes)
 	p_mute_present   playback mute control enable
diff --git a/drivers/usb/gadget/function/f_uac2.c b/drivers/usb/gadget/function/f_uac2.c
index 46255ad0a9cf..c7801bb8765e 100644
--- a/drivers/usb/gadget/function/f_uac2.c
+++ b/drivers/usb/gadget/function/f_uac2.c
@@ -50,8 +50,17 @@
 #define UNFLW_CTRL	8
 #define OVFLW_CTRL	10
 
-#define EPIN_EN(_opts) ((_opts)->p_chmask != 0)
-#define EPOUT_EN(_opts) ((_opts)->c_chmask != 0)
+static unsigned int uac2_num_channels(bool is_playback,
+				      const struct f_uac2_opts *opts)
+{
+	u8 channels = is_playback ? opts->p_channels : opts->c_channels;
+	u32 chmask = is_playback ? opts->p_chmask : opts->c_chmask;
+
+	return channels ? channels : num_channels(chmask);
+}
+
+#define EPIN_EN(_opts) (uac2_num_channels(true, (_opts)) != 0)
+#define EPOUT_EN(_opts) (uac2_num_channels(false, (_opts)) != 0)
 #define FUIN_EN(_opts) (EPIN_EN(_opts) \
 				&& ((_opts)->p_mute_present \
 				|| (_opts)->p_volume_present))
@@ -671,15 +680,16 @@ static int get_max_srate(const int *srates)
 static int get_max_bw_for_bint(const struct f_uac2_opts *uac2_opts,
 	u8 bint, unsigned int factor, bool is_playback)
 {
-	int chmask, srate, ssize;
+	unsigned int channels;
+	int srate, ssize;
 	u16 max_size_bw;
 
 	if (is_playback) {
-		chmask = uac2_opts->p_chmask;
+		channels = uac2_num_channels(true, uac2_opts);
 		srate = get_max_srate(uac2_opts->p_srates);
 		ssize = uac2_opts->p_ssize;
 	} else {
-		chmask = uac2_opts->c_chmask;
+		channels = uac2_num_channels(false, uac2_opts);
 		srate = get_max_srate(uac2_opts->c_srates);
 		ssize = uac2_opts->c_ssize;
 	}
@@ -689,11 +699,11 @@ static int get_max_bw_for_bint(const struct f_uac2_opts *uac2_opts,
 		// Win10 requires max packet size + 1 frame
 		srate = srate * (1000 + uac2_opts->fb_max) / 1000;
 		// updated srate is always bigger, therefore DIV_ROUND_UP always yields +1
-		max_size_bw = num_channels(chmask) * ssize *
+		max_size_bw = channels * ssize *
 			(DIV_ROUND_UP(srate, factor / (1 << (bint - 1))));
 	} else {
 		// adding 1 frame provision for Win10
-		max_size_bw = num_channels(chmask) * ssize *
+		max_size_bw = channels * ssize *
 			(DIV_ROUND_UP(srate, factor / (1 << (bint - 1))) + 1);
 	}
 	return max_size_bw;
@@ -764,10 +774,11 @@ static int set_ep_max_packet_size_bint(struct device *dev, const struct f_uac2_o
 	return 0;
 }
 
-static struct uac2_feature_unit_descriptor *build_fu_desc(int chmask)
+static struct uac2_feature_unit_descriptor *build_fu_desc(bool is_playback,
+							  const struct f_uac2_opts *opts)
 {
 	struct uac2_feature_unit_descriptor *fu_desc;
-	int channels = num_channels(chmask);
+	unsigned int channels = uac2_num_channels(is_playback, opts);
 	int fu_desc_size = UAC2_DT_FEATURE_UNIT_SIZE(channels);
 
 	fu_desc = kzalloc(fu_desc_size, GFP_KERNEL);
@@ -976,13 +987,27 @@ static int afunc_validate_opts(struct g_audio *agdev, struct device *dev)
 {
 	struct f_uac2_opts *opts = g_audio_to_uac2_opts(agdev);
 	const char *msg = NULL;
+	unsigned int p_channels = uac2_num_channels(true, opts);
+	unsigned int c_channels = uac2_num_channels(false, opts);
 
-	if (!opts->p_chmask && !opts->c_chmask)
-		msg = "no playback and capture channels";
-	else if (opts->p_chmask & ~UAC2_CHANNEL_MASK)
+	if (opts->p_chmask & ~UAC2_CHANNEL_MASK)
 		msg = "unsupported playback channels mask";
 	else if (opts->c_chmask & ~UAC2_CHANNEL_MASK)
 		msg = "unsupported capture channels mask";
+	else if (!p_channels && !c_channels)
+		msg = "no playback and capture channels";
+	else if (opts->p_channels && opts->p_chmask &&
+		 opts->p_channels != num_channels(opts->p_chmask))
+		msg = "playback channel count does not match channel mask";
+	else if (opts->c_channels && opts->c_chmask &&
+		 opts->c_channels != num_channels(opts->c_chmask))
+		msg = "capture channel count does not match channel mask";
+	else if (FUIN_EN(opts) &&
+		 UAC2_DT_FEATURE_UNIT_SIZE(p_channels) > U8_MAX)
+		msg = "too many playback channels for feature unit descriptor";
+	else if (FUOUT_EN(opts) &&
+		 UAC2_DT_FEATURE_UNIT_SIZE(c_channels) > U8_MAX)
+		msg = "too many capture channels for feature unit descriptor";
 	else if ((opts->p_ssize < 1) || (opts->p_ssize > 4))
 		msg = "incorrect playback sample size";
 	else if ((opts->c_ssize < 1) || (opts->c_ssize > 4))
@@ -1059,12 +1084,12 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
 		return PTR_ERR(us);
 
 	if (FUOUT_EN(uac2_opts)) {
-		out_feature_unit_desc = build_fu_desc(uac2_opts->c_chmask);
+		out_feature_unit_desc = build_fu_desc(false, uac2_opts);
 		if (!out_feature_unit_desc)
 			return -ENOMEM;
 	}
 	if (FUIN_EN(uac2_opts)) {
-		in_feature_unit_desc = build_fu_desc(uac2_opts->p_chmask);
+		in_feature_unit_desc = build_fu_desc(true, uac2_opts);
 		if (!in_feature_unit_desc) {
 			ret = -ENOMEM;
 			goto err_free_fu;
@@ -1099,13 +1124,13 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
 
 
 	/* Initialize the configurable parameters */
-	usb_out_it_desc.bNrChannels = num_channels(uac2_opts->c_chmask);
+	usb_out_it_desc.bNrChannels = uac2_num_channels(false, uac2_opts);
 	usb_out_it_desc.bmChannelConfig = cpu_to_le32(uac2_opts->c_chmask);
-	io_in_it_desc.bNrChannels = num_channels(uac2_opts->p_chmask);
+	io_in_it_desc.bNrChannels = uac2_num_channels(true, uac2_opts);
 	io_in_it_desc.bmChannelConfig = cpu_to_le32(uac2_opts->p_chmask);
-	as_out_hdr_desc.bNrChannels = num_channels(uac2_opts->c_chmask);
+	as_out_hdr_desc.bNrChannels = uac2_num_channels(false, uac2_opts);
 	as_out_hdr_desc.bmChannelConfig = cpu_to_le32(uac2_opts->c_chmask);
-	as_in_hdr_desc.bNrChannels = num_channels(uac2_opts->p_chmask);
+	as_in_hdr_desc.bNrChannels = uac2_num_channels(true, uac2_opts);
 	as_in_hdr_desc.bmChannelConfig = cpu_to_le32(uac2_opts->p_chmask);
 	as_out_fmt1_desc.bSubslotSize = uac2_opts->c_ssize;
 	as_out_fmt1_desc.bBitResolution = uac2_opts->c_ssize * 8;
@@ -1308,7 +1333,7 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
 	agdev->gadget = gadget;
 
 	agdev->params.p_chmask = uac2_opts->p_chmask;
-	agdev->params.p_channels = num_channels(uac2_opts->p_chmask);
+	agdev->params.p_channels = uac2_num_channels(true, uac2_opts);
 	memcpy(agdev->params.p_srates, uac2_opts->p_srates,
 			sizeof(agdev->params.p_srates));
 	agdev->params.p_ssize = uac2_opts->p_ssize;
@@ -1321,7 +1346,7 @@ afunc_bind(struct usb_configuration *cfg, struct usb_function *fn)
 		agdev->params.p_fu.volume_res = uac2_opts->p_volume_res;
 	}
 	agdev->params.c_chmask = uac2_opts->c_chmask;
-	agdev->params.c_channels = num_channels(uac2_opts->c_chmask);
+	agdev->params.c_channels = uac2_num_channels(false, uac2_opts);
 	memcpy(agdev->params.c_srates, uac2_opts->c_srates,
 			sizeof(agdev->params.c_srates));
 	agdev->params.c_ssize = uac2_opts->c_ssize;
@@ -2091,10 +2116,12 @@ end:									\
 CONFIGFS_ATTR(f_uac2_opts_, name)
 
 UAC2_ATTRIBUTE(u32, p_chmask);
+UAC2_ATTRIBUTE(u8, p_channels);
 UAC2_RATE_ATTRIBUTE(p_srate);
 UAC2_ATTRIBUTE(u32, p_ssize);
 UAC2_ATTRIBUTE(u8, p_hs_bint);
 UAC2_ATTRIBUTE(u32, c_chmask);
+UAC2_ATTRIBUTE(u8, c_channels);
 UAC2_RATE_ATTRIBUTE(c_srate);
 UAC2_ATTRIBUTE_SYNC(c_sync);
 UAC2_ATTRIBUTE(u32, c_ssize);
@@ -2134,10 +2161,12 @@ UAC2_ATTRIBUTE(s16, c_terminal_type);
 
 static struct configfs_attribute *f_uac2_attrs[] = {
 	&f_uac2_opts_attr_p_chmask,
+	&f_uac2_opts_attr_p_channels,
 	&f_uac2_opts_attr_p_srate,
 	&f_uac2_opts_attr_p_ssize,
 	&f_uac2_opts_attr_p_hs_bint,
 	&f_uac2_opts_attr_c_chmask,
+	&f_uac2_opts_attr_c_channels,
 	&f_uac2_opts_attr_c_srate,
 	&f_uac2_opts_attr_c_ssize,
 	&f_uac2_opts_attr_c_hs_bint,
@@ -2207,10 +2236,12 @@ static struct usb_function_instance *afunc_alloc_inst(void)
 				    &f_uac2_func_type);
 
 	opts->p_chmask = UAC2_DEF_PCHMASK;
+	opts->p_channels = UAC2_DEF_PCHANNELS;
 	opts->p_srates[0] = UAC2_DEF_PSRATE;
 	opts->p_ssize = UAC2_DEF_PSSIZE;
 	opts->p_hs_bint = UAC2_DEF_PHSBINT;
 	opts->c_chmask = UAC2_DEF_CCHMASK;
+	opts->c_channels = UAC2_DEF_CCHANNELS;
 	opts->c_srates[0] = UAC2_DEF_CSRATE;
 	opts->c_ssize = UAC2_DEF_CSSIZE;
 	opts->c_hs_bint = UAC2_DEF_CHSBINT;
diff --git a/drivers/usb/gadget/function/u_uac2.h b/drivers/usb/gadget/function/u_uac2.h
index 0df808289ded..bacd841c9df6 100644
--- a/drivers/usb/gadget/function/u_uac2.h
+++ b/drivers/usb/gadget/function/u_uac2.h
@@ -17,10 +17,12 @@
 #include "uac_common.h"
 
 #define UAC2_DEF_PCHMASK 0x3
+#define UAC2_DEF_PCHANNELS 0  /* 0 = derive from p_chmask */
 #define UAC2_DEF_PSRATE 48000
 #define UAC2_DEF_PSSIZE 2
 #define UAC2_DEF_PHSBINT 0
 #define UAC2_DEF_CCHMASK 0x3
+#define UAC2_DEF_CCHANNELS 0  /* 0 = derive from c_chmask */
 #define UAC2_DEF_CSRATE 64000
 #define UAC2_DEF_CSSIZE 2
 #define UAC2_DEF_CHSBINT 0
@@ -46,11 +48,13 @@ struct f_uac2_opts {
 	int				p_srates[UAC_MAX_RATES];
 	int				p_ssize;
 	u8				p_hs_bint;
+	u8				p_channels;
 	int				c_chmask;
 	int				c_srates[UAC_MAX_RATES];
 	int				c_ssize;
 	int				c_sync;
 	u8				c_hs_bint;
+	u8				c_channels;
 
 	bool			p_mute_present;
 	bool			p_volume_present;
-- 
2.34.1