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

[Kyle Farrell <kaf@nwlink.com>]

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