[PATCH 0/3] UAC2 Gadget: feedback endpoint support

[Ruslan Bilovol <ruslan.bilovol@gmail.com>]

Current UAC2 gadget implements capture/sync paths
as two USB ISO ASYNC endpoints (IN and OUT).

This violates USB spec which says that ISO ASYNC OUT endpoint
should have feedback companion endpoint.
See USB2.0 spec  "5.12.4.1 Synchronization Type": asynchronous
sink provides explicit feedback (isochronous pipe).
Interesting that for ISO ASYNC *IN* endpoint respective
feedback isn't required since source provides implicit
feedforward (data stream).

While it's not an issue if UAC2 Gadget is connected to
Linux host (Linux ignores missing feedback endpoint),
with other hosts like Windows or MacOS the UAC2 Gadget
isn't enumerated due to missing feedback endpoint.

This patch series adds feedback endpoint support to
UAC2 function, new control to UAC2 mixer which can
be used by userspace tools (like alsaloop from alsa-utils)
for updating feedback frequency reported to the host.
This is useful for usecases when UAC2 Gadget's audio
samples are played to another codec or audio card
with its own internal freerunning clock so host can
be notified that more/less samples are required.

The alsaloop tool requires some (relatively small)
modifications in order to start support driving
feedback frequency through UAC2 mixer control.
That change will be sent as a separate patch
to ALSA community.

Also added ability to switch ISO ASYNC OUT endpoint into
adaptive endpoint which doesn't require feedback endpoint
(as per USB spec).

Ruslan Bilovol (3):
  usb: gadget: f_uac2/u_audio: add feedback endpoint support
  usb: gadget: f_uac2: add adaptive sync support for capture
  usb: gadget: u_audio: add real feedback implementation

 Documentation/ABI/testing/configfs-usb-gadget-uac2 |   1 +
 Documentation/usb/gadget-testing.rst               |   1 +
 drivers/usb/gadget/function/f_uac2.c               | 118 ++++++++++-
 drivers/usb/gadget/function/u_audio.c              | 217 ++++++++++++++++++++-
 drivers/usb/gadget/function/u_audio.h              |  10 +
 drivers/usb/gadget/function/u_uac2.h               |   2 +
 6 files changed, 345 insertions(+), 4 deletions(-)

-- 
1.9.1