[PATCH v3 0/3] USB Audio Gadget refactoring

[Ruslan Bilovol]

I came to this patch series when wanted to do two things:
 - use UAC1 as virtual ALSA sound card on gadget side,
   just like UAC2 is used so it's possible to do rate
   resampling
 - have both playback/capture support in UAC1

Since I wanted to have same behavior for both UAC1/UAC2,
obviously I've got an utility part (u_audio.c) for
virtual ALSA sound card handling like we have
for ethernet(u_ether) or serial(u_serial) functions.
Function-specific parts (f_uac1/f_uac2) became almost 
as storage for class-specific USB descriptors, some
boilerplate for configfs, binding and few USB
config request handling.

Originally in RFC [1] I've posted before, there was
major change to f_uac1 after that it couldn't do
direct play to existing ALSA sound card anymore,
representing audio on gadget side as virtual
ALSA sound card where audio streams are simply
sinked to and sourced from it, so it may break
current usecase for some people (and that's why
it was RFC).

During RFC discussion, it was agreed to not touch
existing f_uac1 implementation and create new one
instead. This patchset (v2) introduced new function
named f_uac1_newapi and doesn't touch current f_uac1
implementation, so people still can use old behavior

Now, it's possible to use existing user-space
applications for audio routing between Audio Gadget
and real sound card. I personally use alsaloop tool
from alsautils and have ability to create PCM
loopback between two different ALSA cards using
rate resampling, which was not possible with previous
"direct play to ALSA card" approach in f_uac1. 

While here, also dropped redundant platform
driver/device creation in f_uac2 driver (as well as
didn't add "never implemented" volume/mute functionality
in f_uac1 to f_uac1_newapi) that made this work even
easier to do.

This series is tested with both legacy g_audio.ko and
modern configfs approaches under Ubuntu 14.04 (UAC1 and
UAC2) and under Windows7 x64 (UAC1 only) having
perfect results in all cases.

Comments, testing are welcome.

v3 changes:
 - renamed u_audio exported symbols so they don't
   conflict with old f_uac1 if both are built-in.

v2 changes:
 - do not touch f_uac1, instead created f_uac1_newapi
 - added documentation for f_uac1_newapi
 - rebased on top of v4.8-rc1

[1] https://lkml.org/lkml/2016/5/23/649

Ruslan Bilovol (3):
  usb: gadget: f_uac2: remove platform driver/device creation
  usb: gadget: f_uac2: split out audio core
  usb: gadget: add f_uac1 variant based on new u_audio api

 .../ABI/testing/configfs-usb-gadget-uac1_newapi    |  12 +
 Documentation/usb/gadget-testing.txt               |  41 ++
 drivers/usb/gadget/Kconfig                         |  25 +
 drivers/usb/gadget/function/Makefile               |   3 +
 drivers/usb/gadget/function/f_uac1_newapi.c        | 795 +++++++++++++++++++++
 drivers/usb/gadget/function/f_uac2.c               | 778 +++-----------------
 drivers/usb/gadget/function/u_audio.c              | 632 ++++++++++++++++
 drivers/usb/gadget/function/u_audio.h              |  93 +++
 drivers/usb/gadget/function/u_uac1_newapi.h        |  39 +
 drivers/usb/gadget/legacy/Kconfig                  |  14 +-
 drivers/usb/gadget/legacy/audio.c                  |  52 ++
 11 files changed, 1801 insertions(+), 683 deletions(-)
 create mode 100644 Documentation/ABI/testing/configfs-usb-gadget-uac1_newapi
 create mode 100644 drivers/usb/gadget/function/f_uac1_newapi.c
 create mode 100644 drivers/usb/gadget/function/u_audio.c
 create mode 100644 drivers/usb/gadget/function/u_audio.h
 create mode 100644 drivers/usb/gadget/function/u_uac1_newapi.h

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1.9.1