[RFC PATCH 00/16] soundwire/ASoC: speed-up downloads with BTP/BRA protocol

[Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>]

This RFC patchset suggests a new API for ASoC codec drivers to use for
firmware/table downloads.

Problem statement:

All existing transfers initiated by codec drivers rely on SoundWire
read/write commands, which can only support ONE byte per frame. With
the typical 48kHz frame rate, this means 384 kbits/s.

In addition, the command/control is typically handled with a FIFO and
interrupts which adds more software overhead. To give a practical
reference, sending 32Kb takes 2.5s on Intel platforms, which means
about 105kbit/s only. Additional non-audio activity is likely to
adversely impact interrupt scheduling and further decrease the
transfer speeds.

New SDCA-based codecs have a need to download tables and DSP firmware
which are typically between 20 and 256 Kb. The slow bus operation has
a direct impact on boot/resume times, and clearly having to wait more
than 300ms is a showstopper in terms of latency requirements and
user-experience.

Suggested solution:

The MIPI specification and most of the new codecs support the Bulk
Transfer Protocol (BTP) and specifically the Bulk Register Access
(BRA) configuration. This mode reclaims the 'audio' data space of the
SoundWire frame to send firmware/coefficients over the DataPort 0
(DP0).

The API suggested is rather simple, with the following sequence
expected:
open(): reserve resources and prepare hardware
send_async(): trigger DMAs and perform SoundWire bank switch
wait(): wait for DMA completion and disable DMAs
close(): release resources

Benefits:

Even after accounting for the protocol overhead, the data can be sent
8x or 16x faster on the link than with the regular commands.

With the use of DMAs, the software overhead becomes limited to the
initialization. Measured results show that transferring the same 32Kb
takes about 100ms, a 25x improvement on the baseline write() commands
with an actual bitrate of 2.6 Mbits/s. These results are a measure of
bus/hardware performance mainly, and will typically not be too
modified by the CPU activity and scheduling.

The performance for reads is similar, with a 25x speedup measured.

Limitations:

Setting up the transfers over DP0 takes time, and the reliance on DMAs
on the host side brings alignment restrictions. The BTP/BRA protocol
is really only relevant for "large" transfers done during boot/resume
*before* audio transfers take place. Mixing BTP/BRA and audio is a
nightmare, this patchset suggests a mutual-exclusion between two
usages.

Scope:

This patchset only exposes the API and a debugfs interface to initiate
commands, validate results and measure performance. The actual use of
the API is left as an exercise for codec driver developers.

This patchset depends on a number of pre-requisite patches and will
not build on top of any for-next branches. The main intent of this RFC
is to gather comments on the usage, API, benefits and restrictions.

The code and functionality was tested on an Intel LunarLake RVP platform
connected to a Realtek RT711-SDCA device.

Acknowledgements:

Thanks to Zeek Tsai at Realtek for providing test sequences that
helped reconcile the data formatted by the host driver with the
expected results on the code side.

Pierre-Louis Bossart (16):
  Documentation: driver: add SoundWire BRA description
  soundwire: cadence: add BTP support for DP0
  soundwire: stream: extend sdw_alloc_stream() to take 'type' parameter
  soundwire: extend sdw_stream_type to BPT
  soundwire: stream: special-case the bus compute_params() routine
  soundwire: stream: reuse existing code for BPT stream
  soundwire: bus: add API for BPT protocol
  soundwire: bus: add bpt_stream pointer
  soundwire: crc8: add constant table
  soundwire: cadence: add BTP/BRA helpers to format data
  soundwire: intel_auxdevice: add indirection for BPT
    open/close/send_async/wait
  ASoC: SOF: Intel: hda-sdw-bpt: add helpers for SoundWire BPT DMA
  soundwire: intel: add BPT context definition
  soundwire: intel_ace2x: add BPT open/close/send_async/wait
  soundwire: debugfs: add interface for BPT/BRA transfers
  ASoC: rt711-sdca: add DP0 support

 Documentation/driver-api/soundwire/bra.rst    | 478 +++++++++++++
 Documentation/driver-api/soundwire/index.rst  |   1 +
 Documentation/driver-api/soundwire/stream.rst |   2 +-
 .../driver-api/soundwire/summary.rst          |   5 +-
 drivers/soundwire/Kconfig                     |   1 +
 drivers/soundwire/Makefile                    |   4 +-
 drivers/soundwire/amd_manager.c               |   2 +-
 drivers/soundwire/bus.c                       |  77 +++
 drivers/soundwire/bus.h                       |  18 +
 drivers/soundwire/cadence_master.c            | 646 +++++++++++++++++-
 drivers/soundwire/cadence_master.h            |  30 +
 drivers/soundwire/crc8.c                      | 277 ++++++++
 drivers/soundwire/crc8.h                      |  11 +
 drivers/soundwire/debugfs.c                   | 122 +++-
 .../soundwire/generic_bandwidth_allocation.c  |  84 ++-
 drivers/soundwire/intel.h                     |  12 +
 drivers/soundwire/intel_ace2x.c               | 377 ++++++++++
 drivers/soundwire/intel_auxdevice.c           |  55 ++
 drivers/soundwire/qcom.c                      |   2 +-
 drivers/soundwire/stream.c                    | 137 +++-
 include/linux/soundwire/sdw.h                 |  91 ++-
 include/linux/soundwire/sdw_intel.h           |  16 +
 include/sound/hda-sdw-bpt.h                   |  76 +++
 sound/soc/codecs/rt711-sdca-sdw.c             |   8 +
 sound/soc/qcom/sdw.c                          |   2 +-
 sound/soc/sof/intel/Kconfig                   |   8 +-
 sound/soc/sof/intel/Makefile                  |   4 +
 sound/soc/sof/intel/hda-sdw-bpt.c             | 328 +++++++++
 28 files changed, 2810 insertions(+), 64 deletions(-)
 create mode 100644 Documentation/driver-api/soundwire/bra.rst
 create mode 100644 drivers/soundwire/crc8.c
 create mode 100644 drivers/soundwire/crc8.h
 create mode 100644 include/sound/hda-sdw-bpt.h
 create mode 100644 sound/soc/sof/intel/hda-sdw-bpt.c

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2.39.2