pw_bluetooth_sapphire#

Battle-tested Bluetooth with rock-solid reliability

Unstable C++17 Code Size Impact: 1.5 to 2 mB

Attention

This module is still under construction. There is no public API yet. Please contact the Pigweed team if you are interested in using this module.

The pw_bluetooth_sapphire module provides a dual-mode Bluetooth Host stack that will implement the pw_bluetooth APIs. Sapphire originated as the Fuchsia operating system’s Bluetooth stack and is used in production. The Sapphire Host stack has been migrated into the Pigweed project for use in embedded projects. However, as it was not written in an embedded context, there is a lot of work to be done to optimize memory usage.

Why use Sapphire?

  • Open source, unlike vendor Bluetooth stacks. This makes debugging and fixing issues much easier!

  • Integrated with Pigweed. Logs, asserts, randomness, time, async, strings, and more are all using Pigweed modules.

  • Excellent test coverage, unlike most Bluetooth stacks. This means fewer bugs and greater maintainability.

  • Certified. Sapphire was certified by the Bluetooth SIG after passing all conformance tests.

  • A great API. Coming 2025. See pw_bluetooth.

Architecture#

Sapphire is composed of multiple libraries that implement different protocols in the Core specification. These libraries include:

HCI: Host-Controller Interface

Processes data (ACL), command, and event packets exchanged between the host and the controller. Note that HCI is currently split into hci, transport, and hci-spec libraries.

L2CAP: Logical Link Control and Adaptation Layer Protocol

Multiplexes a single ACL connection into multiple channels and manages channel configuration and flow control.

SDP: Service Discovery Protocol

Serves the BR/EDR service database and searches peers for BR/EDR services.

SM: Security Manager

Implements LE security, including pairing.

ATT: Attribute Protocol

Implements the ATT protocol PDUs, flow control, and database.

GATT: Generic Attribute Profile

Implements the GATT server and client.

GAP: Generic Access Profile

Responsible for device discovery and connection functionality.

SCO: Synchronous Connection Oriented link

Used for audio data by the Hands-Free Profile.

ISO: Isochronous Channels

Used for audio data in LE Audio.

These libraries build on top of each other as shown in the following diagram. HCI is the foundation, L2CAP builds on top of HCI, ATT builds on top of L2CAP, etc.

flowchart TB subgraph HCI["HCI"] TRA["Transport"] HCI2["HCI"] SPEC["HCI-Spec"] CONT["Controllers"] end subgraph Host["Host"] COM["Common"] GATT["GATT"] ISO["ISO"] SCO["SCO"] SM["SM"] L2CAP["L2CAP"] SDP["SDP"] GAP["GAP"] ATT["ATT"] HCI end GAP --> GATT & ISO & SCO & SM & L2CAP & SDP & HCI2 GATT --> ATT ATT --> L2CAP L2CAP --> TRA SCO --> HCI2 & TRA SM --> L2CAP SDP --> L2CAP ISO --> TRA HCI2 --> TRA TRA --> CONT CONT -- UART/USB --> Controller["Controller"]

Certification#

The Sapphire Host stack was certified as implementing the Bluetooth Core Specification 5.0 by the Bluetooth SIG in 2020 as “Google Sapphire 1.0 Bluetooth Core Host Solution”. You can find its public listing and add it to your product certification at the Qualification Workspace.

Products running Sapphire#

Sapphire is running on millions of the following devices:

  • Google Nest Hub (1st Gen)

  • Google Nest Hub (2nd Gen)

  • Google Nest Hub Max

Supported features#

GAP#

Feature

Supported?

Notes

BR/EDR/LE Device Configuration

✅

BR/EDR Device Configuration

❌

Tracking bug

LE Device Configuration

❌

Tracking bug

Secure Simple Pairing

✅

Legacy Pairing

✅

LE Central Role

✅

LE Peripheral Role

✅

LE Advertising

✅

LE Active Scanning

✅

LE Passive Scanning

✅

LE General Connection Establishment

✅

LE Auto Connection Establishment

✅

BR/EDR Inquiry

✅

BR/EDR Connection Establishment

✅

L2CAP#

Feature

Supported?

Notes

Fragmentation/Recombination

✅

Segmentation/Reassembly

❌

Tracking bug Clients must respect the peer’s MTU size, and Sapphire supports receiving the max SDU size.

BR/EDR Dynamic Channels

✅

LE Connection-Oriented Channels (CoC)

✅

Basic Mode (ERTM)

✅

Enhanced Retransmission Mode (ERTM)

✅

LE Credit-Based Flow Control Mode

✅

Retransmission Mode

❌

Tracking bug

Flow Control Mode

❌

Tracking bug

Streaming Mode

❌

Tracking bug

ATT#

Feature

Supported?

Notes

ATT over LE

✅

ATT over BR/EDR

❌

Tracking bug

Enhanced Attribute Protocol (EATT)

🚧

Tracking bug Most of the requirements are implemented. A few new PDUs are missing and there is no API yet.

Contributing#

Running tests#

Run all tests:

bazelisk test //pw_bluetooth_sapphire/host/... \
  --platforms=//pw_unit_test:googletest_platform \
  --@pigweed//pw_unit_test:backend=@pigweed//pw_unit_test:googletest

Run l2cap tests with a test filter, logs, and log level filter:

bazelisk test //pw_bluetooth_sapphire/host/l2cap:l2cap_test \
  --platforms=//pw_unit_test:googletest_platform \
  --@pigweed//pw_unit_test:backend=@pigweed//pw_unit_test:googletest \
  --test_arg=--gtest_filter="*InboundChannelFailure" \
  --test_output=all \
  --copt=-DPW_LOG_LEVEL_DEFAULT=PW_LOG_LEVEL_ERROR

The easiest way to run GN tests is with pw presubmit, which will install dependencies and set GN args correctly.

$ pw presubmit --step gn_chre_googletest_nanopb_sapphire_build

You can also use pw watch if you install all the packages and set your GN args to match the GN presubmit step.

Roadmap#

  • Support CMake

  • Implement pw_bluetooth APIs

  • Optimize memory footprint

  • Add snoop log capture support

  • Add metrics

  • Add configuration options (LE only, Classic only, etc.)

  • Add CLI for controlling stack over RPC

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