Hi Chengkai and Vela team,
First, thank you for writing this proposal. I think this is a good start:
the community is now discussing the Bluetooth host stack direction
seriously, and that is very helpful for NuttX.
I agree that we should converge on one Zephyr-based Bluetooth host instead
of maintaining two unrelated solutions. However, I have a different view on
the long-term implementation model.
My current opinion is that once we import the Zephyr Bluetooth host into
NuttX, we should treat it more like a forked NuttX Bluetooth stack, not as
a subtree that keeps rebasing/syncing with Zephyr forever.
The reason is that zblue is deeply coupled with the Zephyr kernel model. If
we want to keep it continuously synchronized with Zephyr, then in practice
we also need to keep a large Zephyr compatibility layer alive. For an
official NuttX solution, I do not think it is ideal to introduce and
maintain a large part of the Zephyr kernel abstraction just for one
protocol stack.
Some comments on the questions:
1. About putting a userspace Zephyr host in nuttx-apps and fetching it
from an external repository while keeping it synchronized with Zephyr:
I still have concerns about this model.
First, zblue is not really an independent userspace library. It was
designed to run inside Zephyr and depends heavily on Zephyr kernel
concepts. Even if it is placed in apps, the porting layer still needs many
NuttX kernel-style primitives, such as nxsem, nxmutex, work queues, ticks,
timers, and internal scheduling behavior. So I do not think simply placing
it in nuttx-apps makes it a clean application-level library.
Second, this does not match the original direction that Greg wanted for
the NuttX Bluetooth stack. My understanding is that the goal was a
Unix-like Bluetooth architecture, where data communication goes through
network/socket APIs. A pure userspace Zephyr-style host over HCI does not
really provide that architecture.
Third, I think keeping full long-term synchronization with Zephyr will
be very difficult unless we preserve too much of Zephyr's own kernel model.
Once we adapt the stack seriously to NuttX APIs, networking, memory
management, driver model, and build modes, it will naturally diverge. So I
prefer to accept that it becomes a NuttX-maintained Bluetooth stack based
on Zephyr, rather than trying to keep it as an almost-unmodified Zephyr
module.
2. About where the external repository should live:
My preference is not to maintain it as a separate external repository in
the long term. I think it should be integrated directly into either
nuttx.git or nuttx-apps.git, depending on the final layering decision. This
makes review, CI, release management, and maintenance clearer for the NuttX
community.
3. About coexistence with PR #20686:
I am very happy to drop my current PR if the community agrees on a
better common direction. My original goal is not to push my own
implementation, but to help NuttX get a complete Bluetooth stack and
improve the kernel-side networking architecture.
If the openvela roadmap is aligned with that goal, especially if it can
eventually support a Unix-like Bluetooth model for NuttX, I would be very
happy to work with you and use your solution as the base.
4. About Kconfig naming:
I discussed this with Alan before. The current compromise is to use a
global switch like CONFIG_ZBLUE_ENABLED, while keeping the internal
Kconfig options close to Zephyr style, such as CONFIG_BT_*. I think this
is friendly to both Zephyr and Linux Bluetooth developers, while still
making the top-level NuttX feature explicit.
5. About the long-term split:
My preference is:
- For protected/kernel builds, move toward a BlueZ/Linux-style split:
kernel-side HCI/core/L2CAP/socket integration, with profiles and tools in
userspace.
- For flat builds, allow a more direct HCI/btdev callback path for
better performance on low-end IoT chips.
In other words, I do not think we need one single boundary for all build
modes. NuttX supports very different deployment models, so the Bluetooth
architecture may also need to support different integration paths.
Again, thanks for proposing this. I think the most important thing is to
reach a common direction first. I am open to working together and adjusting
my PR if we can agree on a roadmap that gives NuttX a complete Bluetooth
stack while also moving toward the right long-term architecture.
BRs,
成凯 via dev <[email protected]> 于2026年10月28日周三 20:46写道：
> Hi all,
>
> Apologies for the garbled characters in my previous mail; my mail
> client
> used a non-UTF-8 encoding. The "??5" / "��5" references mean "Section
> 5"
> (Optional path to a BlueZ/Linux-style split), "Section 7" is the Plan,
> and
> the dashes in the phase titles are plain hyphens. The content is
> otherwise
> unchanged.
>
> Please also ignore the confidentiality footer added by our mail
> gateway;
> this proposal is intended for public discussion.
>
> Best regards,
> Kai Cheng
>
>
>
> ________________________________
> From: 成凯 via dev <[email protected]>
> Sent: Wednesday, October 28, 2026 5:54 PM
> To: [email protected]
> Cc: 黄玉龙; 高子豪; 梁超众; 李剑; 陈建; 成凯
> Subject: [External Mail][DISCUSS] Proposal: integrate a Zephyr-based
> Bluetooth host (openvela zblue) as a userspace stack in nuttx-apps, kept in
> sync with upstream Zephyr
>
> [外部邮件] 此邮件来源于小米公司外部，请谨慎处理。若对邮件安全性存疑，请将邮件转发给[email protected]进行反馈
>
> Hi all,
>
> Following the discussion on PR #20686 [1] and the thread "PR #20686 —
> Introducing
> a Zephyr-based Bluetooth host stack under net/bt" [2], I would like to
> propose an
> alternative integration model for a full-featured (BLE + BR/EDR Classic)
> Bluetooth host on NuttX, and ask for the community's opinion before we
> start
> sending patches.
>
> First, thanks to Archer (anchao) for pushing this topic forward. Everyone
> in the
> thread seems to agree that NuttX needs a more complete Bluetooth host. The
> open
> question is where it should live and how it should be maintained. This
> proposal
> only covers that question.
>
>
> ------------------------------------------------------------------------------
> 1. Summary
>
> ------------------------------------------------------------------------------
>
> - Integrate the Zephyr Bluetooth host as a userspace library in nuttx-apps,
> under wireless/bluetooth/, the same way NimBLE is integrated today [3].
> - The stack source lives in a standalone repository and is fetched at a
> pinned
> revision at build time. Zephyr's subsys/bluetooth is kept unmodified; all
> NuttX adaptation goes into a separate, small port layer.
> - At the start, the NuttX kernel only provides the HCI transport (the
> existing drivers/wireless/bluetooth bt_driver_s drivers, exposed to
> userspace through the H4 character device or the HCI user-channel
> socket).
> If the community prefers a BlueZ/Linux-style split, the L2/L3 layers can
> be
> moved into the kernel step by step from this base (see §5).
> - The stack is an optional choice selected by Kconfig, next to the native
> wireless/bluetooth stack and NimBLE. It does not replace them, and it is
> not
> presented as the default NuttX Bluetooth stack.
> - Fixes go to upstream Zephyr first and come back to NuttX through regular
> rebases.
>
> If the community agrees with this direction, we will upstream our existing,
> production-proven stack step by step (stack integration first, then tests,
> then the optional Bluetooth service layer).
>
>
> ------------------------------------------------------------------------------
> 2. What we already have
>
> ------------------------------------------------------------------------------
>
> The openvela team at Xiaomi has run a Zephyr-based Bluetooth host on
> NuttX/openvela in production for some time:
>
> - Baseline: Zephyr v4.0 (upstream commit 2efc8598e39, 2024-11), plus later
> upstream fixes and our own work.
> - Features: LE GAP/GATT/ATT/SMP, and BR/EDR Classic with A2DP (SRC/SNK),
> AVRCP (CT/TG), HFP (AG/HF), SPP/RFCOMM, SDP, HID Device/Host and
> GATT over BR/EDR. LE Audio and Mesh come from upstream.
> - Products: shipping in smart speakers (since 2025), smart panels, and
> wearables (watches and bands).
> - Qualification: Bluetooth SIG qualification against Core 6.2 (Declaration
> ID
> Q386483).
> - Qualification testing: the stack plugs directly into AutoPTS through the
> Zephyr BTP tester. A full dual-mode qualification test run takes about
> 2 person-days.
> - Simulation: conformance tests can run in simulation on BSWorld or a
> software RF controller, without real hardware or phones. This is key to
> keeping the stack's quality up as it evolves.
> - Performance: the host runs in userspace in a flat build and reaches about
> 210 kB/s SPP throughput on a wearable SoC. Running outside the kernel
> has not
> been a bottleneck for us.
> - Upstream work: our team is one of the major contributors to Classic
> Bluetooth in the Zephyr community.
> - Public code: https://github.com/open-vela/external_zblue (stack) and
> https://github.com/open-vela/frameworks_bluetooth (service layer). These
> repositories contain our 2025 changes. The 2026 changes are still in our
> internal tree and will be released and upstreamed.
>
>
> ------------------------------------------------------------------------------
> 3. Points raised in the PR #20686 discussion
>
> ------------------------------------------------------------------------------
>
> My reading of the review comments and the mailing-list thread:
>
> a) Profiles, shell commands and other application-level code do not belong
> in
> kernel space (Xiang Xiao, raiden00pl).
> b) A monolithic third-party host stack should live in nuttx-apps or a
> separate
> git, integrated by download like NimBLE (Xiang Xiao).
> c) A stack of this size is a large attack surface. Keeping it out of the
> kernel
> protects at least protected and kernel builds (Alan Carvalho de Assis,
> casaroli).
> d) It should be one selectable option, not the implicit "official" NuttX
> Bluetooth stack (Alan Carvalho de Assis).
> e) Tests are required, e.g. sim-based or BSWorld (LingaoM, cederom).
> f) Copyright and license headers of the imported code must be handled
> correctly (jerpelea, acassis, Xiang Xiao).
> g) Unfinished work should not land on master (raiden00pl, Xiang Xiao).
>
> The proposal below tries to meet all of these.
>
>
> ------------------------------------------------------------------------------
> 4. Proposed architecture
>
> ------------------------------------------------------------------------------
>
> +----------------------------------------------------------+
> | Applications / btshell / (optional) Bluetooth service | userspace
> +----------------------------------------------------------+
> | Zephyr Bluetooth host (unmodified subsys/bluetooth) | userspace
> | GAP/GATT/SMP/L2CAP/RFCOMM/SDP/A2DP/AVRCP/HFP/HID/... |
> (nuttx-apps)
> +----------------------------------------------------------+
> | Port layer: k_thread/k_sem/k_work/k_timer, net_buf, | userspace
> | settings backend, logging, iterable sections, HCI glue |
> +=========================== HCI =========================+
> | HCI transport: /dev/ttyHCIx (H4) or HCI user channel | kernel
> | bt_uart / bt_rpmsghci / bt_slip / sim HCI socket ... |
> (existing)
> +----------------------------------------------------------+
> | Controller |
> +----------------------------------------------------------+
>
> Design rules:
>
> 1. Upstream code stays unmodified. Zephyr's subsys/bluetooth is not
> reformatted
> or edited in place. Like NimBLE, it is third-party code and not subject
> to
> nxstyle. Any change the stack needs is sent to Zephyr first. (If parts
> are
> later moved into the kernel as described in §5, those parts are
> maintained
> in NuttX; the rest keeps this rule.)
> 2. NuttX adaptation stays in the port layer. Zephyr kernel APIs are
> implemented
> on top of NuttX. Upstream Bluetooth code keeps calling the Zephyr API,
> so
> upstream patches still apply.
> 3. The initial kernel/user boundary is HCI. The only kernel component is
> the
> HCI transport, which NuttX already has. The host stack uses public
> interfaces only (open/read/write/poll or the HCI socket).
> 4. Kconfig is namespaced. One top-level option (e.g. CONFIG_ZBLUE, like
> CONFIG_NIMBLE) selects the stack. The Zephyr CONFIG_BT_* feature options
> live under it inside nuttx-apps. If a kernel-side stack also uses
> CONFIG_BT_*, the two must be made mutually exclusive or renamed; we are
> happy to work this out with Archer.
> 5. Build modes: flat build first. Because only public interfaces are used,
> we
> plan protected and kernel build support as a follow-up phase (see §7).
>
> Why we start with the split at HCI:
>
> The Zephyr host is not layered like Linux. L2CAP is tightly coupled with
> conn.c, ATT, SMP and the buffer and work-queue model. Splitting at HCI
> keeps
> the upstream code untouched and gives NuttX a complete, working stack from
> day one. It is the same model Android uses (a userspace host on top of an
> HCI
> transport) and the model NimBLE already uses on NuttX. Moving L2CAP and
> below
> into the kernel (the BlueZ model) is possible, but it means restructuring
> those layers, so we propose it as a separate, step-by-step path (§5).
>
>
>
> ------------------------------------------------------------------------------
> 5. Optional path to a BlueZ/Linux-style split
>
> ------------------------------------------------------------------------------
>
> Some community members prefer the Linux/BlueZ layering, where the lower
> protocol layers (HCI core, connection management, L2CAP, and possibly SMP)
> live in the kernel and are exposed through AF_BLUETOOTH sockets, while
> profiles, services and tools stay in userspace. This proposal is compatible
> with that goal, and our stack can serve as the base for it.
>
> The difference from PR #20686 is the direction of migration:
>
> - PR #20686 puts the whole host into the kernel first and plans to move
> parts back to userspace later.
> - We start with a complete, product-proven host in userspace, and move
> the
> L2/L3 layers into the kernel step by step, only where the community
> agrees.
>
> Every intermediate state is a working stack, every step can be reviewed and
> tested on its own, and the code that has not moved yet keeps tracking
> upstream Zephyr.
>
> Possible steps, each a separate patch series:
>
> 1. Define the kernel/user interface first: AF_BLUETOOTH sockets for L2CAP
> (BTPROTO_L2CAP, struct sockaddr_l2), building on the existing NuttX
> definitions in include/netpacket/bluetooth.h, plus an HCI management
> interface.
> 2. Move the HCI core and connection management into the kernel, next to
> the existing HCI drivers.
> 3. Move L2CAP (LE and BR/EDR) into the kernel behind the socket
> interface.
> 4. Decide case by case for SMP, ATT/GATT and RFCOMM, based on review and
> measurements.
>
> What stays in nuttx-apps in all cases: profiles (A2DP, AVRCP, HFP, HID,
> ...), GATT services, the Bluetooth service layer, shell and tools.
>
> Trade-off: code moved into the kernel will diverge from upstream Zephyr and
> must be maintained in NuttX. Moving it layer by layer, and keeping the
> Zephyr API at the boundary where possible, limits that cost and lets the
> community decide how far to go.
>
>
> ------------------------------------------------------------------------------
> 6. Comparison with PR #20686
>
> ------------------------------------------------------------------------------
>
> PR #20686 (current) This proposal
> Location nuttx (net/bt, now libs/) nuttx-apps +
> external repo
> Upstream code reformatted, Zephyr APIs unmodified; port
> layer only
> replaced in place
> Zephyr sync close to a re-port per rebase onto new
> Zephyr
> release (author describes releases
> it as a fork)
> Security fixes backported by hand picked from upstream
> Kernel attack surface whole host in kernel HCI transport only
> Shell / profiles inside kernel tree userspace
> Build modes flat only flat first, then
> protected/kernel
> License headers need rewriting in tree upstream files keep
> their
> headers, not in
> nuttx.git
> Evolution path whole host in kernel first, working host in
> userspace
> then move parts out first, then move
> L2/L3
> into kernel if
> agreed
>
> Notes:
>
> - Sync cost: the PR reformats the code to nxstyle and replaces Zephyr
> kernel
> APIs inside protocol files. A whitespace-insensitive diff against
> Zephyr v4.5.0-rc1 still differs in roughly 19% of lines, so upstream
> patches
> no longer apply directly. Archer also wrote on the PR that he does not
> expect
> to keep synchronizing with Zephyr ("more like forking"). For a code base
> this
> size, with a steady stream of upstream CVE fixes (Alan listed more than
> 70
> since 2019), we think staying close to upstream is the main long-term
> value.
> - Performance: as Xiang pointed out, in a flat build a userspace stack has
> the
> same direct-call cost as an in-kernel one. The remaining per-packet
> overhead
> is the HCI transport (VFS + one copy). Our static estimate on a wearable
> SoC
> is a few KB of RAM and negligible CPU. Very constrained parts (for
> example
> the 80 MHz M0 / 64 KB case Archer described) can get an optional
> flat-build-only direct HCI binding later, without moving the host into
> the
> kernel.
>
>
> ------------------------------------------------------------------------------
> 7. Plan
>
> ------------------------------------------------------------------------------
>
> Phase 0 – Consensus (this thread)
> Agree on location, naming, Kconfig namespace and coexistence with PR
> #20686.
>
> Phase 1 – Stack integration (nuttx-apps)
> - wireless/bluetooth/zblue: Makefile/CMake download at a pinned ref,
> Kconfig,
> port layer.
> - Uses the existing HCI transports; no kernel changes expected (only
> small,
> separate fixes if any).
> - btshell as an NSH builtin (the Zephyr shell, Classic and LE commands).
> - sim configuration with a host HCI dongle, built in CI.
>
> Phase 2 – Tests
> - sim-based tests for LE and Classic in CI (BSWorld integration).
> - AutoPTS support through the Zephyr BTP tester.
> - Samples: LE peripheral/central, A2DP sink, HFP HF, SPP.
>
> Phase 3 – Protected / kernel build
> - Port layer audited to use only public NuttX/POSIX interfaces.
> - Validated on a protected and a kernel build target.
>
> Phase 4 – Bluetooth service layer (optional)
> - Upstream the openvela Bluetooth service ("btservice"): a stable C API
> for
> applications, multi-client IPC, and a stack abstraction layer, so
> applications do not depend on the host stack API.
>
> Phase 5 – Boards
> - Board configurations with real controllers (UART / RPMsg HCI).
>
> Phase 6 – L2/L3 migration to the kernel (optional, if agreed)
> - Follow the steps in §5, one layer per patch series.
>
> Ongoing
> - Track Zephyr releases and send fixes upstream first.
> - We will provide maintainers for the integration.
>
> The main goal of this mail is to collect feedback on the direction. We will
> follow up with a detailed effort estimate and schedule for each phase once
> the
> direction is agreed.
>
>
> ------------------------------------------------------------------------------
> 8. Licensing
>
> ------------------------------------------------------------------------------
>
> - Zephyr-derived files stay in the external repository with their original
> Apache-2.0 headers and SPDX tags. They are not copied into apache/nuttx
> or
> apache/nuttx-apps.
> - New glue code in nuttx-apps uses the standard ASF header and is
> contributed
> under ICLA/CCLA.
>
>
> ------------------------------------------------------------------------------
> 9. Questions for the community
>
> ------------------------------------------------------------------------------
>
> 1. Is a userspace Zephyr-based host in nuttx-apps, fetched from an external
> repository and kept in sync with Zephyr, acceptable as the integration
> model?
> 2. Where should the external repository live (open-vela GitHub
> organization,
> or another location the PMC prefers)?
> 3. How should this coexist with PR #20686? We would rather converge on one
> Zephyr-based host than ship two. We are glad to work with Archer on a
> common approach.
> 4. Any preference on the top-level Kconfig name (CONFIG_ZBLUE or other)?
> 5. Should the long-term target stay at the HCI split, or move towards the
> BlueZ/Linux-style split described in §5?
>
> Feedback is very welcome.
>
> [1] https://github.com/apache/nuttx/pull/20686
> [2] https://www.mail-archive.com/[email protected]/msg15328.html
> [3]
> https://github.com/apache/nuttx-apps/tree/master/wireless/bluetooth/nimble
>
> Best regards,
>
> Kai Cheng
>
>
> #/******本邮件及其附件含有小米公司的保密信息，仅限于发送给上面地址中列出的个人或群组。禁止任何其他人以任何形式使用（包括但不限于全部或部分地泄露、复制、或散发）本邮件中的信息。如果您错收了本邮件，请您立即电话或邮件通知发件人并删除本邮件！
> This e-mail and its attachments contain confidential information from
> XIAOMI, which is intended only for the person or entity whose address is
> listed above. Any use of the information contained herein in any way
> (including, but not limited to, total or partial disclosure, reproduction,
> or dissemination) by persons other than the intended recipient(s) is
> prohibited. If you receive this e-mail in error, please notify the sender
> by phone or email immediately and delete it!******/#
> #/******本邮件及其附件含有小米公司的保密信息，仅限于发送给上面地址中列出的个人或群组。禁止任何其他人以任何形式使用（包括但不限于全部或部分地泄露、复制、或散发）本邮件中的信息。如果您错收了本邮件，请您立即电话或邮件通知发件人并删除本邮件！
> This e-mail and its attachments contain confidential information from
> XIAOMI, which is intended only for the person or entity whose address is
> listed above. Any use of the information contained herein in any way
> (including, but not limited to, total or partial disclosure, reproduction,
> or dissemination) by persons other than the intended recipient(s) is
> prohibited. If you receive this e-mail in error, please notify the sender
> by phone or email immediately and delete it!******/#
>
