Re: [RFC PATCH v4 00/38] NTB transport backed by PCI EP embedded DMA

On 1/18/26 6:54 AM, Koichiro Den wrote:
> Hi,
> 
> This is RFC v4 of the NTB/PCI/dmaengine series that introduces an
> optional NTB transport variant where payload data is moved by a PCI
> embedded-DMA engine (eDMA) residing on the endpoint side.

Just a fly by comment. This series is huge. I do suggest break it down to something more manageable to prevent review fatigue from patch reviewers. For example, linux network sub-system has a rule to restrict patch series to no more than 15 patches. NTB sub-system does not have that rule. But maybe split out the dmaengine changes and the hardware specific dw-edma bits from the ntb core changes.

DJ
 
> 
> The primary target is Synopsys DesignWare PCIe endpoint controllers that
> integrate a DesignWare eDMA instance (dw-edma). In the remote
> embedded-DMA mode, payload is transferred by DMA directly between the
> two systems' memory, and NTB Memory Windows are used primarily for
> control/metadata and for exposing the endpoint eDMA resources (register
> window + linked-list rings) to the host.
> 
> Compared to the existing cpu/dma memcpy-based implementation, this
> approach avoids window-backed payload rings and the associated extra
> copies, and it is less sensitive to scarce MW space. This also enables
> scaling out to multiple queue pairs, which is particularly beneficial
> for ntb_netdev. On R-Car S4, preliminary iperf3 results show 10~20x
> throughput improvement. Latency improvements are also observed.
> 
> RFC history:
>   RFC v3: https://lore.kernel.org/all/[email protected]/
>   RFC v2: https://lore.kernel.org/all/[email protected]/
>   RFC v1: https://lore.kernel.org/all/[email protected]/
> 
> Parts of RFC v3 series have already been split out and posted separately
> (see "Kernel base / dependencies" section below). However, feedback on
> the remaining parts led to substantial restructuring and code changes,
> so I am sending an RFC v4 as a refreshed version of the full series.
> 
> RFC v4 is still a large, cross-subsystem series. At this RFC stage,
> I am sending the full picture in a single set to make it easier to
> review the overall direction and architecture. Once the direction is
> agreed upon and no further large restructuring appears necessary, I will stop
> posting the new RFC-tagged revisions and continue development on
> separate threads, split by sub-topic.
> 
> Many thanks for all the reviews and feedback from multiple perspectives.
> 
> 
> Software architecture overview (RFC v4)
> =======================================
> 
> A major change in RFC v4 is the software layering and module split.
> 
> The existing memcpy-based transport and the new remote embedded-DMA
> transport are implemented as two independent NTB client drivers on top
> of a shared core library:
> 
>                        +--------------------+
>                        | ntb_transport_core |
>                        +--------------------+
>                            ^            ^
>                            |            |
>         ntb_transport -----+            +----- ntb_transport_edma
>        (cpu/dma memcpy)                   (remote embedded DMA transfer)
>                                                        |
>                                                        v
>                                                  +-----------+
>                                                  |  ntb_edma |
>                                                  +-----------+
>                                                        ^
>                                                        |
>                                                +----------------+
>                                                |                |
>                                           ntb_dw_edma         [...]
> 
> Key points:
>   * ntb_transport_core provides the queue-pair abstraction used by upper
>     layer clients (e.g. ntb_netdev).
>   * ntb_transport is the legacy shared-memory transport client (CPU/DMA
>     memcpy).
>   * ntb_transport_edma is the remote embedded-DMA transport client.
>   * ntb_transport_edma relies on an ntb_edma backend registry.
>     This RFC provides an initial DesignWare backend (ntb_dw_edma).
>   * Transport selection is per-NTB device via the standard
>     driver_override mechanism. To enable that, this RFC adds
>     driver_override support to ntb_bus. This allows mixing transports
>     across multiple NTB ports and provides an explicit fallback path to
>     the legacy transport.
> 
> So, if ntb_transport / ntb_transport_edma are built as loadable modules,
> you can just run modprobe ntb_transport as before and the original cpu/dma
> memcpy-based implementation will be active. If they are built-in, whether
> ntb_transport or ntb_transport_edma are bound by default depends on
> initcall order. Regarding how to switch the driver, please see Patch 34
> ("Documentation: driver-api: ntb: Document remote embedded-DMA transport")
> for details.
> 
> 
> Data flow overview (remote embedded-DMA transport)
> ==================================================
> 
> At a high level:
>   * One MW is reserved as an "eDMA window". The endpoint exposes the
>     eDMA register block plus LL descriptor rings through that window, so
>     the peer can ioremap it and drive DMA reads remotely.
>   * Remaining MWs carry only small control-plane rings used to exchange
>     buffer addresses and completion information.
>   * For RC->EP traffic, the RC drives endpoint DMA read channels through
>     the peer-visible eDMA window.
>   * For EP->RC traffic, the endpoint uses its local DMA write channels.
> 
> The following figures illustrate the data flow when ntb_netdev sits on
> top of the transport:
> 
>      Figure 1. RC->EP traffic via ntb_netdev + ntb_transport_edma
>                    backed by ntb_edma/ntb_dw_edma
> 
>              EP                                   RC
>           phys addr                            phys addr
>             space                                space
>              +-+                                  +-+
>              | |                                  | |
>              | |                ||                | |
>              +-+-----.          ||                | |
>     EDMA REG | |      \     [A] ||                | |
>              +-+----.  '---+-+  ||                | |
>              | |     \     | |<---------[0-a]----------
>              +-+-----------| |<----------[2]----------.
>      EDMA LL | |           | |  ||                | | :
>              | |           | |  ||                | | :
>              +-+-----------+-+  ||  [B]           | | :
>              | |                ||  ++            | | :
>           ---------[0-b]----------->||----------------'
>              | |            ++  ||  ||            | |
>              | |            ||  ||  ++            | |
>              | |            ||<----------[4]-----------
>              | |            ++  ||                | |
>              | |           [C]  ||                | |
>           .--|#|<------------------------[3]------|#|<-.
>           :  |#|                ||                |#|  :
>          [5] | |                ||                | | [1]
>           :  | |                ||                | |  :
>           '->|#|                                  |#|--'
>              |#|                                  |#|
>              | |                                  | |
> 
>      Figure 2. EP->RC traffic via ntb_netdev + ntb_transport_edma
>                   backed by ntb_edma/ntb_dw_edma
> 
>              EP                                   RC
>           phys addr                            phys addr
>             space                                space
>              +-+                                  +-+
>              | |                                  | |
>              | |                ||                | |
>              +-+                ||                | |
>     EDMA REG | |                ||                | |
>              +-+                ||                | |
>     ^        | |                ||                | |
>     :        +-+                ||                | |
>     : EDMA LL| |                ||                | |
>     :        | |                ||                | |
>     :        +-+                ||  [C]           | |
>     :        | |                ||  ++            | |
>     :     -----------[4]----------->||            | |
>     :        | |            ++  ||  ||            | |
>     :        | |            ||  ||  ++            | |
>     '----------------[2]-----||<--------[0-b]-----------
>              | |            ++  ||                | |
>              | |           [B]  ||                | |
>           .->|#|--------[3]---------------------->|#|--.
>           :  |#|                ||                |#|  :
>          [1] | |                ||                | | [5]
>           :  | |                ||                | |  :
>           '--|#|                                  |#|<-'
>              |#|                                  |#|
>              | |                                  | |
> 
>     0-a. configure remote embedded DMA (program endpoint DMA registers)
>     0-b. DMA-map and publish destination address (DAR)
>     1.   network stack builds skb (copy from application/user memory)
>     2.   consume DAR, DMA-map source address (SAR) and kick DMA transfer
>     3.   DMA transfer (payload moves between RC/EP memory)
>     4.   consume completion (commit)
>     5.   network stack delivers data to application/user memory
> 
>     [A]: Dedicated MW that aggregates DMA regs and LL (peer ioremaps it)
>     [B]: Control-plane ring buffer for "produce"
>     [C]: Control-plane ring buffer for "consume"
> 
> 
> Kernel base / dependencies
> ==========================
> 
> This series is based on:
> 
>   - next-20260114 (commit b775e489bec7)
> 
> plus the following seven unmerged patch series or standalone patches:
> 
>   - [PATCH v4 0/7] PCI: endpoint/NTB: Harden vNTB resource management
>     https://lore.kernel.org/all/[email protected]/
> 
>   - [PATCH v2 0/2] NTB: ntb_transport: debugfs cleanups
>     https://lore.kernel.org/all/[email protected]/
> 
>   - [PATCH v3 0/9] dmaengine: Add new API to combine configuration and descriptor preparation
>     https://lore.kernel.org/all/[email protected]/
> 
>   - [PATCH v8 0/5] PCI: endpoint: BAR subrange mapping support
>     https://lore.kernel.org/all/[email protected]/
> 
>   - [PATCH] PCI: endpoint: pci-epf-vntb: Use array_index_nospec() on mws_size[] access
>     https://lore.kernel.org/all/[email protected]/
> 
>   - [PATCH] dmaengine: dw-edma: Fix MSI data values for multi-vector IMWr interrupts
>     https://lore.kernel.org/all/[email protected]/
> 
>   - [PATCH v2 01/11] dmaengine: dw-edma: Add spinlock to protect DONE_INT_MASK and ABORT_INT_MASK
>     https://lore.kernel.org/imx/[email protected]/
>     (only this single commit is cherry-picked from the series)
> 
> 
> Patch layout
> ============
> 
>   1. dw-edma / DesignWare EP helpers needed for remote embedded-DMA (export
>      register/LL windows, IRQ routing control, etc.)
> 
>      Patch 01 : dmaengine: dw-edma: Export helper to get integrated register window
>      Patch 02 : dmaengine: dw-edma: Add per-channel interrupt routing control
>      Patch 03 : dmaengine: dw-edma: Poll completion when local IRQ handling is disabled
>      Patch 04 : dmaengine: dw-edma: Add notify-only channels support
>      Patch 05 : dmaengine: dw-edma: Add a helper to query linked-list region
> 
>   2. NTB EPF/core + vNTB prep (mwN_offset + versioning, MSI vector
>      management, new ntb_dev_ops helpers, driver_override, vntb glue)
> 
>      Patch 06 : NTB: epf: Add mwN_offset support and config region versioning
>      Patch 07 : NTB: epf: Reserve a subset of MSI vectors for non-NTB users
>      Patch 08 : NTB: epf: Provide db_vector_count/db_vector_mask callbacks
>      Patch 09 : NTB: core: Add mw_set_trans_ranges() for subrange programming
>      Patch 10 : NTB: core: Add .get_private_data() to ntb_dev_ops
>      Patch 11 : NTB: core: Add .get_dma_dev() to ntb_dev_ops
>      Patch 12 : NTB: core: Add driver_override support for NTB devices
>      Patch 13 : PCI: endpoint: pci-epf-vntb: Support BAR subrange mappings for MWs
>      Patch 14 : PCI: endpoint: pci-epf-vntb: Implement .get_private_data() callback
>      Patch 15 : PCI: endpoint: pci-epf-vntb: Implement .get_dma_dev()
> 
>   3. ntb_transport refactor/modularization and backend infrastructure
> 
>      Patch 16 : NTB: ntb_transport: Move TX memory window setup into setup_qp_mw()
>      Patch 17 : NTB: ntb_transport: Dynamically determine qp count
>      Patch 18 : NTB: ntb_transport: Use ntb_get_dma_dev()
>      Patch 19 : NTB: ntb_transport: Rename ntb_transport.c to ntb_transport_core.c
>      Patch 20 : NTB: ntb_transport: Move internal types to ntb_transport_internal.h
>      Patch 21 : NTB: ntb_transport: Export common helpers for modularization
>      Patch 22 : NTB: ntb_transport: Split core library and default NTB client
>      Patch 23 : NTB: ntb_transport: Add transport backend infrastructure
>      Patch 24 : NTB: ntb_transport: Run ntb_set_mw() before link-up negotiation
> 
>   4. ntb_edma backend registry + DesignWare backend + transport client
> 
>      Patch 25 : NTB: hw: Add remote eDMA backend registry and DesignWare backend
>      Patch 26 : NTB: ntb_transport: Add remote embedded-DMA transport client
> 
>   5. ntb_netdev multi-queue support
> 
>      Patch 27 : ntb_netdev: Multi-queue support
> 
>   6. Renesas R-Car S4 enablement (IOMMU, DTs, quirks)
> 
>      Patch 28 : iommu: ipmmu-vmsa: Add PCIe ch0 to devices_allowlist
>      Patch 29 : iommu: ipmmu-vmsa: Add support for reserved regions
>      Patch 30 : arm64: dts: renesas: Add Spider RC/EP DTs for NTB with remote DW PCIe eDMA
>      Patch 31 : NTB: epf: Add per-SoC quirk to cap MRRS for DWC eDMA (128B for R-Car)
>      Patch 32 : NTB: epf: Add an additional memory window (MW2) barno mapping on Renesas R-Car
> 
>   7. Documentation updates
> 
>      Patch 33 : Documentation: PCI: endpoint: pci-epf-vntb: Update and add mwN_offset usage
>      Patch 34 : Documentation: driver-api: ntb: Document remote embedded-DMA transport
> 
>   8. pci-epf-test / pci_endpoint_test / kselftest coverage for remote eDMA
> 
>      Patch 35 : PCI: endpoint: pci-epf-test: Add pci_epf_test_next_free_bar() helper
>      Patch 36 : PCI: endpoint: pci-epf-test: Add remote eDMA-backed mode
>      Patch 37 : misc: pci_endpoint_test: Add remote eDMA transfer test mode
>      Patch 38 : selftests: pci_endpoint: Add remote eDMA transfer coverage
> 
> 
> Tested on
> =========
> 
> * 2x Renesas R-Car S4 Spider (RC<->EP connected with OCuLink cable)
> * Kernel base as described above
> 
> 
> Performance notes
> =================
> 
> The primary motivation remains improving throughput/latency for ntb_transport
> users (typically ntb_netdev). On R-Car S4, the earlier prototype (RFC v3)
> showed roughly 10-20x throughput improvement in preliminary iperf3 tests and
> lower ping RTT. I have not yet re-measured after the v4 refactor and
> module split.
> 
> 
> Changelog
> =========
> 
> RFCv3->RFCv4 changes:
>   - Major refactor of the transport layering:
>     - Introduce ntb_transport_core as a shared library module.
>     - Split the legacy shared-memory transport client (ntb_transport) and the
>       remote embedded-DMA transport client (ntb_transport_edma).
>     - Add driver_override support for ntb_bus and use it for per-port transport
>       selection.
>   - Introduce a vendor-agnostic remote embedded-DMA backend registry (ntb_edma)
>     and add the initial DesignWare backend (ntb_dw_edma).
>   - Rebase to next-20260114 and move several prerequisite/fixup patchsets into
>     separate threads (listed above), including BAR subrange mapping support and
>     dw-edma fixes.
>   - Add PCI endpoint test coverage for the remote embedded-DMA path:
>     - extend pci-epf-test / pci_endpoint_test
>     - add a kselftest variant to exercise remote-eDMA transfers
>     Note: to keep the changes as small as possible, I added a few #ifdefs
>     in the main test code. Feedback on whether/how/to what extent this
>     should be split into separate modules would be appreciated.
>   - Expand documentation (Documentation/driver-api/ntb.rst) to describe transport
>     variants, the new module structure, and the remote embedded-DMA data flow.
>   - Addressed other feedbacks from the RFC v3 thread.
> 
> RFCv2->RFCv3 changes:
>   - Architecture
>     - Have EP side use its local write channels, while leaving RC side to
>       use remote read channels.
>     - Abstraction/HW-specific stuff encapsulation improved.
>   - Added control/config region versioning for the vNTB/EPF control region
>     so that mismatched RC/EP kernels fail early instead of silently using an
>     incompatible layout.
>   - Reworked BAR subrange / multi-region mapping support:
>     - Dropped the v2 approach that added new inbound mapping ops in the EPC
>       core.
>     - Introduced `struct pci_epf_bar.submap` and extended DesignWare EP to
>       support BAR subrange inbound mapping via Address Match Mode IB iATU.
>     - pci-epf-vntb now provides a subrange mapping hint to the EPC driver
>       when offsets are used.
>   - Changed .get_pci_epc() to .get_private_data()
>   - Dropped two commits from RFC v2 that should be submitted separately:
>     (1) ntb_transport debugfs seq_file conversion
>     (2) DWC EP outbound iATU MSI mapping/cache fix (will be re-posted separately)
>   - Added documentation updates.
>   - Addressed assorted review nits from the RFC v2 thread (naming/structure).
> 
> RFCv1->RFCv2 changes:
>   - Architecture
>     - Drop the generic interrupt backend + DW eDMA test-interrupt backend
>       approach and instead adopt the remote eDMA-backed ntb_transport mode
>       proposed by Frank Li. The BAR-sharing / mwN_offset / inbound
>       mapping (Address Match Mode) infrastructure from RFC v1 is largely
>       kept, with only minor refinements and code motion where necessary
>       to fit the new transport-mode design.
>   - For Patch 01
>     - Rework the array_index_nospec() conversion to address review
>       comments on "[RFC PATCH 01/25]".
> 
> RFCv3: https://lore.kernel.org/all/[email protected]/
> RFCv2: https://lore.kernel.org/all/[email protected]/
> RFCv1: https://lore.kernel.org/all/[email protected]/
> 
> Thank you for reviewing,
> 
> 
> Koichiro Den (38):
>   dmaengine: dw-edma: Export helper to get integrated register window
>   dmaengine: dw-edma: Add per-channel interrupt routing control
>   dmaengine: dw-edma: Poll completion when local IRQ handling is
>     disabled
>   dmaengine: dw-edma: Add notify-only channels support
>   dmaengine: dw-edma: Add a helper to query linked-list region
>   NTB: epf: Add mwN_offset support and config region versioning
>   NTB: epf: Reserve a subset of MSI vectors for non-NTB users
>   NTB: epf: Provide db_vector_count/db_vector_mask callbacks
>   NTB: core: Add mw_set_trans_ranges() for subrange programming
>   NTB: core: Add .get_private_data() to ntb_dev_ops
>   NTB: core: Add .get_dma_dev() to ntb_dev_ops
>   NTB: core: Add driver_override support for NTB devices
>   PCI: endpoint: pci-epf-vntb: Support BAR subrange mappings for MWs
>   PCI: endpoint: pci-epf-vntb: Implement .get_private_data() callback
>   PCI: endpoint: pci-epf-vntb: Implement .get_dma_dev()
>   NTB: ntb_transport: Move TX memory window setup into setup_qp_mw()
>   NTB: ntb_transport: Dynamically determine qp count
>   NTB: ntb_transport: Use ntb_get_dma_dev()
>   NTB: ntb_transport: Rename ntb_transport.c to ntb_transport_core.c
>   NTB: ntb_transport: Move internal types to ntb_transport_internal.h
>   NTB: ntb_transport: Export common helpers for modularization
>   NTB: ntb_transport: Split core library and default NTB client
>   NTB: ntb_transport: Add transport backend infrastructure
>   NTB: ntb_transport: Run ntb_set_mw() before link-up negotiation
>   NTB: hw: Add remote eDMA backend registry and DesignWare backend
>   NTB: ntb_transport: Add remote embedded-DMA transport client
>   ntb_netdev: Multi-queue support
>   iommu: ipmmu-vmsa: Add PCIe ch0 to devices_allowlist
>   iommu: ipmmu-vmsa: Add support for reserved regions
>   arm64: dts: renesas: Add Spider RC/EP DTs for NTB with remote DW PCIe
>     eDMA
>   NTB: epf: Add per-SoC quirk to cap MRRS for DWC eDMA (128B for R-Car)
>   NTB: epf: Add an additional memory window (MW2) barno mapping on
>     Renesas R-Car
>   Documentation: PCI: endpoint: pci-epf-vntb: Update and add mwN_offset
>     usage
>   Documentation: driver-api: ntb: Document remote embedded-DMA transport
>   PCI: endpoint: pci-epf-test: Add pci_epf_test_next_free_bar() helper
>   PCI: endpoint: pci-epf-test: Add remote eDMA-backed mode
>   misc: pci_endpoint_test: Add remote eDMA transfer test mode
>   selftests: pci_endpoint: Add remote eDMA transfer coverage
> 
>  Documentation/PCI/endpoint/pci-vntb-howto.rst |   19 +-
>  Documentation/driver-api/ntb.rst              |  193 ++
>  arch/arm64/boot/dts/renesas/Makefile          |    2 +
>  .../boot/dts/renesas/r8a779f0-spider-ep.dts   |   37 +
>  .../boot/dts/renesas/r8a779f0-spider-rc.dts   |   52 +
>  drivers/dma/dw-edma/dw-edma-core.c            |  207 +-
>  drivers/dma/dw-edma/dw-edma-core.h            |   10 +
>  drivers/dma/dw-edma/dw-edma-v0-core.c         |   26 +-
>  drivers/iommu/ipmmu-vmsa.c                    |    7 +-
>  drivers/misc/pci_endpoint_test.c              |  633 +++++
>  drivers/net/ntb_netdev.c                      |  341 ++-
>  drivers/ntb/Kconfig                           |   13 +
>  drivers/ntb/Makefile                          |    2 +
>  drivers/ntb/core.c                            |   68 +
>  drivers/ntb/hw/Kconfig                        |    1 +
>  drivers/ntb/hw/Makefile                       |    1 +
>  drivers/ntb/hw/edma/Kconfig                   |   28 +
>  drivers/ntb/hw/edma/Makefile                  |    5 +
>  drivers/ntb/hw/edma/backend.c                 |   87 +
>  drivers/ntb/hw/edma/backend.h                 |  102 +
>  drivers/ntb/hw/edma/ntb_dw_edma.c             |  977 +++++++
>  drivers/ntb/hw/epf/ntb_hw_epf.c               |  199 +-
>  drivers/ntb/ntb_transport.c                   | 2458 +---------------
>  drivers/ntb/ntb_transport_core.c              | 2523 +++++++++++++++++
>  drivers/ntb/ntb_transport_edma.c              | 1110 ++++++++
>  drivers/ntb/ntb_transport_internal.h          |  261 ++
>  drivers/pci/controller/dwc/pcie-designware.c  |   26 +
>  drivers/pci/endpoint/functions/pci-epf-test.c |  497 +++-
>  drivers/pci/endpoint/functions/pci-epf-vntb.c |  380 ++-
>  include/linux/dma/edma.h                      |  106 +
>  include/linux/ntb.h                           |   88 +
>  include/uapi/linux/pcitest.h                  |    3 +-
>  .../pci_endpoint/pci_endpoint_test.c          |   17 +
>  33 files changed, 7855 insertions(+), 2624 deletions(-)
>  create mode 100644 arch/arm64/boot/dts/renesas/r8a779f0-spider-ep.dts
>  create mode 100644 arch/arm64/boot/dts/renesas/r8a779f0-spider-rc.dts
>  create mode 100644 drivers/ntb/hw/edma/Kconfig
>  create mode 100644 drivers/ntb/hw/edma/Makefile
>  create mode 100644 drivers/ntb/hw/edma/backend.c
>  create mode 100644 drivers/ntb/hw/edma/backend.h
>  create mode 100644 drivers/ntb/hw/edma/ntb_dw_edma.c
>  create mode 100644 drivers/ntb/ntb_transport_core.c
>  create mode 100644 drivers/ntb/ntb_transport_edma.c
>  create mode 100644 drivers/ntb/ntb_transport_internal.h
>