[PATCH 15/15] Documentation: PCI: endpoint: Add vNTB DMA export HOWTO

Document the new pci-epf-vntb DMA export model.

Describe the configfs placement knobs for the DMA slice, the versioned
control layout, the two-stage BAR programming model used for shared
BARs, and the host-side discovery path through ntb_hw_epf and
dw-edma-aux.

Also show how to exercise the feature with the ntb_ep_dma test client.

Signed-off-by: Koichiro Den <[email protected]>
---
 Documentation/PCI/endpoint/index.rst          |  1 +
 .../PCI/endpoint/pci-vntb-dma-howto.rst       | 83 +++++++++++++++++++
 2 files changed, 84 insertions(+)
 create mode 100644 Documentation/PCI/endpoint/pci-vntb-dma-howto.rst

diff --git a/Documentation/PCI/endpoint/index.rst b/Documentation/PCI/endpoint/index.rst
index dd1f62e731c9..c3f93f1da34b 100644
--- a/Documentation/PCI/endpoint/index.rst
+++ b/Documentation/PCI/endpoint/index.rst
@@ -15,6 +15,7 @@ PCI Endpoint Framework
    pci-ntb-howto
    pci-vntb-function
    pci-vntb-howto
+   pci-vntb-dma-howto
    pci-nvme-function
 
    function/binding/pci-test
diff --git a/Documentation/PCI/endpoint/pci-vntb-dma-howto.rst b/Documentation/PCI/endpoint/pci-vntb-dma-howto.rst
new file mode 100644
index 000000000000..c3acc4e6ab37
--- /dev/null
+++ b/Documentation/PCI/endpoint/pci-vntb-dma-howto.rst
@@ -0,0 +1,83 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============================
+pci-epf-vntb DMA export HOWTO
+=============================
+
+``pci-epf-vntb`` can expose a virtual NTB device to the host while also
+exporting an endpoint-integrated DMA instance. The logical DMA export slice may
+occupy its own BAR or share a physical BAR with one or more memory windows.
+
+Configuration knobs
+===================
+
+The endpoint function instance exposes the following placement controls via
+configfs:
+
+- ``mwN_bar``
+- ``mwN_offset``
+- ``dma_bar``
+- ``dma_offset``
+- ``dma_num_chans``
+
+The offsets are BAR-relative. Regions packed into the same physical BAR must be
+non-overlapping and must cover the complete programmed BAR aperture.
+
+``dma_num_chans`` controls how many DMA read channels are delegated to the
+peer. If fewer channels are available at runtime, the endpoint falls back to
+the maximum available number and reports the effective delegation count in the
+kernel log.
+
+DMA ABI v1 is currently defined for READ channels only. The exported channels
+must form a dense prefix of the endpoint READ-channel space, and the
+``chans[]`` entries in the ABI header are ordered by remote hardware READ
+channel index starting at 0. ``dw-edma-aux`` relies on that ordering when it
+reconstructs the remote engine on the host side.
+
+Control-layout compatibility
+============================
+
+Offset ``0x0c`` in the control block is used as a control-layout version. The
+legacy layout is version 0. Version 1 extends the block with per-MW
+``offset/size`` tuples and the DMA locator.
+
+``pci-epf-vntb`` emits the legacy layout whenever the current configuration can
+still be represented by it. This allows a new endpoint kernel to interoperate
+with an older ``ntb_hw_epf`` host as long as the deployment uses the historical
+one-MW-per-BAR layout and does not enable DMA export.
+
+Deferred BAR activation
+=======================
+
+BAR subrange mapping cannot be programmed at bind time because the host has not
+assigned BAR addresses yet. Shared ``MW/DMA`` BARs therefore use a two-stage
+programming model:
+
+1. bind time: configure the BAR aperture with a temporary plain mapping
+2. first host command (``CONFIGURE_MW`` or ``LINK_UP``): re-issue
+   ``pci_epc_set_bar()`` with the final subrange layout
+
+This matches the requirements for EPC subrange mapping feature.
+
+DMA export and ``ntb_ep_dma``
+=============================
+
+When DMA export is enabled, ``pci-epf-vntb`` publishes a logical DMA locator in
+the control block. ``ntb_hw_epf`` parses that locator during probe, reserves an
+IRQ slice for the DMA provider, and registers the auxiliary child only after
+``LINK_UP`` succeeds. As a vendor-specific frontend example, ``dw-edma-aux``
+binds to that child and exposes a DMA engine provider for the remote endpoint
+DMA instance.
+
+The ``ntb_ep_dma`` test client exercises that path. Both peers load the
+module. Each side allocates a local test buffer and publishes the DMA address
+and size through scratchpads. The responder only waits and verifies, while the
+initiator triggers one transfer with::
+
+    # cat /sys/kernel/debug/ntb_ep_dma/0000:01:00.0/ready
+    # echo 1 > /sys/kernel/debug/ntb_ep_dma/0000:01:00.0/run
+    # cat /sys/kernel/debug/ntb_ep_dma/0000:01:00.0/result
+
+The initiator side uses the remote exported DMA instance to transfer into the
+peer-published buffer. The responder verifies the contents locally and reports
+PASS/FAIL back through scratchpads and a doorbell.
-- 
2.51.0