[PATCH 3/3] Documentation: PCI: Add PCI DMA endpoint function documentation

[Koichiro Den]

Add a function description and a user guide for pci-epf-dma. Describe
the BAR-resident metadata consumed by dw-edma-pcie, the configfs
attributes, endpoint controller requirements and the host-side DMAengine
usage model.

Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
---
 Documentation/PCI/endpoint/index.rst          |   2 +
 .../PCI/endpoint/pci-dma-function.rst         | 182 ++++++++++++++++
 Documentation/PCI/endpoint/pci-dma-howto.rst  | 200 ++++++++++++++++++
 3 files changed, 384 insertions(+)
 create mode 100644 Documentation/PCI/endpoint/pci-dma-function.rst
 create mode 100644 Documentation/PCI/endpoint/pci-dma-howto.rst

diff --git a/Documentation/PCI/endpoint/index.rst b/Documentation/PCI/endpoint/index.rst
index dd1f62e731c9..cd4107e02ec2 100644
--- a/Documentation/PCI/endpoint/index.rst
+++ b/Documentation/PCI/endpoint/index.rst
@@ -15,6 +15,8 @@ PCI Endpoint Framework
    pci-ntb-howto
    pci-vntb-function
    pci-vntb-howto
+   pci-dma-function
+   pci-dma-howto
    pci-nvme-function
 
    function/binding/pci-test
diff --git a/Documentation/PCI/endpoint/pci-dma-function.rst b/Documentation/PCI/endpoint/pci-dma-function.rst
new file mode 100644
index 000000000000..54caf4fafe00
--- /dev/null
+++ b/Documentation/PCI/endpoint/pci-dma-function.rst
@@ -0,0 +1,182 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================
+PCI DMA Function
+================
+
+:Author: Koichiro Den <den@xxxxxxxxxxxxx>
+
+The PCI DMA endpoint function exposes an endpoint-integrated DMA controller
+to the PCI host as a PCI DMA controller.  A matching host-side driver
+discovers the endpoint DMA metadata and registers the delegated channels with
+the Linux DMAengine framework, so host DMAengine clients can submit
+transfers.
+
+An endpoint Linux system can already use an endpoint-integrated DMA
+controller locally through the normal DMAengine API, for example to transfer
+data between endpoint memory and host addresses reachable over PCI.  The PCI
+DMA function provides a different ownership model: it delegates selected
+local DMA channels to the host, so a host DMAengine client can request and
+program those endpoint-side channels through the host's DMAengine API.
+
+To make that possible, the endpoint function publishes the DMA controller
+register window and descriptor memory layout to the host, reserves the
+selected local DMA channels on the endpoint side, and lets the host program
+those channels directly.
+
+Constructs Used for Implementing DMA
+====================================
+
+The PCI DMA function uses the following endpoint-side resources and
+configuration:
+
+	1) DMA controller register window
+	2) DMA descriptor memory for endpoint-to-RC channels
+	3) DMA descriptor memory for RC-to-endpoint channels
+	4) MSI or MSI-X interrupt vectors selected through configfs
+	5) One endpoint BAR used to publish metadata
+	6) If needed, one endpoint BAR used for dynamically mapped DMA windows
+
+The endpoint controller reports the DMA controller register and descriptor
+resources through the endpoint auxiliary resource interface.  The PCI DMA
+function uses those descriptions to build the host-visible metadata and to map
+resources that are not already visible to the host.
+
+DMA Controller Register Window:
+-------------------------------
+
+It contains the DMA controller registers programmed by the host-side driver
+to submit transfers, control channels and handle DMA interrupts.
+
+DMA Descriptor Memory:
+----------------------
+
+It contains the descriptor memory used by the DMA controller.  The PCI DMA
+function exposes descriptor memory for the delegated endpoint-to-RC and
+RC-to-endpoint channels.
+
+MSI/MSI-X Interrupt Vectors:
+----------------------------
+
+They are used by the delegated DMA channels to signal completion and error
+conditions to the host-side driver.
+
+Metadata BAR:
+-------------
+
+It is the endpoint BAR used to publish the endpoint DMA metadata and handshake
+bits.  The BAR remains stable while the endpoint function programs the DMA
+windows.
+
+DMA Window BAR:
+---------------
+
+It is the endpoint BAR used for DMA resources that are not already visible
+through a fixed BAR.  The endpoint function may switch this BAR to subrange
+mapping after the host-side driver has found the metadata BAR.
+
+BAR Metadata
+============
+
+The endpoint function places a small metadata block at the beginning of the
+selected metadata BAR.  The format is defined in
+``include/linux/pci-ep-dma.h``.
+
+The host-side driver scans the function's assigned memory BARs, looks for the
+endpoint DMA metadata magic, requests DMA window programming, waits for the
+READY bit, and then parses the metadata to find the DMA register window and
+descriptor windows.
+
+::
+
+	+----------------------+ metadata BAR offset 0
+	| endpoint DMA metadata|
+	+----------------------+
+	| optional padding     |
+	+----------------------+
+
+	+----------------------+ DMA window BAR offset 0
+	| mapped DMA resources |
+	+----------------------+
+	| optional padding     |
+	+----------------------+
+
+The metadata can also reference resources that are already host-visible
+through fixed BARs.  For example, an endpoint controller may expose the DMA
+controller register window at a fixed BAR offset while descriptor memories
+are mapped into the DMA window BAR by the endpoint function.
+
+The metadata is BAR-resident instead of a self-contained PCI Vendor-Specific
+Extended Capability (VSEC).  Some endpoint controllers do not provide writable
+configuration-space backing storage large enough for a new VSEC payload, while
+they can map endpoint memory and controller resources into a BAR.
+
+Channel Ownership
+=================
+
+The ``wr_chans`` attribute exposes endpoint-to-RC DMA write channels.  The
+``rd_chans`` attribute exposes RC-to-endpoint DMA read channels.  The function
+reserves the selected endpoint-side DMAengine channels so that endpoint-side
+DMAengine clients cannot allocate and use the same hardware channels while
+they are delegated to the host.
+
+The current metadata revision describes channels in dense, zero-based order.
+For example, ``wr_chans = 2`` exposes write channels 0 and 1.  Skipping a
+hardware channel in the middle of the exposed range is not supported.
+
+The current DesignWare eDMA unroll and HDMA compatible support also requires
+each exposed direction to be delegated as a whole.  For example, on a controller
+with two write channels, ``wr_chans`` must be either 0 or 2.
+
+Interrupts
+==========
+
+The PCI DMA function exposes DMA interrupts through MSI or MSI-X.  The common
+endpoint function ``msi_interrupts`` and ``msix_interrupts`` configfs attributes
+select the interrupt vector counts programmed into endpoint config space.  At
+least one MSI or MSI-X vector must be configured before the function is bound
+to an endpoint controller.
+
+Transfer Addressing
+===================
+
+The host-side DMAengine client supplies the endpoint memory address as the
+DMA slave address.  For example, the ``dw-edma-pcie`` endpoint DMA metadata
+parser passes that slave address to the DMA controller as a raw endpoint-side
+address instead of translating it through a host PCI BAR resource.
+
+The host memory buffer used as the other side of the transfer is still mapped
+using the normal DMA mapping API on the host.
+
+Endpoint Controller Requirements
+================================
+
+The endpoint controller driver must expose the DMA controller register
+window and per-channel descriptor memories through the endpoint auxiliary
+resource API.  Endpoint controllers with other DMA register layouts also need
+matching metadata and host-side DMAengine driver support.
+
+If any DMA resource is not already host-visible through a fixed BAR, the
+endpoint controller must also support BAR subrange mapping and dynamic inbound
+mapping, because the DMA window BAR is assembled from those resources.
+
+Current Support
+===============
+
+The current host-side support is implemented in ``dw-edma-pcie`` for
+DesignWare eDMA unroll and HDMA compatible layouts.  Other PCIe controller
+DMA implementations need corresponding host-side DMAengine driver support.
+
+The ``dw-edma-pcie`` PCI ID table does not contain a generic endpoint DMA PCI
+ID entry.  Users need to bind the host-side driver explicitly using
+``driver_override``.
+
+The current metadata revision requires the exposed channels to be a dense
+prefix of the hardware channel numbers.
+
+Security Model
+==============
+
+The interface is intended for trusted endpoint/host deployments.  A delegated
+DMA channel can access endpoint memory addresses supplied by a host DMAengine
+client.
diff --git a/Documentation/PCI/endpoint/pci-dma-howto.rst b/Documentation/PCI/endpoint/pci-dma-howto.rst
new file mode 100644
index 000000000000..84f322881aa7
--- /dev/null
+++ b/Documentation/PCI/endpoint/pci-dma-howto.rst
@@ -0,0 +1,200 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================================
+PCI DMA Endpoint Function (EPF) User Guide
+==========================================
+
+:Author: Koichiro Den <den@xxxxxxxxxxxxx>
+
+This guide shows how to configure the ``pci-epf-dma`` endpoint function driver.
+It uses ``dw-edma-pcie`` as the currently available host-side driver.  For the
+hardware model and layout see Documentation/PCI/endpoint/pci-dma-function.rst.
+
+Endpoint Device
+===============
+
+Endpoint Controller Devices
+---------------------------
+
+To find the list of endpoint controller devices in the system::
+
+	# ls /sys/class/pci_epc/
+	e65d0000.pcie-ep
+
+If ``PCI_ENDPOINT_CONFIGFS`` is enabled::
+
+	# ls /sys/kernel/config/pci_ep/controllers
+	e65d0000.pcie-ep
+
+Endpoint Function Drivers
+-------------------------
+
+To find the list of endpoint function drivers in the system::
+
+	# ls /sys/bus/pci-epf/drivers
+	pci_epf_dma  pci_epf_test
+
+If ``PCI_ENDPOINT_CONFIGFS`` is enabled::
+
+	# ls /sys/kernel/config/pci_ep/functions
+	pci_epf_dma  pci_epf_test
+
+Creating pci-epf-dma Device
+---------------------------
+
+Create a ``pci-epf-dma`` device with configfs::
+
+	# mount -t configfs none /sys/kernel/config
+	# cd /sys/kernel/config/pci_ep/
+	# mkdir functions/pci_epf_dma/dma0
+
+The "mkdir dma0" above creates the ``pci-epf-dma`` function device that will
+be probed by the ``pci_epf_dma`` driver.
+
+The PCI endpoint framework populates the directory with the common
+configurable fields::
+
+	# ls functions/pci_epf_dma/dma0
+	baseclass_code   msi_interrupts   progif_code    subsys_id
+	cache_line_size  msix_interrupts  revid          subsys_vendor_id
+	deviceid         pci_epf_dma.0    secondary      vendorid
+	interrupt_pin    primary          subclass_code
+
+The PCI DMA function driver also creates a function-specific sub-directory.
+The numeric suffix depends on the endpoint function instance number::
+
+	# ls functions/pci_epf_dma/dma0/pci_epf_dma.0/
+	dma_window_bar  metadata_bar  rd_chans  wr_chans
+
+Configuring pci-epf-dma Device
+------------------------------
+
+The host-side ``dw-edma-pcie`` PCI ID table does not contain a generic
+endpoint DMA PCI ID entry.  Choose a PCI vendor/device ID for the endpoint
+device::
+
+	# echo <vendor-id> > functions/pci_epf_dma/dma0/vendorid
+	# echo <device-id> > functions/pci_epf_dma/dma0/deviceid
+	# echo 1 > functions/pci_epf_dma/dma0/msi_interrupts
+
+The PCI class defaults to ``PCI_BASE_CLASS_SYSTEM`` and
+``PCI_CLASS_SYSTEM_DMA``.
+
+The function-specific attributes are:
+
+============== ============================================================
+Attribute      Description
+============== ============================================================
+metadata_bar   BAR used to publish the endpoint DMA metadata and handshake
+               bits.  It is kept as a stable BAR while the DMA windows are
+               programmed.  If this is left unset, the first usable BAR that
+               does not already contain a fixed DMA resource is used.
+dma_window_bar BAR used for DMA resources that are not already host-visible,
+               such as the DMA register window or descriptor windows.  This
+               BAR may be switched to subrange mapping after the host driver
+               has found the metadata.  If this is left unset and a DMA
+               window is needed, the first usable BAR different from
+               ``metadata_bar`` and not already occupied by a fixed DMA
+               resource is used.
+wr_chans       Number of endpoint-to-RC DMA write channels to expose.
+rd_chans       Number of RC-to-endpoint DMA read channels to expose.
+============== ============================================================
+
+A sample configuration for a DesignWare eDMA/HDMA compatible controller with
+two write channels and two read channels is given below::
+
+	# echo 0 > functions/pci_epf_dma/dma0/pci_epf_dma.0/metadata_bar
+	# echo 2 > functions/pci_epf_dma/dma0/pci_epf_dma.0/dma_window_bar
+	# echo 2 > functions/pci_epf_dma/dma0/pci_epf_dma.0/wr_chans
+	# echo 2 > functions/pci_epf_dma/dma0/pci_epf_dma.0/rd_chans
+
+``wr_chans`` and ``rd_chans`` default to 0.  At least one channel direction
+must be configured.  The selected channels are exposed in dense, zero-based
+order; for example, ``wr_chans = 2`` exposes write channels 0 and 1.
+Current DesignWare eDMA unroll and HDMA compatible support requires each
+exposed direction to be delegated as a whole, so set a direction to either 0 or
+the number of hardware channels in that direction.  If ``dma_window_bar`` is
+configured, it must be different from ``metadata_bar``.
+
+The common ``msi_interrupts`` and ``msix_interrupts`` attributes select the
+number of MSI and MSI-X vectors exposed to the host.  At least one MSI or
+MSI-X vector must be configured.
+
+The function-specific attributes can only be changed before the endpoint
+function is bound to an endpoint controller.
+
+Binding pci-epf-dma Device to EP Controller
+-------------------------------------------
+
+The DMA function device should be attached to a PCI endpoint controller
+connected to the host::
+
+	# ln -s controllers/e65d0000.pcie-ep \
+		functions/pci_epf_dma/dma0/primary/
+
+Once the above step is completed, the PCI endpoint controller is ready to
+establish a link with the host.
+
+Start the Link
+--------------
+
+Start the endpoint controller by writing 1 to ``start``::
+
+	# echo 1 > controllers/e65d0000.pcie-ep/start
+
+Root Complex Device
+===================
+
+lspci Output
+------------
+
+Note that the device listed here corresponds to the values populated in the
+endpoint configuration above::
+
+	# lspci -nk
+	01:00.1 0801: <vendor-id>:<device-id>
+
+If the host was already running while the endpoint function was configured,
+rescan the PCI bus after the endpoint side has completed the configfs setup
+and started the endpoint controller, if the platform supports it.
+
+Bind the endpoint DMA function to ``dw-edma-pcie`` explicitly with
+``driver_override``::
+
+	# modprobe dw_edma_pcie
+	# echo dw-edma-pcie > /sys/bus/pci/devices/0000:01:00.1/driver_override
+	# echo 0000:01:00.1 > /sys/bus/pci/drivers_probe
+
+The device should then be bound to ``dw-edma-pcie``::
+
+	# lspci -nk -s 01:00.1
+	01:00.1 0801: <vendor-id>:<device-id>
+		Kernel driver in use: dw-edma-pcie
+
+Using pci-epf-dma Device
+------------------------
+
+The host side software uses the standard Linux DMAengine API.  A DMAengine
+client driver running on the host must request one of the channels provided by
+``dw-edma-pcie`` and submit a transfer.
+
+For an endpoint-to-RC write transfer, the DMAengine client uses a host DMA
+buffer as the destination and an endpoint-side address as the slave source
+address.  For an RC-to-endpoint read transfer, the DMAengine client uses a
+host DMA buffer as the source and an endpoint-side address as the slave
+destination address.
+
+Troubleshooting
+===============
+
+``pci-epf-dma`` requires endpoint controller support for DMA auxiliary
+resources and MSI or MSI-X.  If any DMA resource must be mapped dynamically,
+the endpoint controller must also support BAR subrange mapping and dynamic
+inbound mapping.  Binding the function to an endpoint controller fails if the
+required capabilities are not available, or if both ``msi_interrupts`` and
+``msix_interrupts`` are zero.
+
+If ``dw-edma-pcie`` fails to probe on the host, check that the endpoint was
+bound to the host driver, that the endpoint BARs were assigned by PCI
+enumeration, and that the endpoint DMA metadata READY bit was set after any
+DMA window BAR submaps were programmed.
-- 
2.51.0