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Co-developed-by: Siddharth Vadapalli Signed-off-by: Siddharth Vadapalli Signed-off-by: Aksh Garg --- Changes from v2 to v3: - Rebased on 7.1-rc1. Changes since v1: - Squashed the patches [1] and [2], and moved the documentation file to Documentation/PCI/endpoint/pci-endpoint-doe.rst to match the existing naming scheme, as suggested by Niklas Cassel - Updated the documentation as per the design and implementaion changes made to previous patches in this series: * Updated for static protocol array instead of dynamic registration * Documented asynchronous callback model * Updated request/response flow with new callback signature * Updated memory ownership: DOE core frees request, driver frees response * Updated initialization and cleanup sections for new APIs v2: https://lore.kernel.org/all/20260401073022.215805-5-a-garg7@ti.com/ v1: [1] https://lore.kernel.org/all/20260213123603.420941-2-a-garg7@ti.com/ [2] https://lore.kernel.org/all/20260213123603.420941-5-a-garg7@ti.com/ Documentation/PCI/endpoint/index.rst | 1 + .../PCI/endpoint/pci-endpoint-doe.rst | 318 ++++++++++++++++++ 2 files changed, 319 insertions(+) create mode 100644 Documentation/PCI/endpoint/pci-endpoint-doe.rst diff --git a/Documentation/PCI/endpoint/index.rst b/Documentation/PCI/endpoint/index.rst index dd1f62e731c9..7c03d5abd2ef 100644 --- a/Documentation/PCI/endpoint/index.rst +++ b/Documentation/PCI/endpoint/index.rst @@ -9,6 +9,7 @@ PCI Endpoint Framework pci-endpoint pci-endpoint-cfs + pci-endpoint-doe pci-test-function pci-test-howto pci-ntb-function diff --git a/Documentation/PCI/endpoint/pci-endpoint-doe.rst b/Documentation/PCI/endpoint/pci-endpoint-doe.rst new file mode 100644 index 000000000000..03b7a69516f3 --- /dev/null +++ b/Documentation/PCI/endpoint/pci-endpoint-doe.rst @@ -0,0 +1,318 @@ +.. SPDX-License-Identifier: GPL-2.0-only or MIT + +.. include:: + +============================================= +Data Object Exchange (DOE) for PCIe Endpoint +============================================= + +:Copyright: |copy| 2026 Texas Instruments Incorporated +:Author: Aksh Garg +:Co-Author: Siddharth Vadapalli + +Overview +======== + +DOE (Data Object Exchange) is a standard PCIe extended capability feature +introduced in the Data Object Exchange (DOE) ECN for PCIe r5.0. It is an optional +mechanism for system firmware/software running on root complex (host) to perform +:ref:`data object ` exchanges with an endpoint function. Each +data object is uniquely identified by the Vendor ID of the vendor publishing the +data object definition and a Data Object Type value assigned by that vendor. + +Think of DOE as a sophisticated mailbox system built into PCIe. The root complex +can send structured requests to the endpoint device through DOE mailboxes, and +the endpoint device responds with appropriate data. DOE mailboxes are implemented +as PCIe Extended Capabilities in endpoint devices, allowing multiple mailboxes +per function, each potentially supporting different data object protocols. + +The DOE support for root complex devices has already been implemented in +``drivers/pci/doe.c``. + +How DOE Works +============= + +The DOE mailbox operates through a simple request-response model: + +1. **Host sends request**: The root complex writes a data object (vendor ID, type, + and payload) to the DOE write mailbox register (one DWORD at a time) of the + endpoint function's config space and sets the GO bit in the DOE Status register + to indicate that a request is ready for processing. +2. **Endpoint processes**: The endpoint function reads the request from DOE write + mailbox register, sets the BUSY bit in the DOE Status register, identifies the + protocol of the data object, and executes the appropriate handler. +3. **Endpoint responds**: The endpoint function writes the response data object to the + DOE read mailbox register (one DWORD at a time), and sets the READY bit in the DOE + Status register to indicate that the response is ready. If an error occurs during + request processing (such as unsupported protocol or handler failure), the endpoint + sets the ERROR bit in the DOE Status register instead of the READY bit. +4. **Host reads response**: The root complex retrieves the response data from the DOE read + mailbox register once the READY bit is set in the DOE Status register, and then writes + any value to this register to indicate a successful read. If the ERROR bit was set, + the root complex discards the response and performs error handling as needed. + +Each mailbox operates independently and can handle one transaction at a time. The +DOE specification supports data objects of size up to 256KB (2\ :sup:`18` dwords). + +For complete DOE capability details, refer to `PCI Express Base Specification Revision 7.0, +Section 6.30 - Data Object Exchange (DOE)`. + +Key Terminologies +================= + +.. _data-object-term: + +**Data Object** + A structured, vendor-defined, or standard-defined message exchanged between + root complex and endpoint function via DOE capability registers in configuration + space of the function. + +**Mailbox** + A DOE capability on the endpoint device, where each physical function can have + multiple mailboxes. + +**Protocol** + A specific type of DOE communication data object identified by a Vendor ID and Type. + +**Handler** + A function that processes DOE requests of a specific protocol and generates responses. + +Architecture of DOE Implementation for Endpoint +=============================================== + +.. code-block:: text + + +------------------+ + | | + | Root Complex | + | | + +--------^---------+ + | + | Config space access + | over PCIe link + | + +----------v-----------+ + | | + | PCIe Controller | + | as Endpoint | + | | + | +-----------------+ | + | | DOE Mailbox | | + | +-------^---------+ | + +----------|-----------+ + +-----------|---------------------------------------------------------------+ + | | +--------------------+ | + | +---------v--------+ Allocate | +--------------+ | | + | | |-------------------------------->| Request | | | + | | EP Controller | +--->| Buffer | | | + | | Driver | Free | | +--------------+ | | + | | |--------------------------+ | | | | + | +--------^---------+ | | | | | + | | | | | | | + | | | | | | | + | | pci_ep_doe_process_request() | | | | | + | | | | | | | + | +--------v---------+ Free | | | | | + | | |----------------------------+ | DDR | | + | | DOE EP Core |<----+ | | | | + | | (doe-ep.c) | | Discovery | | | | + | | |-----+ Protocol Handler | | | | + | +--------^---------+ | | | | + | | | | | | + | | protocol_handler() | | | | + | | | | | | + | +--------v---------+ | | | | + | | | | | +--------------+ | | + | | Protocol Handler | +----->| Response | | | + | | Module |-------------------------------->| Buffer | | | + | | (CMA/SPDM/Other) | Allocate | +--------------+ | | + | | | | | | + | +------------------+ | | | + | +--------------------+ | + +---------------------------------------------------------------------------+ + +Initialization and Cleanup +-------------------------- + +**Framework Initialization and DOE Setup** + +The EPC core provides the ``pci_epc_doe_setup(epc)`` API for centralized DOE +mailbox discovery and registration. The controller driver calls this API during +its probe sequence if DOE is supported. + +This API performs the following steps: + +1. Calls ``pci_ep_doe_init(epc)``, which initializes the xarray data structure + (a resizable array data structure defined in linux) named ``doe_mbs`` that + stores metadata of DOE mailboxes for the controller in ``struct pci_epc``. +2. Discovers all DOE capabilities in the endpoint function's configuration space + for each function. For each discovered DOE capability, calls + ``pci_ep_doe_add_mailbox(epc, func_no, cap_offset)`` to register the mailbox. + +Each DOE mailbox structure created by ``pci_ep_doe_add_mailbox()`` gets an +ordered workqueue allocated for processing DOE requests sequentially for that +mailbox, enabling concurrent request handling across different mailboxes. Each +mailbox is uniquely identified by the combination of physical function number +and capability offset for that controller. + +**Cleanup** + +The EPC core provides the ``pci_epc_doe_destroy(epc)`` API for centralized DOE +cleanup. The controller driver calls this API during its remove sequence +if DOE is supported. + +This API calls ``pci_ep_doe_destroy(epc)``, which destroys all registered +mailboxes, cancels any pending tasks, flushes and destroys the workqueues, +and frees all memory allocated to the mailboxes. + +Protocol Handler Support +------------------------ + +Protocol implementations (such as CMA, SPDM, or vendor-specific protocols) are +supported through a static array of protocol handlers. + +When a new DOE protocol library is introduced, its handler function is added to +the static ``pci_doe_protocols`` array in ``drivers/pci/endpoint/pci-ep-doe.c``. +The discovery protocol (VID = 0x0001 (PCI-SIG vendor ID), Type = 0x00 (discovery +protocol)) is included in this static array and handled internally by the +DOE EP core. + +Request Handling +---------------- + +The complete flow of a DOE request from the root complex to the response: + +**Step 1: Root Complex → EP Controller Driver** + +The root complex writes a DOE request (Vendor ID, Type, and Payload) to the +DOE write mailbox register in the endpoint function's configuration space and sets +the GO bit in the DOE Control register, indicating that the request is ready for +processing. + +**Step 2: EP Controller Driver → DOE EP Core** + +The controller driver reads the request header to determine the data object +length. Based on this length field, it allocates a request buffer in memory +(DDR) of the appropriate size. The driver then reads the complete request +payload from the DOE write mailbox register and converts the data from +little-endian format (the format followed in the PCIe transactions over the +link) to CPU-native format using ``le32_to_cpu()``. The driver defines a +completion callback function with signature ``void (*complete)(u8 func_no, +u16 cap_offset, int status, u16 vendor, u8 type, void *response_pl, +size_t response_pl_sz)`` to be invoked when the request processing completes. +The driver then calls ``pci_ep_doe_process_request(epc, func_no, cap_offset, +vendor, type, request, request_sz, complete)`` to hand off the request to the +DOE EP core. This function returns immediately after queuing the work +(without blocking), and the driver sets the BUSY bit in the DOE Status register. + +**Step 3: DOE EP Core Processing** + +The DOE EP core creates a task structure and submits it to the mailbox's ordered +workqueue. This ensures that requests for each mailbox are processed +sequentially, one at a time, as required by the DOE specification. It looks up +the protocol handler based on the Vendor ID and Type from the request header, +and executes the handler function. + +**Step 4: Protocol Handler Execution** + +The workqueue executes the task by calling the registered protocol handler: +``handler(request, request_sz, &response, &response_sz)``. The handler processes +the request, allocates a response buffer in memory (DDR), builds the response +data, and returns the response pointer and size. For the discovery protocol, +the DOE EP core handles this directly without invoking an external handler. + +**Step 5: DOE EP Core → EP Controller Driver** + +After the protocol handler completes, the DOE EP core frees the request buffer, +and invokes the completion callback provided by the controller driver asynchronously. +The callback receives the function number, capability offset (to identify the mailbox), +status code indicating the result of request processing, vendor ID and type of the data +object, the response buffer, and its size. + +**Step 6: EP Controller Driver → Root Complex** + +The controller driver converts the response from CPU-native format to +little-endian format using ``cpu_to_le32()``, writes the response to DOE read +mailbox register, and sets the READY bit in the DOE Status register. The root +complex then reads the response from the read mailbox register. Finally, the controller +driver frees the response buffer (which the handler allocated). + +Asynchronous Request Processing +------------------------------- + +The DOE-EP framework implements asynchronous request processing because an +endpoint function can have multiple instances of DOE mailboxes, and requests may +be interleaved across these mailboxes. Request processing of one mailbox should +not result in blocking request processing of other mailboxes. Hence, requests +on each mailbox need to be handled in parallel for optimization. + +For the EP controller driver to handle requests on multiple mailboxes in +parallel, ``pci_ep_doe_process_request()`` must be asynchronous. The function +returns immediately after submitting the request to the mailbox's workqueue, +without waiting for the request to complete. A completion callback provided by +the controller driver is invoked asynchronously when request processing +finishes. This asynchronous design enables concurrent processing of requests +across different mailboxes. + +Abort Handling +-------------- + +The DOE specification allows the root complex to abort ongoing DOE operations +by setting the ABORT bit in the DOE Control register. + +**Trigger** + +When the root complex sets the ABORT bit, the EP controller driver detects this +condition (typically in an interrupt handler or register polling routine). The +action taken depends on the timing of the abort: + +- **ABORT during request transfer**: If the ABORT bit is set while the root complex + is still transferring the request to the mailbox registers, the controller driver + discards the request and no call to ``pci_ep_doe_abort()`` is needed. + +- **ABORT after request submission**: If the ABORT bit is set after the request + has been fully received and submitted to the DOE EP core via + ``pci_ep_doe_process_request()``, the controller driver must call + ``pci_ep_doe_abort(epc, func_no, cap_offset)`` for the affected mailbox to + perform abort sequence in the DOE EP core. + +**Abort Sequence** + +The abort function performs the following actions: + +1. Sets the CANCEL flag on the mailbox to prevent queued requests from starting +2. Flushes the workqueue to wait for any currently executing handler to complete + (handlers cannot be interrupted mid-execution) +3. Clears the CANCEL flag to allow the mailbox to accept new requests + +Queued requests that have not started execution will be aborted with an error +status. The currently executing request will complete normally, and the controller +will reject the response if it arrives after the abort sequence has been triggered. + +.. note:: + Independent of when the ABORT bit is triggered, the controller driver must + clear the ERROR, BUSY, and READY bits in the DOE Status register after + completing the abort operation to reset the mailbox to an idle state. + +Error Handling +-------------- + +Errors can occur during DOE request processing for various reasons, such as +unsupported protocols, handler failures, or memory allocation failures. + +**Error Detection** + +When an error occurs during DOE request processing, the DOE EP core propagates this error +back to the controller driver either through the ``pci_ep_doe_process_request()`` return value, +or the status code passed to the completion callback. + +**Error Response** + +When the controller driver receives an error code, it sets the ERROR bit in the DOE Status +register instead of writing a response to the read mailbox register, and frees the buffers. + +API Reference +============= + +.. kernel-doc:: drivers/pci/endpoint/pci-ep-doe.c + :export: -- 2.34.1