.. SPDX-License-Identifier: GPL-2.0

:Author: Kishon Vijay Abraham I <kishon@ti.com>

This document is a guide to use the PCI Endpoint Framework in order to create
endpoint controller driver, endpoint function driver, and using configfs
interface to bind the function driver to the controller driver.

Introduction
============

Linux has a comprehensive PCI subsystem to support PCI controllers that
operates in Root Complex mode. The subsystem has capability to scan PCI bus,
assign memory resources and IRQ resources, load PCI driver (based on
vendor ID, device ID), support other services like hot-plug, power management,
advanced error reporting and virtual channels.

However the PCI controller IP integrated in some SoCs is capable of operating
either in Root Complex mode or Endpoint mode. PCI Endpoint Framework will
add endpoint mode support in Linux. This will help to run Linux in an
EP system which can have a wide variety of use cases from testing or
validation, co-processor accelerator, etc.

PCI Endpoint Core
=================

The PCI Endpoint Core layer comprises 3 components: the Endpoint Controller
library, the Endpoint Function library, and the configfs layer to bind the
endpoint function with the endpoint controller.

PCI Endpoint Controller(EPC) Library
------------------------------------

The EPC library provides APIs to be used by the controller that can operate
in endpoint mode. It also provides APIs to be used by function driver/library
in order to implement a particular endpoint function.

APIs for the PCI controller Driver
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This section lists the APIs that the PCI Endpoint core provides to be used
by the PCI controller driver.

* devm_pci_epc_create()/pci_epc_create()

   The PCI controller driver should implement the following ops:

	 * write_header: ops to populate configuration space header
	 * set_bar: ops to configure the BAR
	 * clear_bar: ops to reset the BAR
	 * alloc_addr_space: ops to allocate in PCI controller address space
	 * free_addr_space: ops to free the allocated address space
	 * raise_irq: ops to raise a legacy, MSI or MSI-X interrupt
	 * start: ops to start the PCI link
	 * stop: ops to stop the PCI link

   The PCI controller driver can then create a new EPC device by invoking
   devm_pci_epc_create()/pci_epc_create().

* pci_epc_destroy()

   The PCI controller driver can destroy the EPC device created by
   pci_epc_create() using pci_epc_destroy().

* pci_epc_linkup()

   In order to notify all the function devices that the EPC device to which
   they are linked has established a link with the host, the PCI controller
   driver should invoke pci_epc_linkup().

* pci_epc_mem_init()

   Initialize the pci_epc_mem structure used for allocating EPC addr space.

* pci_epc_mem_exit()

   Cleanup the pci_epc_mem structure allocated during pci_epc_mem_init().


EPC APIs for the PCI Endpoint Function Driver
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This section lists the APIs that the PCI Endpoint core provides to be used
by the PCI endpoint function driver.

* pci_epc_write_header()

   The PCI endpoint function driver should use pci_epc_write_header() to
   write the standard configuration header to the endpoint controller.

* pci_epc_set_bar()

   The PCI endpoint function driver should use pci_epc_set_bar() to configure
   the Base Address Register in order for the host to assign PCI addr space.
   Register space of the function driver is usually configured
   using this API.

   Some endpoint controllers also support calling pci_epc_set_bar() again
   for the same BAR (without calling pci_epc_clear_bar()) to update inbound
   address translations after the host has programmed the BAR base address.
   Endpoint function drivers can check this capability via the
   dynamic_inbound_mapping EPC feature bit.

   When pci_epf_bar.num_submap is non-zero, the endpoint function driver is
   requesting BAR subrange mapping using pci_epf_bar.submap. This requires
   the EPC to advertise support via the subrange_mapping EPC feature bit.

   When an EPF driver wants to make use of the inbound subrange mapping
   feature, it requires that the BAR base address has been programmed by
   the host during enumeration. Thus, it needs to call pci_epc_set_bar()
   twice for the same BAR (requires dynamic_inbound_mapping): first with
   num_submap set to zero and configuring the BAR size, then after the PCIe
   link is up and the host enumerates the endpoint and programs the BAR
   base address, again with num_submap set to non-zero value.

   Note that when making use of the inbound subrange mapping feature, the
   EPF driver must not call pci_epc_clear_bar() between the two
   pci_epc_set_bar() calls, because clearing the BAR can clear/disable the
   BAR register or BAR decode on the endpoint while the host still expects
   the assigned BAR address to remain valid.

* pci_epc_clear_bar()

   The PCI endpoint function driver should use pci_epc_clear_bar() to reset
   the BAR.

* pci_epc_raise_irq()

   The PCI endpoint function driver should use pci_epc_raise_irq() to raise
   Legacy Interrupt, MSI or MSI-X Interrupt.

* pci_epc_mem_alloc_addr()

   The PCI endpoint function driver should use pci_epc_mem_alloc_addr(), to
   allocate memory address from EPC addr space which is required to access
   RC's buffer

* pci_epc_mem_free_addr()

   The PCI endpoint function driver should use pci_epc_mem_free_addr() to
   free the memory space allocated using pci_epc_mem_alloc_addr().

* pci_epc_map_addr()

  A PCI endpoint function driver should use pci_epc_map_addr() to map to a RC
  PCI address the CPU address of local memory obtained with
  pci_epc_mem_alloc_addr().

* pci_epc_unmap_addr()

  A PCI endpoint function driver should use pci_epc_unmap_addr() to unmap the
  CPU address of local memory mapped to a RC address with pci_epc_map_addr().

* pci_epc_mem_map()

  A PCI endpoint controller may impose constraints on the RC PCI addresses that
  can be mapped. The function pci_epc_mem_map() allows endpoint function
  drivers to allocate and map controller memory while handling such
  constraints. This function will determine the size of the memory that must be
  allocated with pci_epc_mem_alloc_addr() for successfully mapping a RC PCI
  address range. This function will also indicate the size of the PCI address
  range that was actually mapped, which can be less than the requested size, as
  well as the offset into the allocated memory to use for accessing the mapped
  RC PCI address range.

* pci_epc_mem_unmap()

  A PCI endpoint function driver can use pci_epc_mem_unmap() to unmap and free
  controller memory that was allocated and mapped using pci_epc_mem_map().


Other EPC APIs
~~~~~~~~~~~~~~

There are other APIs provided by the EPC library. These are used for binding
the EPF device with EPC device. pci-ep-cfs.c can be used as reference for
using these APIs.

* pci_epc_get()

   Get a reference to the PCI endpoint controller based on the device name of
   the controller.

* pci_epc_put()

   Release the reference to the PCI endpoint controller obtained using
   pci_epc_get()

* pci_epc_add_epf()

   Add a PCI endpoint function to a PCI endpoint controller. A PCIe device
   can have up to 8 functions according to the specification.

* pci_epc_remove_epf()

   Remove the PCI endpoint function from PCI endpoint controller.

* pci_epc_start()

   The PCI endpoint function driver should invoke pci_epc_start() once it
   has configured the endpoint function and wants to start the PCI link.

* pci_epc_stop()

   The PCI endpoint function driver should invoke pci_epc_stop() to stop
   the PCI LINK.


PCI Endpoint Function(EPF) Library
----------------------------------

The EPF library provides APIs to be used by the function driver and the EPC
library to provide endpoint mode functionality.

EPF APIs for the PCI Endpoint Function Driver
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This section lists the APIs that the PCI Endpoint core provides to be used
by the PCI endpoint function driver.

* pci_epf_register_driver()

   The PCI Endpoint Function driver should implement the following ops:
	 * bind: ops to perform when an EPC device has been bound to EPF device
	 * unbind: ops to perform when a binding has been lost between an EPC
	   device and EPF device
	 * add_cfs: optional ops to create function specific configfs
	   attributes

  The PCI Function driver can then register the PCI EPF driver by using
  pci_epf_register_driver().

* pci_epf_unregister_driver()

  The PCI Function driver can unregister the PCI EPF driver by using
  pci_epf_unregister_driver().

* pci_epf_alloc_space()

  The PCI Function driver can allocate space for a particular BAR using
  pci_epf_alloc_space().

* pci_epf_free_space()

  The PCI Function driver can free the allocated space
  (using pci_epf_alloc_space) by invoking pci_epf_free_space().

APIs for the PCI Endpoint Controller Library
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This section lists the APIs that the PCI Endpoint core provides to be used
by the PCI endpoint controller library.

* pci_epf_linkup()

   The PCI endpoint controller library invokes pci_epf_linkup() when the
   EPC device has established the connection to the host.

Other EPF APIs
~~~~~~~~~~~~~~

There are other APIs provided by the EPF library. These are used to notify
the function driver when the EPF device is bound to the EPC device.
pci-ep-cfs.c can be used as reference for using these APIs.

* pci_epf_create()

   Create a new PCI EPF device by passing the name of the PCI EPF device.
   This name will be used to bind the EPF device to a EPF driver.

* pci_epf_destroy()

   Destroy the created PCI EPF device.

* pci_epf_bind()

   pci_epf_bind() should be invoked when the EPF device has been bound to
   an EPC device.

* pci_epf_unbind()

   pci_epf_unbind() should be invoked when the binding between EPC device
   and EPF device is lost.
