9.1. Introduction — The Linux Kernel 7.0.11-gnu-zen1-1-zen documentation

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.

# 9.1. 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.

# 9.2. 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.

## 9.2.1. 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.

### 9.2.1.1. 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()`.

### 9.2.1.2. 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()`.

### 9.2.1.3. 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.

## 9.2.2. 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.

### 9.2.2.1. 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()`.

### 9.2.2.2. 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.

### 9.2.2.3. 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.