12. Raw Device Library

12.1. Introduction

In terms of device flavor (type) support, DPDK currently has ethernet (lib_ether), cryptodev (libcryptodev), eventdev (libeventdev) and vdev (virtual device) support.

For a new type of device, for example an accelerator, there are not many options except:

  1. create another lib/MySpecialDev, driver/MySpecialDrv and use it through Bus/PMD model.

  2. Or, create a vdev and implement necessary custom APIs which are directly exposed from driver layer. However this may still require changes in bus code in DPDK.

The DPDK Rawdev library is an abstraction that provides the DPDK framework a way to manage such devices in a generic manner without expecting changes to library or EAL for each device type. This library provides a generic set of operations and APIs for framework and Applications to use, respectively, for interfacing with such type of devices.

12.2. Design

Key factors guiding design of the Rawdevice library:

  1. Following are some generic operations which can be treated as applicable to a large subset of device types. None of the operations are mandatory to be implemented by a driver. Application should also be designed for proper handling for unsupported APIs.

    • Device Start/Stop - In some cases, ‘reset’ might also be required which has different semantics than a start-stop-start cycle.

    • Configuration - Device, Queue or any other sub-system configuration

    • I/O - Sending a series of buffers which can enclose any arbitrary data

    • Statistics - Fetch arbitrary device statistics

    • Firmware Management - Firmware load/unload/status

  2. Application API should be able to pass along arbitrary state information to/from device driver. This can be achieved by maintaining context information through opaque data or pointers.

Figure below outlines the layout of the rawdevice library and device vis-a-vis other well known device types like eth and crypto:

    +-----------------------------------------------------------+
    |                        Application(s)                     |
    +------------------------------.----------------------------+
                                   |
                                   |
    +------------------------------'----------------------------+
    |                     DPDK Framework (APIs)                 |
    +--------------|----|-----------------|---------------------+
                  /      \                 \
           (crypto ops)  (eth ops)      (rawdev ops)        +----+
           /               \                 \              |DrvA|
    +-----'---+        +----`----+        +---'-----+       +----+
    | crypto  |        | ethdev  |        | raw     |
    +--/------+        +---/-----+        +----/----+       +----+
      /\                __/\                  /   ..........|DrvB|
     /  \              /    \                / ../    \     +----+
 +====+ +====+    +====+ +====+            +==/=+      ```Bus Probe
 |DevA| |DevB|    |DevC| |DevD|            |DevF|
 +====+ +====+    +====+ +====+            +====+
   |      |        |      |                 |
 ``|``````|````````|``````|`````````````````|````````Bus Scan
  (PCI)   |       (PCI)  (PCI)            (PCI)
        (BusA)

* It is assumed above that DrvB is a PCI type driver which registers itself
  with PCI Bus
* Thereafter, when the PCI scan is done, during probe DrvB would match the
  rawdev DevF ID and take control of device
* Applications can then continue using the device through rawdev API
  interfaces

12.2.1. Device Identification

Physical rawdev devices are discovered during the Bus scan executed at DPDK initialization, based on their identification and probing with corresponding driver. Thus, a generic device needs to have an identifier and a driver capable of identifying it through this identifier.

Virtual devices can be created by two mechanisms, either using the EAL command line options or from within the application using an EAL API directly.

From the command line using the –vdev EAL option

--vdev 'rawdev_dev1'

Or using the rte_vdev_init API within the application code.

rte_vdev_init("rawdev_dev1", NULL)