bus: fsl-mc: dpio: add DPIO driver overview document

add document describing the dpio driver and it's role, components
and major interfaces
Signed-off-by: Stuart Yoder <stuart.yoder@nxp.com>
Signed-off-by: Roy Pledge <roy.pledge@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fsl-mc/bus/dpio/dpio-driver.txt

+Copyright 2016 NXP
+
+Introduction
+------------
+
+A DPAA2 DPIO (Data Path I/O) is a hardware object that provides
+interfaces to enqueue and dequeue frames to/from network interfaces
+and other accelerators.  A DPIO also provides hardware buffer
+pool management for network interfaces.
+
+This document provides an overview the Linux DPIO driver, its
+subcomponents, and its APIs.
+
+See Documentation/dpaa2/overview.txt for a general overview of DPAA2
+and the general DPAA2 driver architecture in Linux.
+
+Driver Overview
+---------------
+
+The DPIO driver is bound to DPIO objects discovered on the fsl-mc bus and
+provides services that:
+  A) allow other drivers, such as the Ethernet driver, to enqueue and dequeue
+     frames for their respective objects
+  B) allow drivers to register callbacks for data availability notifications
+     when data becomes available on a queue or channel
+  C) allow drivers to manage hardware buffer pools
+
+The Linux DPIO driver consists of 3 primary components--
+   DPIO object driver-- fsl-mc driver that manages the DPIO object
+   DPIO service-- provides APIs to other Linux drivers for services
+   QBman portal interface-- sends portal commands, gets responses
+
+          fsl-mc          other
+           bus           drivers
+            |               |
+        +---+----+   +------+-----+
+        |DPIO obj|   |DPIO service|
+        | driver |---|  (DPIO)    |
+        +--------+   +------+-----+
+                            |
+                     +------+-----+
+                     |    QBman   |
+                     | portal i/f |
+                     +------------+
+                            |
+                         hardware
+
+The diagram below shows how the DPIO driver components fit with the other
+DPAA2 Linux driver components:
+                                                   +------------+
+                                                   | OS Network |
+                                                   |   Stack    |
+                 +------------+                    +------------+
+                 | Allocator  |. . . . . . .       |  Ethernet  |
+                 |(DPMCP,DPBP)|                    |   (DPNI)   |
+                 +-.----------+                    +---+---+----+
+                  .          .                         ^   |
+                 .            .           <data avail, |   |<enqueue,
+                .              .           tx confirm> |   | dequeue>
+    +-------------+             .                      |   |
+    | DPRC driver |              .    +--------+ +------------+
+    |   (DPRC)    |               . . |DPIO obj| |DPIO service|
+    +----------+--+                   | driver |-|  (DPIO)    |
+               |                      +--------+ +------+-----+
+               |<dev add/remove>                 +------|-----+
+               |                                 |   QBman    |
+          +----+--------------+                  | portal i/f |
+          |   MC-bus driver   |                  +------------+
+          |                   |                     |
+          | /soc/fsl-mc       |                     |
+          +-------------------+                     |
+                                                    |
+ =========================================|=========|========================
+                                        +-+--DPIO---|-----------+
+                                        |           |           |
+                                        |        QBman Portal   |
+                                        +-----------------------+
+
+ ============================================================================
+
+
+DPIO Object Driver (dpio-driver.c)
+----------------------------------
+
+   The dpio-driver component registers with the fsl-mc bus to handle objects of
+   type "dpio".  The implementation of probe() handles basic initialization
+   of the DPIO including mapping of the DPIO regions (the QBman SW portal)
+   and initializing interrupts and registering irq handlers.  The dpio-driver
+   registers the probed DPIO with dpio-service.
+
+DPIO service  (dpio-service.c, dpaa2-io.h)
+------------------------------------------
+
+   The dpio service component provides queuing, notification, and buffers
+   management services to DPAA2 drivers, such as the Ethernet driver.  A system
+   will typically allocate 1 DPIO object per CPU to allow queuing operations
+   to happen simultaneously across all CPUs.
+
+   Notification handling
+      dpaa2_io_service_register()
+      dpaa2_io_service_deregister()
+      dpaa2_io_service_rearm()
+
+   Queuing
+      dpaa2_io_service_pull_fq()
+      dpaa2_io_service_pull_channel()
+      dpaa2_io_service_enqueue_fq()
+      dpaa2_io_service_enqueue_qd()
+      dpaa2_io_store_create()
+      dpaa2_io_store_destroy()
+      dpaa2_io_store_next()
+
+   Buffer pool management
+      dpaa2_io_service_release()
+      dpaa2_io_service_acquire()
+
+QBman portal interface (qbman-portal.c)
+---------------------------------------
+
+   The qbman-portal component provides APIs to do the low level hardware
+   bit twiddling for operations such as:
+      -initializing Qman software portals
+      -building and sending portal commands
+      -portal interrupt configuration and processing
+
+   The qbman-portal APIs are not public to other drivers, and are
+   only used by dpio-service.
+
+Other (dpaa2-fd.h, dpaa2-global.h)
+----------------------------------
+
+   Frame descriptor and scatter-gather definitions and the APIs used to
+   manipulate them are defined in dpaa2-fd.h.
+
+   Dequeue result struct and parsing APIs are defined in dpaa2-global.h.