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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+============
+Introduction
+============
+
+The RapidIO standard is a packet-based fabric interconnect standard designed for
+use in embedded systems. Development of the RapidIO standard is directed by the
+RapidIO Trade Association (RTA). The current version of the RapidIO specification
+is publicly available for download from the RTA web-site [1].
+
+This document describes the basics of the Linux RapidIO subsystem and provides
+information on its major components.
+
+1 Overview
+==========
+
+Because the RapidIO subsystem follows the Linux device model it is integrated
+into the kernel similarly to other buses by defining RapidIO-specific device and
+bus types and registering them within the device model.
+
+The Linux RapidIO subsystem is architecture independent and therefore defines
+architecture-specific interfaces that provide support for common RapidIO
+subsystem operations.
+
+2. Core Components
+==================
+
+A typical RapidIO network is a combination of endpoints and switches.
+Each of these components is represented in the subsystem by an associated data
+structure. The core logical components of the RapidIO subsystem are defined
+in include/linux/rio.h file.
+
+2.1 Master Port
+---------------
+
+A master port (or mport) is a RapidIO interface controller that is local to the
+processor executing the Linux code. A master port generates and receives RapidIO
+packets (transactions). In the RapidIO subsystem each master port is represented
+by a rio_mport data structure. This structure contains master port specific
+resources such as mailboxes and doorbells. The rio_mport also includes a unique
+host device ID that is valid when a master port is configured as an enumerating
+host.
+
+RapidIO master ports are serviced by subsystem specific mport device drivers
+that provide functionality defined for this subsystem. To provide a hardware
+independent interface for RapidIO subsystem operations, rio_mport structure
+includes rio_ops data structure which contains pointers to hardware specific
+implementations of RapidIO functions.
+
+2.2 Device
+----------
+
+A RapidIO device is any endpoint (other than mport) or switch in the network.
+All devices are presented in the RapidIO subsystem by corresponding rio_dev data
+structure. Devices form one global device list and per-network device lists
+(depending on number of available mports and networks).
+
+2.3 Switch
+----------
+
+A RapidIO switch is a special class of device that routes packets between its
+ports towards their final destination. The packet destination port within a
+switch is defined by an internal routing table. A switch is presented in the
+RapidIO subsystem by rio_dev data structure expanded by additional rio_switch
+data structure, which contains switch specific information such as copy of the
+routing table and pointers to switch specific functions.
+
+The RapidIO subsystem defines the format and initialization method for subsystem
+specific switch drivers that are designed to provide hardware-specific
+implementation of common switch management routines.
+
+2.4 Network
+-----------
+
+A RapidIO network is a combination of interconnected endpoint and switch devices.
+Each RapidIO network known to the system is represented by corresponding rio_net
+data structure. This structure includes lists of all devices and local master
+ports that form the same network. It also contains a pointer to the default
+master port that is used to communicate with devices within the network.
+
+2.5 Device Drivers
+------------------
+
+RapidIO device-specific drivers follow Linux Kernel Driver Model and are
+intended to support specific RapidIO devices attached to the RapidIO network.
+
+2.6 Subsystem Interfaces
+------------------------
+
+RapidIO interconnect specification defines features that may be used to provide
+one or more common service layers for all participating RapidIO devices. These
+common services may act separately from device-specific drivers or be used by
+device-specific drivers. Example of such service provider is the RIONET driver
+which implements Ethernet-over-RapidIO interface. Because only one driver can be
+registered for a device, all common RapidIO services have to be registered as
+subsystem interfaces. This allows to have multiple common services attached to
+the same device without blocking attachment of a device-specific driver.
+
+3. Subsystem Initialization
+===========================
+
+In order to initialize the RapidIO subsystem, a platform must initialize and
+register at least one master port within the RapidIO network. To register mport
+within the subsystem controller driver's initialization code calls function
+rio_register_mport() for each available master port.
+
+After all active master ports are registered with a RapidIO subsystem,
+an enumeration and/or discovery routine may be called automatically or
+by user-space command.
+
+RapidIO subsystem can be configured to be built as a statically linked or
+modular component of the kernel (see details below).
+
+4. Enumeration and Discovery
+============================
+
+4.1 Overview
+------------
+
+RapidIO subsystem configuration options allow users to build enumeration and
+discovery methods as statically linked components or loadable modules.
+An enumeration/discovery method implementation and available input parameters
+define how any given method can be attached to available RapidIO mports:
+simply to all available mports OR individually to the specified mport device.
+
+Depending on selected enumeration/discovery build configuration, there are
+several methods to initiate an enumeration and/or discovery process:
+
+  (a) Statically linked enumeration and discovery process can be started
+  automatically during kernel initialization time using corresponding module
+  parameters. This was the original method used since introduction of RapidIO
+  subsystem. Now this method relies on enumerator module parameter which is
+  'rio-scan.scan' for existing basic enumeration/discovery method.
+  When automatic start of enumeration/discovery is used a user has to ensure
+  that all discovering endpoints are started before the enumerating endpoint
+  and are waiting for enumeration to be completed.
+  Configuration option CONFIG_RAPIDIO_DISC_TIMEOUT defines time that discovering
+  endpoint waits for enumeration to be completed. If the specified timeout
+  expires the discovery process is terminated without obtaining RapidIO network
+  information. NOTE: a timed out discovery process may be restarted later using
+  a user-space command as it is described below (if the given endpoint was
+  enumerated successfully).
+
+  (b) Statically linked enumeration and discovery process can be started by
+  a command from user space. This initiation method provides more flexibility
+  for a system startup compared to the option (a) above. After all participating
+  endpoints have been successfully booted, an enumeration process shall be
+  started first by issuing a user-space command, after an enumeration is
+  completed a discovery process can be started on all remaining endpoints.
+
+  (c) Modular enumeration and discovery process can be started by a command from
+  user space. After an enumeration/discovery module is loaded, a network scan
+  process can be started by issuing a user-space command.
+  Similar to the option (b) above, an enumerator has to be started first.
+
+  (d) Modular enumeration and discovery process can be started by a module
+  initialization routine. In this case an enumerating module shall be loaded
+  first.
+
+When a network scan process is started it calls an enumeration or discovery
+routine depending on the configured role of a master port: host or agent.
+
+Enumeration is performed by a master port if it is configured as a host port by
+assigning a host destination ID greater than or equal to zero. The host
+destination ID can be assigned to a master port using various methods depending
+on RapidIO subsystem build configuration:
+
+  (a) For a statically linked RapidIO subsystem core use command line parameter
+  "rapidio.hdid=" with a list of destination ID assignments in order of mport
+  device registration. For example, in a system with two RapidIO controllers
+  the command line parameter "rapidio.hdid=-1,7" will result in assignment of
+  the host destination ID=7 to the second RapidIO controller, while the first
+  one will be assigned destination ID=-1.
+
+  (b) If the RapidIO subsystem core is built as a loadable module, in addition
+  to the method shown above, the host destination ID(s) can be specified using
+  traditional methods of passing module parameter "hdid=" during its loading:
+
+  - from command line: "modprobe rapidio hdid=-1,7", or
+  - from modprobe configuration file using configuration command "options",
+    like in this example: "options rapidio hdid=-1,7". An example of modprobe
+    configuration file is provided in the section below.
+
+NOTES:
+  (i) if "hdid=" parameter is omitted all available mport will be assigned
+  destination ID = -1;
+
+  (ii) the "hdid=" parameter in systems with multiple mports can have
+  destination ID assignments omitted from the end of list (default = -1).
+
+If the host device ID for a specific master port is set to -1, the discovery
+process will be performed for it.
+
+The enumeration and discovery routines use RapidIO maintenance transactions
+to access the configuration space of devices.
+
+NOTE: If RapidIO switch-specific device drivers are built as loadable modules
+they must be loaded before enumeration/discovery process starts.
+This requirement is cased by the fact that enumeration/discovery methods invoke
+vendor-specific callbacks on early stages.
+
+4.2 Automatic Start of Enumeration and Discovery
+------------------------------------------------
+
+Automatic enumeration/discovery start method is applicable only to built-in
+enumeration/discovery RapidIO configuration selection. To enable automatic
+enumeration/discovery start by existing basic enumerator method set use boot
+command line parameter "rio-scan.scan=1".
+
+This configuration requires synchronized start of all RapidIO endpoints that
+form a network which will be enumerated/discovered. Discovering endpoints have
+to be started before an enumeration starts to ensure that all RapidIO
+controllers have been initialized and are ready to be discovered. Configuration
+parameter CONFIG_RAPIDIO_DISC_TIMEOUT defines time (in seconds) which
+a discovering endpoint will wait for enumeration to be completed.
+
+When automatic enumeration/discovery start is selected, basic method's
+initialization routine calls rio_init_mports() to perform enumeration or
+discovery for all known mport devices.
+
+Depending on RapidIO network size and configuration this automatic
+enumeration/discovery start method may be difficult to use due to the
+requirement for synchronized start of all endpoints.
+
+4.3 User-space Start of Enumeration and Discovery
+-------------------------------------------------
+
+User-space start of enumeration and discovery can be used with built-in and
+modular build configurations. For user-space controlled start RapidIO subsystem
+creates the sysfs write-only attribute file '/sys/bus/rapidio/scan'. To initiate
+an enumeration or discovery process on specific mport device, a user needs to
+write mport_ID (not RapidIO destination ID) into that file. The mport_ID is a
+sequential number (0 ... RIO_MAX_MPORTS) assigned during mport device
+registration. For example for machine with single RapidIO controller, mport_ID
+for that controller always will be 0.
+
+To initiate RapidIO enumeration/discovery on all available mports a user may
+write '-1' (or RIO_MPORT_ANY) into the scan attribute file.
+
+4.4 Basic Enumeration Method
+----------------------------
+
+This is an original enumeration/discovery method which is available since
+first release of RapidIO subsystem code. The enumeration process is
+implemented according to the enumeration algorithm outlined in the RapidIO
+Interconnect Specification: Annex I [1].
+
+This method can be configured as statically linked or loadable module.
+The method's single parameter "scan" allows to trigger the enumeration/discovery
+process from module initialization routine.
+
+This enumeration/discovery method can be started only once and does not support
+unloading if it is built as a module.
+
+The enumeration process traverses the network using a recursive depth-first
+algorithm. When a new device is found, the enumerator takes ownership of that
+device by writing into the Host Device ID Lock CSR. It does this to ensure that
+the enumerator has exclusive right to enumerate the device. If device ownership
+is successfully acquired, the enumerator allocates a new rio_dev structure and
+initializes it according to device capabilities.
+
+If the device is an endpoint, a unique device ID is assigned to it and its value
+is written into the device's Base Device ID CSR.
+
+If the device is a switch, the enumerator allocates an additional rio_switch
+structure to store switch specific information. Then the switch's vendor ID and
+device ID are queried against a table of known RapidIO switches. Each switch
+table entry contains a pointer to a switch-specific initialization routine that
+initializes pointers to the rest of switch specific operations, and performs
+hardware initialization if necessary. A RapidIO switch does not have a unique
+device ID; it relies on hopcount and routing for device ID of an attached
+endpoint if access to its configuration registers is required. If a switch (or
+chain of switches) does not have any endpoint (except enumerator) attached to
+it, a fake device ID will be assigned to configure a route to that switch.
+In the case of a chain of switches without endpoint, one fake device ID is used
+to configure a route through the entire chain and switches are differentiated by
+their hopcount value.
+
+For both endpoints and switches the enumerator writes a unique component tag
+into device's Component Tag CSR. That unique value is used by the error
+management notification mechanism to identify a device that is reporting an
+error management event.
+
+Enumeration beyond a switch is completed by iterating over each active egress
+port of that switch. For each active link, a route to a default device ID
+(0xFF for 8-bit systems and 0xFFFF for 16-bit systems) is temporarily written
+into the routing table. The algorithm recurs by calling itself with hopcount + 1
+and the default device ID in order to access the device on the active port.
+
+After the host has completed enumeration of the entire network it releases
+devices by clearing device ID locks (calls rio_clear_locks()). For each endpoint
+in the system, it sets the Discovered bit in the Port General Control CSR
+to indicate that enumeration is completed and agents are allowed to execute
+passive discovery of the network.
+
+The discovery process is performed by agents and is similar to the enumeration
+process that is described above. However, the discovery process is performed
+without changes to the existing routing because agents only gather information
+about RapidIO network structure and are building an internal map of discovered
+devices. This way each Linux-based component of the RapidIO subsystem has
+a complete view of the network. The discovery process can be performed
+simultaneously by several agents. After initializing its RapidIO master port
+each agent waits for enumeration completion by the host for the configured wait
+time period. If this wait time period expires before enumeration is completed,
+an agent skips RapidIO discovery and continues with remaining kernel
+initialization.
+
+4.5 Adding New Enumeration/Discovery Method
+-------------------------------------------
+
+RapidIO subsystem code organization allows addition of new enumeration/discovery
+methods as new configuration options without significant impact to the core
+RapidIO code.
+
+A new enumeration/discovery method has to be attached to one or more mport
+devices before an enumeration/discovery process can be started. Normally,
+method's module initialization routine calls rio_register_scan() to attach
+an enumerator to a specified mport device (or devices). The basic enumerator
+implementation demonstrates this process.
+
+4.6 Using Loadable RapidIO Switch Drivers
+-----------------------------------------
+
+In the case when RapidIO switch drivers are built as loadable modules a user
+must ensure that they are loaded before the enumeration/discovery starts.
+This process can be automated by specifying pre- or post- dependencies in the
+RapidIO-specific modprobe configuration file as shown in the example below.
+
+File /etc/modprobe.d/rapidio.conf::
+
+  # Configure RapidIO subsystem modules
+
+  # Set enumerator host destination ID (overrides kernel command line option)
+  options rapidio hdid=-1,2
+
+  # Load RapidIO switch drivers immediately after rapidio core module was loaded
+  softdep rapidio post: idt_gen2 idtcps tsi57x
+
+  # OR :
+
+  # Load RapidIO switch drivers just before rio-scan enumerator module is loaded
+  softdep rio-scan pre: idt_gen2 idtcps tsi57x
+
+  --------------------------
+
+NOTE:
+  In the example above, one of "softdep" commands must be removed or
+  commented out to keep required module loading sequence.
+
+5. References
+=============
+
+[1] RapidIO Trade Association. RapidIO Interconnect Specifications.
+    http://www.rapidio.org.
+
+[2] Rapidio TA. Technology Comparisons.
+    http://www.rapidio.org/education/technology_comparisons/
+
+[3] RapidIO support for Linux.
+    https://lwn.net/Articles/139118/
+
+[4] Matt Porter. RapidIO for Linux. Ottawa Linux Symposium, 2005
+    https://www.kernel.org/doc/ols/2005/ols2005v2-pages-43-56.pdf