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+                          The Linux RapidIO Subsystem
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+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.
+
+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 initialization code calls function
+rio_register_mport() for each available master port. After all active master
+ports are registered with a RapidIO subsystem, the rio_init_mports() routine
+is called to perform enumeration and discovery.
+
+In the current PowerPC-based implementation a subsys_initcall() is specified to
+perform controller initialization and mport registration. At the end it directly
+calls rio_init_mports() to execute RapidIO enumeration and discovery.
+
+4. Enumeration and Discovery
+----------------------------
+
+When rio_init_mports() is called it scans a list of registered master ports and
+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 device ID greater than or equal to zero. A host device ID is
+assigned to a master port through the kernel command line parameter "riohdid=",
+or can be configured in a platform-specific manner. 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.
+
+The enumeration process is implemented according to the enumeration algorithm
+outlined in the RapidIO Interconnect Specification: Annex I [1].
+
+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 Master Enable 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.
+
+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.
+    http://lwn.net/Articles/139118/
+[4] Matt Porter. RapidIO for Linux. Ottawa Linux Symposium, 2005
+    http://www.kernel.org/doc/ols/2005/ols2005v2-pages-43-56.pdf