Documentation/driver-api/rapidio/rapidio.rst - kernel/common - Git at Google

 ============
 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
	============
	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