174 lines
8.6 KiB
Plaintext
174 lines
8.6 KiB
Plaintext
The Linux RapidIO Subsystem
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The RapidIO standard is a packet-based fabric interconnect standard designed for
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use in embedded systems. Development of the RapidIO standard is directed by the
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RapidIO Trade Association (RTA). The current version of the RapidIO specification
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is publicly available for download from the RTA web-site [1].
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This document describes the basics of the Linux RapidIO subsystem and provides
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information on its major components.
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1 Overview
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----------
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Because the RapidIO subsystem follows the Linux device model it is integrated
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into the kernel similarly to other buses by defining RapidIO-specific device and
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bus types and registering them within the device model.
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The Linux RapidIO subsystem is architecture independent and therefore defines
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architecture-specific interfaces that provide support for common RapidIO
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subsystem operations.
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2. Core Components
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------------------
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A typical RapidIO network is a combination of endpoints and switches.
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Each of these components is represented in the subsystem by an associated data
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structure. The core logical components of the RapidIO subsystem are defined
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in include/linux/rio.h file.
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2.1 Master Port
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A master port (or mport) is a RapidIO interface controller that is local to the
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processor executing the Linux code. A master port generates and receives RapidIO
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packets (transactions). In the RapidIO subsystem each master port is represented
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by a rio_mport data structure. This structure contains master port specific
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resources such as mailboxes and doorbells. The rio_mport also includes a unique
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host device ID that is valid when a master port is configured as an enumerating
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host.
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RapidIO master ports are serviced by subsystem specific mport device drivers
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that provide functionality defined for this subsystem. To provide a hardware
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independent interface for RapidIO subsystem operations, rio_mport structure
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includes rio_ops data structure which contains pointers to hardware specific
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implementations of RapidIO functions.
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2.2 Device
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A RapidIO device is any endpoint (other than mport) or switch in the network.
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All devices are presented in the RapidIO subsystem by corresponding rio_dev data
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structure. Devices form one global device list and per-network device lists
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(depending on number of available mports and networks).
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2.3 Switch
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A RapidIO switch is a special class of device that routes packets between its
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ports towards their final destination. The packet destination port within a
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switch is defined by an internal routing table. A switch is presented in the
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RapidIO subsystem by rio_dev data structure expanded by additional rio_switch
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data structure, which contains switch specific information such as copy of the
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routing table and pointers to switch specific functions.
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The RapidIO subsystem defines the format and initialization method for subsystem
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specific switch drivers that are designed to provide hardware-specific
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implementation of common switch management routines.
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2.4 Network
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A RapidIO network is a combination of interconnected endpoint and switch devices.
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Each RapidIO network known to the system is represented by corresponding rio_net
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data structure. This structure includes lists of all devices and local master
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ports that form the same network. It also contains a pointer to the default
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master port that is used to communicate with devices within the network.
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3. Subsystem Initialization
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---------------------------
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In order to initialize the RapidIO subsystem, a platform must initialize and
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register at least one master port within the RapidIO network. To register mport
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within the subsystem controller driver initialization code calls function
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rio_register_mport() for each available master port. After all active master
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ports are registered with a RapidIO subsystem, the rio_init_mports() routine
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is called to perform enumeration and discovery.
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In the current PowerPC-based implementation a subsys_initcall() is specified to
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perform controller initialization and mport registration. At the end it directly
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calls rio_init_mports() to execute RapidIO enumeration and discovery.
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4. Enumeration and Discovery
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----------------------------
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When rio_init_mports() is called it scans a list of registered master ports and
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calls an enumeration or discovery routine depending on the configured role of a
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master port: host or agent.
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Enumeration is performed by a master port if it is configured as a host port by
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assigning a host device ID greater than or equal to zero. A host device ID is
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assigned to a master port through the kernel command line parameter "riohdid=",
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or can be configured in a platform-specific manner. If the host device ID for
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a specific master port is set to -1, the discovery process will be performed
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for it.
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The enumeration and discovery routines use RapidIO maintenance transactions
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to access the configuration space of devices.
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The enumeration process is implemented according to the enumeration algorithm
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outlined in the RapidIO Interconnect Specification: Annex I [1].
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The enumeration process traverses the network using a recursive depth-first
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algorithm. When a new device is found, the enumerator takes ownership of that
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device by writing into the Host Device ID Lock CSR. It does this to ensure that
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the enumerator has exclusive right to enumerate the device. If device ownership
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is successfully acquired, the enumerator allocates a new rio_dev structure and
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initializes it according to device capabilities.
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If the device is an endpoint, a unique device ID is assigned to it and its value
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is written into the device's Base Device ID CSR.
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If the device is a switch, the enumerator allocates an additional rio_switch
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structure to store switch specific information. Then the switch's vendor ID and
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device ID are queried against a table of known RapidIO switches. Each switch
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table entry contains a pointer to a switch-specific initialization routine that
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initializes pointers to the rest of switch specific operations, and performs
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hardware initialization if necessary. A RapidIO switch does not have a unique
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device ID; it relies on hopcount and routing for device ID of an attached
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endpoint if access to its configuration registers is required. If a switch (or
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chain of switches) does not have any endpoint (except enumerator) attached to
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it, a fake device ID will be assigned to configure a route to that switch.
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In the case of a chain of switches without endpoint, one fake device ID is used
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to configure a route through the entire chain and switches are differentiated by
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their hopcount value.
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For both endpoints and switches the enumerator writes a unique component tag
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into device's Component Tag CSR. That unique value is used by the error
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management notification mechanism to identify a device that is reporting an
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error management event.
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Enumeration beyond a switch is completed by iterating over each active egress
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port of that switch. For each active link, a route to a default device ID
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(0xFF for 8-bit systems and 0xFFFF for 16-bit systems) is temporarily written
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into the routing table. The algorithm recurs by calling itself with hopcount + 1
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and the default device ID in order to access the device on the active port.
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After the host has completed enumeration of the entire network it releases
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devices by clearing device ID locks (calls rio_clear_locks()). For each endpoint
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in the system, it sets the Master Enable bit in the Port General Control CSR
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to indicate that enumeration is completed and agents are allowed to execute
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passive discovery of the network.
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The discovery process is performed by agents and is similar to the enumeration
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process that is described above. However, the discovery process is performed
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without changes to the existing routing because agents only gather information
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about RapidIO network structure and are building an internal map of discovered
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devices. This way each Linux-based component of the RapidIO subsystem has
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a complete view of the network. The discovery process can be performed
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simultaneously by several agents. After initializing its RapidIO master port
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each agent waits for enumeration completion by the host for the configured wait
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time period. If this wait time period expires before enumeration is completed,
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an agent skips RapidIO discovery and continues with remaining kernel
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initialization.
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5. References
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-------------
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[1] RapidIO Trade Association. RapidIO Interconnect Specifications.
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http://www.rapidio.org.
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[2] Rapidio TA. Technology Comparisons.
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http://www.rapidio.org/education/technology_comparisons/
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[3] RapidIO support for Linux.
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http://lwn.net/Articles/139118/
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[4] Matt Porter. RapidIO for Linux. Ottawa Linux Symposium, 2005
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http://www.kernel.org/doc/ols/2005/ols2005v2-pages-43-56.pdf
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