1 The Linux RapidIO Subsystem
3 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5 The RapidIO standard is a packet-based fabric interconnect standard designed for
6 use in embedded systems. Development of the RapidIO standard is directed by the
7 RapidIO Trade Association (RTA). The current version of the RapidIO specification
8 is publicly available for download from the RTA web-site [1].
10 This document describes the basics of the Linux RapidIO subsystem and provides
11 information on its major components.
16 Because the RapidIO subsystem follows the Linux device model it is integrated
17 into the kernel similarly to other buses by defining RapidIO-specific device and
18 bus types and registering them within the device model.
20 The Linux RapidIO subsystem is architecture independent and therefore defines
21 architecture-specific interfaces that provide support for common RapidIO
27 A typical RapidIO network is a combination of endpoints and switches.
28 Each of these components is represented in the subsystem by an associated data
29 structure. The core logical components of the RapidIO subsystem are defined
30 in include/linux/rio.h file.
34 A master port (or mport) is a RapidIO interface controller that is local to the
35 processor executing the Linux code. A master port generates and receives RapidIO
36 packets (transactions). In the RapidIO subsystem each master port is represented
37 by a rio_mport data structure. This structure contains master port specific
38 resources such as mailboxes and doorbells. The rio_mport also includes a unique
39 host device ID that is valid when a master port is configured as an enumerating
42 RapidIO master ports are serviced by subsystem specific mport device drivers
43 that provide functionality defined for this subsystem. To provide a hardware
44 independent interface for RapidIO subsystem operations, rio_mport structure
45 includes rio_ops data structure which contains pointers to hardware specific
46 implementations of RapidIO functions.
50 A RapidIO device is any endpoint (other than mport) or switch in the network.
51 All devices are presented in the RapidIO subsystem by corresponding rio_dev data
52 structure. Devices form one global device list and per-network device lists
53 (depending on number of available mports and networks).
57 A RapidIO switch is a special class of device that routes packets between its
58 ports towards their final destination. The packet destination port within a
59 switch is defined by an internal routing table. A switch is presented in the
60 RapidIO subsystem by rio_dev data structure expanded by additional rio_switch
61 data structure, which contains switch specific information such as copy of the
62 routing table and pointers to switch specific functions.
64 The RapidIO subsystem defines the format and initialization method for subsystem
65 specific switch drivers that are designed to provide hardware-specific
66 implementation of common switch management routines.
70 A RapidIO network is a combination of interconnected endpoint and switch devices.
71 Each RapidIO network known to the system is represented by corresponding rio_net
72 data structure. This structure includes lists of all devices and local master
73 ports that form the same network. It also contains a pointer to the default
74 master port that is used to communicate with devices within the network.
76 3. Subsystem Initialization
77 ---------------------------
79 In order to initialize the RapidIO subsystem, a platform must initialize and
80 register at least one master port within the RapidIO network. To register mport
81 within the subsystem controller driver initialization code calls function
82 rio_register_mport() for each available master port. After all active master
83 ports are registered with a RapidIO subsystem, the rio_init_mports() routine
84 is called to perform enumeration and discovery.
86 In the current PowerPC-based implementation a subsys_initcall() is specified to
87 perform controller initialization and mport registration. At the end it directly
88 calls rio_init_mports() to execute RapidIO enumeration and discovery.
90 4. Enumeration and Discovery
91 ----------------------------
93 When rio_init_mports() is called it scans a list of registered master ports and
94 calls an enumeration or discovery routine depending on the configured role of a
95 master port: host or agent.
97 Enumeration is performed by a master port if it is configured as a host port by
98 assigning a host device ID greater than or equal to zero. A host device ID is
99 assigned to a master port through the kernel command line parameter "riohdid=",
100 or can be configured in a platform-specific manner. If the host device ID for
101 a specific master port is set to -1, the discovery process will be performed
104 The enumeration and discovery routines use RapidIO maintenance transactions
105 to access the configuration space of devices.
107 The enumeration process is implemented according to the enumeration algorithm
108 outlined in the RapidIO Interconnect Specification: Annex I [1].
110 The enumeration process traverses the network using a recursive depth-first
111 algorithm. When a new device is found, the enumerator takes ownership of that
112 device by writing into the Host Device ID Lock CSR. It does this to ensure that
113 the enumerator has exclusive right to enumerate the device. If device ownership
114 is successfully acquired, the enumerator allocates a new rio_dev structure and
115 initializes it according to device capabilities.
117 If the device is an endpoint, a unique device ID is assigned to it and its value
118 is written into the device's Base Device ID CSR.
120 If the device is a switch, the enumerator allocates an additional rio_switch
121 structure to store switch specific information. Then the switch's vendor ID and
122 device ID are queried against a table of known RapidIO switches. Each switch
123 table entry contains a pointer to a switch-specific initialization routine that
124 initializes pointers to the rest of switch specific operations, and performs
125 hardware initialization if necessary. A RapidIO switch does not have a unique
126 device ID; it relies on hopcount and routing for device ID of an attached
127 endpoint if access to its configuration registers is required. If a switch (or
128 chain of switches) does not have any endpoint (except enumerator) attached to
129 it, a fake device ID will be assigned to configure a route to that switch.
130 In the case of a chain of switches without endpoint, one fake device ID is used
131 to configure a route through the entire chain and switches are differentiated by
132 their hopcount value.
134 For both endpoints and switches the enumerator writes a unique component tag
135 into device's Component Tag CSR. That unique value is used by the error
136 management notification mechanism to identify a device that is reporting an
137 error management event.
139 Enumeration beyond a switch is completed by iterating over each active egress
140 port of that switch. For each active link, a route to a default device ID
141 (0xFF for 8-bit systems and 0xFFFF for 16-bit systems) is temporarily written
142 into the routing table. The algorithm recurs by calling itself with hopcount + 1
143 and the default device ID in order to access the device on the active port.
145 After the host has completed enumeration of the entire network it releases
146 devices by clearing device ID locks (calls rio_clear_locks()). For each endpoint
147 in the system, it sets the Master Enable bit in the Port General Control CSR
148 to indicate that enumeration is completed and agents are allowed to execute
149 passive discovery of the network.
151 The discovery process is performed by agents and is similar to the enumeration
152 process that is described above. However, the discovery process is performed
153 without changes to the existing routing because agents only gather information
154 about RapidIO network structure and are building an internal map of discovered
155 devices. This way each Linux-based component of the RapidIO subsystem has
156 a complete view of the network. The discovery process can be performed
157 simultaneously by several agents. After initializing its RapidIO master port
158 each agent waits for enumeration completion by the host for the configured wait
159 time period. If this wait time period expires before enumeration is completed,
160 an agent skips RapidIO discovery and continues with remaining kernel
166 [1] RapidIO Trade Association. RapidIO Interconnect Specifications.
167 http://www.rapidio.org.
168 [2] Rapidio TA. Technology Comparisons.
169 http://www.rapidio.org/education/technology_comparisons/
170 [3] RapidIO support for Linux.
171 http://lwn.net/Articles/139118/
172 [4] Matt Porter. RapidIO for Linux. Ottawa Linux Symposium, 2005
173 http://www.kernel.org/doc/ols/2005/ols2005v2-pages-43-56.pdf