1 .. SPDX-License-Identifier: GPL-2.0 2 3 ===================================== 4 Generic System Interconnect Subsystem 5 ===================================== 6 7 Introduction 8 ------------ 9 10 This framework is designed to provide a standard kernel interface to control 11 the settings of the interconnects on an SoC. These settings can be throughput, 12 latency and priority between multiple interconnected devices or functional 13 blocks. This can be controlled dynamically in order to save power or provide 14 maximum performance. 15 16 The interconnect bus is hardware with configurable parameters, which can be 17 set on a data path according to the requests received from various drivers. 18 An example of interconnect buses are the interconnects between various 19 components or functional blocks in chipsets. There can be multiple interconnects 20 on an SoC that can be multi-tiered. 21 22 Below is a simplified diagram of a real-world SoC interconnect bus topology. 23 24 :: 25 26 +----------------+ +----------------+ 27 | HW Accelerator |--->| M NoC |<---------------+ 28 +----------------+ +----------------+ | 29 | | +------------+ 30 +-----+ +-------------+ V +------+ | | 31 | DDR | | +--------+ | PCIe | | | 32 +-----+ | | Slaves | +------+ | | 33 ^ ^ | +--------+ | | C NoC | 34 | | V V | | 35 +------------------+ +------------------------+ | | +-----+ 36 | |-->| |-->| |-->| CPU | 37 | |-->| |<--| | +-----+ 38 | Mem NoC | | S NoC | +------------+ 39 | |<--| |---------+ | 40 | |<--| |<------+ | | +--------+ 41 +------------------+ +------------------------+ | | +-->| Slaves | 42 ^ ^ ^ ^ ^ | | +--------+ 43 | | | | | | V 44 +------+ | +-----+ +-----+ +---------+ +----------------+ +--------+ 45 | CPUs | | | GPU | | DSP | | Masters |-->| P NoC |-->| Slaves | 46 +------+ | +-----+ +-----+ +---------+ +----------------+ +--------+ 47 | 48 +-------+ 49 | Modem | 50 +-------+ 51 52 Terminology 53 ----------- 54 55 Interconnect provider is the software definition of the interconnect hardware. 56 The interconnect providers on the above diagram are M NoC, S NoC, C NoC, P NoC 57 and Mem NoC. 58 59 Interconnect node is the software definition of the interconnect hardware 60 port. Each interconnect provider consists of multiple interconnect nodes, 61 which are connected to other SoC components including other interconnect 62 providers. The point on the diagram where the CPUs connect to the memory is 63 called an interconnect node, which belongs to the Mem NoC interconnect provider. 64 65 Interconnect endpoints are the first or the last element of the path. Every 66 endpoint is a node, but not every node is an endpoint. 67 68 Interconnect path is everything between two endpoints including all the nodes 69 that have to be traversed to reach from a source to destination node. It may 70 include multiple master-slave pairs across several interconnect providers. 71 72 Interconnect consumers are the entities which make use of the data paths exposed 73 by the providers. The consumers send requests to providers requesting various 74 throughput, latency and priority. Usually the consumers are device drivers, that 75 send request based on their needs. An example for a consumer is a video decoder 76 that supports various formats and image sizes. 77 78 Interconnect providers 79 ---------------------- 80 81 Interconnect provider is an entity that implements methods to initialize and 82 configure interconnect bus hardware. The interconnect provider drivers should 83 be registered with the interconnect provider core. 84 85 .. kernel-doc:: include/linux/interconnect-provider.h 86 87 Interconnect consumers 88 ---------------------- 89 90 Interconnect consumers are the clients which use the interconnect APIs to 91 get paths between endpoints and set their bandwidth/latency/QoS requirements 92 for these interconnect paths. These interfaces are not currently 93 documented. 94 95 Interconnect debugfs interfaces 96 ------------------------------- 97 98 Like several other subsystems interconnect will create some files for debugging 99 and introspection. Files in debugfs are not considered ABI so application 100 software shouldn't rely on format details change between kernel versions. 101 102 ``/sys/kernel/debug/interconnect/interconnect_summary``: 103 104 Show all interconnect nodes in the system with their aggregated bandwidth 105 request. Indented under each node show bandwidth requests from each device. 106 107 ``/sys/kernel/debug/interconnect/interconnect_graph``: 108 109 Show the interconnect graph in the graphviz dot format. It shows all 110 interconnect nodes and links in the system and groups together nodes from the 111 same provider as subgraphs. The format is human-readable and can also be piped 112 through dot to generate diagrams in many graphical formats:: 113 114 $ cat /sys/kernel/debug/interconnect/interconnect_graph | \ 115 dot -Tsvg > interconnect_graph.svg 116 117 The ``test-client`` directory provides interfaces for issuing BW requests to 118 any arbitrary path. Note that for safety reasons, this feature is disabled by 119 default without a Kconfig to enable it. Enabling it requires code changes to 120 ``#define INTERCONNECT_ALLOW_WRITE_DEBUGFS``. Example usage:: 121 122 cd /sys/kernel/debug/interconnect/test-client/ 123 124 # Configure node endpoints for the path from CPU to DDR on 125 # qcom/sm8550. 126 echo chm_apps > src_node 127 echo ebi > dst_node 128 129 # Get path between src_node and dst_node. This is only 130 # necessary after updating the node endpoints. 131 echo 1 > get 132 133 # Set desired BW to 1GBps avg and 2GBps peak. 134 echo 1000000 > avg_bw 135 echo 2000000 > peak_bw 136 137 # Vote for avg_bw and peak_bw on the latest path from "get". 138 # Voting for multiple paths is possible by repeating this 139 # process for different nodes endpoints. 140 echo 1 > commit
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