1.. _arm_scmi: 2 3ARM System Control and Management Interface 4########################################### 5 6Overview 7******** 8 9What is SCMI? 10************* 11 12System Control and Management Interface (SCMI) is a specification developed by 13ARM, which describes a set of OS-agnostic software interfaces used to perform 14system management (e.g: clock control, pinctrl, etc...). 15 16 17Agent, platform, protocol and transport 18*************************************** 19 20The SCMI specification defines **four** key terms, which will also be used throughout 21this documentation: 22 23 #. Agent 24 Entity that performs SCMI requests (e.g: gating a clock or configuring 25 a pin). In this context, Zephyr itself is an agent. 26 #. Platform 27 This refers to a set of hardware components that handle the requests from 28 agents and provide the necessary functionality. In some cases, the requests 29 are handled by a firmware, running on a core dedicated to performing system 30 management tasks. 31 #. Protocol 32 A protocol is a set of messages grouped by functionality. Intuitively, a message 33 can be thought of as a remote procedure call. 34 35 The SCMI specification defines ten standard protocols: 36 37 #. **Base** (0x10) 38 #. **Power domain management** (0x11) 39 #. **System power management** (0x12) 40 #. **Performance domain management** (0x13) 41 #. **Clock management** (0x14) 42 #. **Sensor management** (0x15) 43 #. **Reset domain management** (0x16) 44 #. **Voltage domain management** (0x17) 45 #. **Power capping and monitoring** (0x18) 46 #. **Pin Control** (0x19) 47 48 where each of these protocols is identified by an unique protocol ID 49 (listed between brackets). 50 51 Apart from the standard protocols, the SCMI specification reserves the 52 **0x80-0xFF** protocol ID range for vendor-specific protocols. 53 54 55 #. Transport 56 This describes how messages are exchanged between agents and the platform. 57 The communication itself happens through channels. 58 59.. note:: 60 A system may have more than one agent. 61 62Channels 63******** 64 65A **channel** is the medium through which agents and the platform exchange messages. 66The structure of a channel and the way it works is solely dependent on the transport. 67 68Each agent has its own distinct set of channels, meaning some channel A cannot be used 69by two different agents for example. 70 71Channels are **bidirectional** (exception: FastChannels), and, depending on which entity 72initiates the communication, can be one of **two** types: 73 74 #. A2P (agent to platform) 75 The agent is the initiator/requester. The messages passed through these 76 channels are known as **commands**. 77 #. P2A (platform to agent) 78 The platform is the initiator/requester. 79 80Messages 81******** 82 83The SCMI specification defines **four** types of messages: 84 85 #. Synchronous 86 These are commands that block until the platform has completed the 87 requested work and are sent over A2P channels. 88 #. Asynchronous 89 For these commands, the platform schedules the requested work to 90 be performed at a later time. As such, they return almost immediately. 91 These commands are sent over A2P channels. 92 #. Delayed response 93 These messages indicate the completion of the work associated 94 with an asynchronous command. These are sent over P2A channels. 95 96 #. Notification 97 These messages are used to notify agents of events that take place on 98 the platform. These are sent over P2A channels. 99 100The Zephyr support for SCMI is based on the documentation provided by ARM: 101`DEN0056E <https://developer.arm.com/documentation/den0056/latest/>`_. For more details 102on the specification, the readers are encouraged to have a look at it. 103 104SCMI support in Zephyr 105********************** 106 107Shared memory and doorbell-based transport 108****************************************** 109 110This form of transport uses shared memory for reading/writing messages 111and doorbells for signaling. The interaction with the shared 112memory area is performed using a driver (:file:`drivers/firmware/scmi/shmem.c`), 113which offers a set of functions for this exact purpose. Furthermore, 114signaling is performed using the Zephyr MBOX API (signaling mode only, no message passing). 115 116Interacting with the shared memory area and signaling are abstracted by the 117transport API, which is implemented by the shared memory and doorbell-based 118transport driver (:file:`drivers/firmware/scmi/mailbox.c`). 119 120The steps below exemplify how the communication between the Zephyr agent 121and the platform may happen using this transport: 122 123 #. Write message to the shared memory area. 124 #. Zephyr rings request doorbell. If in ``PRE_KERNEL_1`` or ``PRE_KERNEL_2`` phase start polling for reply, otherwise wait for reply doorbell ring. 125 #. Platform reads message from shared memory area, processes it, writes the reply back to the same area and rings the reply doorbell. 126 #. Zephyr reads reply from the shared memory area. 127 128In the context of this transport, a channel is comprised of a **single** shared 129memory area and one or more mailbox channels. This is because users may need/want 130to use different mailbox channels for the request/reply doorbells. 131 132 133Protocols 134********* 135 136Currently, Zephyr has support for the following standard protocols: 137 138 #. **Clock management** 139 #. **Pin Control** 140 141 142Clock management protocol 143************************* 144 145This protocol is used to perform clock management operations. This is done 146via a driver (:file:`drivers/clock_control/clock_control_arm_scmi.c`), which 147implements the Zephyr clock control subsystem API. As such, from the user's 148perspective, using this driver is no different than using any other clock 149management driver. 150 151.. note:: 152 This driver is vendor-agnostic. As such, it may be used on any 153 system that uses SCMI for clock management operations. 154 155Pin Control protocol 156******************** 157 158This protocol is used to perform pin configuration operations. This is done 159via a set of functions implementing various commands. Currently, the only 160supported command is ``PINCTRL_SETTINGS_CONFIGURE``. 161 162.. note:: 163 The support for this protocol **does not** include a definition for 164 the :code:`pinctrl_configure_pins` function. Each vendor should use 165 their own definition of :code:`pinctrl_configure_pins`, which should 166 call into the SCMI pin control protocol function implementing the 167 ``PINCTRL_SETTINGS_CONFIGURE`` command. 168 169 170Enabling the SCMI support 171************************* 172 173To use the SCMI support, each vendor is required to add an ``scmi`` DT 174node (used for transport driver binding) and a ``protocol`` node under the ``scmi`` 175node for each supported protocol. 176 177.. note:: 178 Zephyr has no support for protocol discovery. As such, if users 179 add a DT node for a certain protocol it's assumed the platform 180 supports said protocol. 181 182The example below shows how a DT may be configured in order to use the 183SCMI support. It's assumed that the only protocol required is the clock 184management protocol. 185 186.. code-block:: devicetree 187 188 #include <mem.h> 189 190 #define MY_CLOCK_CONSUMER_CLK_ID 123 191 192 scmi_res0: memory@cafebabe { 193 /* mandatory to use shared memory driver */ 194 compatible = "arm,scmi-shmem"; 195 reg = <0xcafebabe DT_SIZE_K(1)>; 196 }; 197 198 scmi { 199 /* compatible for shared memory and doorbell-based transport */ 200 compatible = "arm,scmi"; 201 202 /* one SCMI channel => A2P/transmit channel */ 203 shmem = <&scmi_res0>; 204 205 /* two mailbox channels */ 206 mboxes = <&my_mbox_ip 0>, <&my_mbox_ip 1>; 207 mbox-names = "tx", "tx_reply"; 208 209 scmi_clk: protocol@14 { 210 compatible = "arm,scmi-clock"; 211 212 /* matches the clock management protocol ID */ 213 reg = <0x14>; 214 215 /* vendor-agnostic - always 1 */ 216 #clock-cells = <1>; 217 }; 218 }; 219 220 my_mbox_ip: mailbox@deadbeef { 221 compatible = "vnd,mbox-ip"; 222 reg = <0xdeadbeef DT_SIZE_K(1)>; 223 #mbox-cells = <1>; 224 }; 225 226 my_clock_consumer_ip: serial@12345678 { 227 compatible = "vnd,consumer-ip"; 228 reg = <0x12345678 DT_SIZE_K(1)>; 229 /* clock ID is vendor specific */ 230 clocks = <&scmi_clk MY_CLOCK_CONSUMER_CLK_ID>; 231 }; 232 233 234Finally, all that's left to do is enable :kconfig:option:`CONFIG_ARM_SCMI`. 235
