LAS16-200: SCMI - System Management and Control Interface

ENGINEERS AND DEVICESWORKING TOGETHER

System Control and Management InterfaceCharles García-‐Tobin


ENGINEERS AND DEVICES WORKING TOGETHER

Agenda• What’s this all about• Requirements of SCMI• What’s the status

ENGINEERS AND DEVICESWORKING TOGETHERENGINEERS AND DEVICESWORKING TOGETHER

A nice future

We’d like a common methodology for system management functions

This requires a method that is:1. Extensible2. OS agnostic:

○Virtualizable so that guest or host drivers look the same○So we can run other OSs

3. Wide coverage: work across segments from mobile to enterprise and anywhere in between

StandardinterfacesProprietary

interfaces

Past Future


We are not really there L but there are opportunities J

● PSCI fixes this for core power management, but not performance management or peripheral device management

● ACPI covers many of the gaps in ACPI but has low level of adoption outside enterprise

● Everybody has to implement the same functions● In many segments everybody is doing this in similar ways

○ There is a strong trend to use system level controllers, all with very similar interfaces

● ARM has published the Power System Control Architecture (PSCA) which covers the use of system controllers with ARM IP


SCMI system control and management interface

● SCMI is an extensible interface covering performance and power functions

● Aim is to create an interface that can be driven from ACPI, or directly from kernel

● Builds on strong trend in the industry towards µC based platform controllers

● Candidate version for review○ http://connect.arm.com/dropzone/systemarch/DEN0056A_S

ystem_Control_and_Management_Interface_candidate_B.pdf

OS

Device Power Sensors

System Management and Control Interface (SCMI)

Platform Controller

Power domain prt

Perf domain protocol

Power domain protocol

Sensors protocol

Mailbox

DoorbelMailbox

DoorbelOther transport

Mailbox

DoorbelMailbox

DoorbelMailbox

Doorbell

PSCI

ACPI / DT

Core Power

Core perf


SCMI structure

SCMI is split into two layers:● Protocols: The services you want to

provide:○ Device Power/Perf management○ Core perf management ○ Sensor management

● Transports: The mechanism by which the service is communicated between the OS and system controllers

OS

Device Power Sensors

System Management and Control Interface (SCMI)

Platform Controller

Power domain prt

Perf domain protocol


Sensors protocol

Mailbox

DoorbelMailbox

DoorbelOther transport

Mailbox

DoorbelMailbox

DoorbelMailbox

Doorbell

PSCI

ACPI / DT

Core Power

Core perf


Power domain protocol•PROTOCOL_ATTRIBUTES•PROTOCOL_VERSION•PROTOCOL_MESSAGE_ATTRIBUTES•POWER_DOMAIN_ATTRIBUTES•POWER_STATE_GET•POWER_STATE_SET•POWERT_STATE_NOTIFY

•POWER_STATE_CHANGED

•Optional Shared Memory for stats

Performance domain protocol•PROTOCOL_ATTRIBUTES•PROTOCOL_VERSION•PROTOCOL_MESSAGE_ATTRIBUTES•PERFORMANCE_DOMAIN_ATTRIBUTES•PERFORMANCE_DESCRIBE_LEVELS•PERFORMANCE_LIMITS_GET•PERFORMANCE_LIMITS_SET•PERFORMANCE_LEVEL_GET•PERFORMANCE_LEVEL_SET•PERFORMANCE_NOTIFY_LIMIT•PERFORMANCE_NOTIFY_LEVEL

•PERFORMANCE_LIMIT_CHANGED•PERFORMANCE_LEVEL_CHANGED

•Optional Shared Memory for stats

Protocols examples

• There are additional protocols to cover sensors, clocks and binary transfer

Discovery protocol•PROTOCOL_ATTRIBUTES•PROTOCOL_VERSION•PROTOCOL_MESSAGE_ATTRIBUTES•DISCOVER_VENDOR•DISCOVER_SUB_VENDOR•DISCOVER_IMPLEMENTATION_VERSION•DISCOVER_LIST_PROTOCOLS•DISCOVER_AGENT


Transports

● Have the notion of channels○ Platform uses these to identify calling

agent○ Different channels depending on who

initiates communication, agent or platform

● Today the specification only defines a mailbox transport○ Uses a shared memory area and doorbell

interrupts○ Supports interrupts or polling○ Very similar to ACPI PCC○ This could be extended in the future

• Delayed responses• Notifications

agent

• Synchronous commands• Asynchronous command


agent

• Synchronous commands• Asynchronous command platform


agent

• Synchronous commands• Asynchronous command

Agent to platform channels

Platform to agent channels

● Method by which parameters/results are passed between the caller of the interface (an agent) and the implementer (the platform)





Extensibility and OS agnosticism

● The design is inherently extensible allowing more protocols or transports to be added over time

● OS agnostic○ Different OSs same Firmware

● Virtualizable○ In practice this generally means memory based transport e.g Mailbox

● The current draft of the specification provides a mailbox transport ○ In future an SMC transport could be added if we enable ways of isolating management from security


Wide coverage: How it all comes together: linux -‐ mobile

Android/ChromeOS/Other user side linux

EAS

Core-idling Sched-util/rfeq

Use case driven / Perf management

EAS-core

IPA

Sched-tune

Hotplug2nd core boot

Sched-freqSched-freq

SCMI

Sensor protocol


Perf protocolPSCI

Power Controller

cpuidle

Kernel API

SMC call

Non secure channel

Secure channel

Sysfs interface


Wide coverage: How it could all come together: enterprise

Core-idling Perf management


SCMIPower domain protocol

Perf protocolPSCI

Power ControllerKernel APISMC call

Non secure channelSecure channel

ACPILPI

Device PM

D-states CPPC Power metering dev.

Hot add/remove2nd core boot

Sensor protocol


Wide coverage: How it could all come together: enterprise

Core-idling Perf management


SCMIPower domain protocol

Perf protocolPSCI

Power ControllerKernel APISMC call

Non secure channelSecure channel

ACPILPI

Device PM

D-states CPPC Power metering dev.

Hot add/remove2nd core boot

Sensor protocol





Status and plans

● There are remaining challenges○ Interaction with ACPI CPPC○ Use of SCMI from ASL

● We are prototyping○ In Juno we have basic power control and performance management working○ We have looked at interaction with CPPC

● We’d like to release first public spec end of this year○ Concentrating on mobile○ Feedback welcome

● We are planning to upstream SCMI code following spec publication○ In linux and ARM TF and SCP reference FW

● Candidate version for review○ http://connect.arm.com/dropzone/systemarch/DEN0056A_System_Control_and_Management_Interface_candidate_B.pdf


Thank You#LAS16

For further information: www.linaro.orgLAS16 keynotes and videos on: connect.linaro.org


Wide coverage: SCMI is not just for application processor cores

• The spec mainly describes communications between AP and platform

• Other system agents could use the same interface, e.g. a modem in mobile or an management controller in enterprise

• Self managed agents must use private channels

Paltform controller

Secure

Application Processor

OSPM

CPU Power Device or CPU performance

Device power domains

Sensors

PSCICPU power

domains

Management

Power capping

Non secure

PrivatePSCI SMC call

Technology

LAS16-200: SCMI - System Management and Control Interface