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System Control and Power Interface (SCPI) Message Protocol
Firmware implementing the SCPI described in ARM document number ARM DUI 0922B
("ARM Compute Subsystem SCP: Message Interface Protocols")[0] can be used
by Linux to initiate various system control and power operations.
Required properties:
- compatible : should be
* "arm,scpi" : For implementations complying to SCPI v1.0 or above
* "arm,scpi-pre-1.0" : For implementations complying to all
unversioned releases prior to SCPI v1.0
- mboxes: List of phandle and mailbox channel specifiers
All the channels reserved by remote SCP firmware for use by
SCPI message protocol should be specified in any order
- shmem : List of phandle pointing to the shared memory(SHM) area between the
processors using these mailboxes for IPC, one for each mailbox
SHM can be any memory reserved for the purpose of this communication
between the processors.
See Documentation/devicetree/bindings/mailbox/mailbox.txt
for more details about the generic mailbox controller and
client driver bindings.
Clock bindings for the clocks based on SCPI Message Protocol
This binding uses the common clock binding[1].
Container Node
Required properties:
- compatible : should be "arm,scpi-clocks"
All the clocks provided by SCP firmware via SCPI message
protocol much be listed as sub-nodes under this node.
Required properties:
- compatible : shall include one of the following
"arm,scpi-dvfs-clocks" - all the clocks that are variable and index based.
These clocks don't provide an entire range of values between the
limits but only discrete points within the range. The firmware
provides the mapping for each such operating frequency and the
index associated with it. The firmware also manages the
voltage scaling appropriately with the clock scaling.
"arm,scpi-variable-clocks" - all the clocks that are variable and provide full
range within the specified range. The firmware provides the
range of values within a specified range.
Other required properties for all clocks(all from common clock binding):
- #clock-cells : Should be 1. Contains the Clock ID value used by SCPI commands.
- clock-output-names : shall be the corresponding names of the outputs.
- clock-indices: The identifying number for the clocks(i.e.clock_id) in the
node. It can be non linear and hence provide the mapping of identifiers
into the clock-output-names array.
SRAM and Shared Memory for SCPI
A small area of SRAM is reserved for SCPI communication between application
processors and SCP.
The properties should follow the generic mmio-sram description found in [3]
Each sub-node represents the reserved area for SCPI.
Required sub-node properties:
- reg : The base offset and size of the reserved area with the SRAM
- compatible : should be "arm,scp-shmem" for Non-secure SRAM based
shared memory
Sensor bindings for the sensors based on SCPI Message Protocol
SCPI provides an API to access the various sensors on the SoC.
Required properties:
- compatible : should be "arm,scpi-sensors".
- #thermal-sensor-cells: should be set to 1. This property follows the
thermal device tree bindings[2].
Valid cell values are raw identifiers (Sensor ID)
as used by the firmware. Refer to platform details
for your implementation for the IDs to use.
Power domain bindings for the power domains based on SCPI Message Protocol
This binding uses the generic power domain binding[4].
PM domain providers
Required properties:
- #power-domain-cells : Should be 1. Contains the device or the power
domain ID value used by SCPI commands.
- num-domains: Total number of power domains provided by SCPI. This is
needed as the SCPI message protocol lacks a mechanism to
query this information at runtime.
PM domain consumers
Required properties:
- power-domains : A phandle and PM domain specifier as defined by bindings of
the power controller specified by phandle.
[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
[2] Documentation/devicetree/bindings/thermal/thermal.txt
[3] Documentation/devicetree/bindings/sram/sram.yaml
[4] Documentation/devicetree/bindings/power/power-domain.yaml
sram: sram@50000000 {
compatible = "arm,juno-sram-ns", "mmio-sram";
reg = <0x0 0x50000000 0x0 0x10000>;
#address-cells = <1>;
#size-cells = <1>;
ranges = <0 0x0 0x50000000 0x10000>;
cpu_scp_lpri: scp-shmem@0 {
compatible = "arm,juno-scp-shmem";
reg = <0x0 0x200>;
cpu_scp_hpri: scp-shmem@200 {
compatible = "arm,juno-scp-shmem";
reg = <0x200 0x200>;
mailbox: mailbox0@40000000 {
#mbox-cells = <1>;
scpi_protocol: scpi@2e000000 {
compatible = "arm,scpi";
mboxes = <&mailbox 0 &mailbox 1>;
shmem = <&cpu_scp_lpri &cpu_scp_hpri>;
clocks {
compatible = "arm,scpi-clocks";
scpi_dvfs: scpi_clocks@0 {
compatible = "arm,scpi-dvfs-clocks";
#clock-cells = <1>;
clock-indices = <0>, <1>, <2>;
clock-output-names = "atlclk", "aplclk","gpuclk";
scpi_clk: scpi_clocks@3 {
compatible = "arm,scpi-variable-clocks";
#clock-cells = <1>;
clock-indices = <3>, <4>;
clock-output-names = "pxlclk0", "pxlclk1";
scpi_sensors0: sensors {
compatible = "arm,scpi-sensors";
#thermal-sensor-cells = <1>;
scpi_devpd: scpi-power-domains {
compatible = "arm,scpi-power-domains";
num-domains = <2>;
#power-domain-cells = <1>;
cpu@0 {
reg = <0 0>;
clocks = <&scpi_dvfs 0>;
hdlcd@7ff60000 {
reg = <0 0x7ff60000 0 0x1000>;
clocks = <&scpi_clk 4>;
power-domains = <&scpi_devpd 1>;
thermal-zones {
soc_thermal {
polling-delay-passive = <100>;
polling-delay = <1000>;
/* sensor ID */
thermal-sensors = <&scpi_sensors0 3>;
In the above example, the #clock-cells is set to 1 as required.
scpi_dvfs has 3 output clocks namely: atlclk, aplclk, and gpuclk with 0,
1 and 2 as clock-indices. scpi_clk has 2 output clocks namely: pxlclk0
and pxlclk1 with 3 and 4 as clock-indices.
The first consumer in the example is cpu@0 and it has '0' as the clock
specifier which points to the first entry in the output clocks of
scpi_dvfs i.e. "atlclk".
Similarly the second example is hdlcd@7ff60000 and it has pxlclk1 as input
clock. '4' in the clock specifier here points to the second entry
in the output clocks of scpi_clocks i.e. "pxlclk1"
The thermal-sensors property in the soc_thermal node uses the
temperature sensor provided by SCP firmware to setup a thermal
zone. The ID "3" is the sensor identifier for the temperature sensor
as used by the firmware.
The num-domains property in scpi-power-domains domain specifies that
SCPI provides 2 power domains. The hdlcd node uses the power domain with
domain ID 1.