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dt-bindings: thermal: Add the idle cooling device
Some devices are not able to cool down by reducing their voltage / frequency because it could be not available or the system does not allow voltage scaling. In this configuration, it is not possible to use this strategy and the idle injection cooling device can be used instead. One idle cooling device is now present for the CPU as implemented by the combination of the idle injection framework belonging to the power capping framework and the thermal cooling device. The missing part is the DT binding providing a way to describe how the cooling device will work on the system. A first iteration was done by making the cooling device to point to the idle state. Unfortunately it does not make sense because it would need to duplicate the idle state description for each CPU in order to have a different phandle and make the thermal internal framework happy. It was proposed to add an cooling-cells to <3>, unfortunately the thermal framework is expecting a value of <2> as stated by the documentation and it is not possible from the cooling device generic code to loop this third value to the back end cooling device. Another proposal was to add a child 'thermal-idle' node as the SCMI does. This approach allows to have a self-contained configuration for the idle cooling device without colliding with the cpufreq cooling device which is based on the CPU node. In addition, it allows to have the cpufreq cooling device and the idle cooling device to co-exist together as shown in the example. Reviewed-by: Rob Herring <robh@kernel.org> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org> Link: https://lore.kernel.org/r/20200429103644.5492-2-daniel.lezcano@linaro.org
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Daniel Lezcano
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# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
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# Copyright 2020 Linaro Ltd.
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%YAML 1.2
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---
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$id: http://devicetree.org/schemas/thermal/thermal-idle.yaml#
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$schema: http://devicetree.org/meta-schemas/core.yaml#
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title: Thermal idle cooling device binding
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maintainers:
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- Daniel Lezcano <daniel.lezcano@linaro.org>
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description: |
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The thermal idle cooling device allows the system to passively
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mitigate the temperature on the device by injecting idle cycles,
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forcing it to cool down.
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This binding describes the thermal idle node.
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properties:
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$nodename:
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const: thermal-idle
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description: |
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A thermal-idle node describes the idle cooling device properties to
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cool down efficiently the attached thermal zone.
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'#cooling-cells':
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const: 2
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description: |
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Must be 2, in order to specify minimum and maximum cooling state used in
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the cooling-maps reference. The first cell is the minimum cooling state
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and the second cell is the maximum cooling state requested.
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duration-us:
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description: |
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The idle duration in microsecond the device should cool down.
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exit-latency-us:
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description: |
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The exit latency constraint in microsecond for the injected
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idle state for the device. It is the latency constraint to
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apply when selecting an idle state from among all the present
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ones.
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required:
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- '#cooling-cells'
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examples:
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- |
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#include <dt-bindings/thermal/thermal.h>
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// Example: Combining idle cooling device on big CPUs with cpufreq cooling device
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cpus {
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#address-cells = <2>;
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#size-cells = <0>;
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/* ... */
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cpu_b0: cpu@100 {
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device_type = "cpu";
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compatible = "arm,cortex-a72";
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reg = <0x0 0x100>;
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enable-method = "psci";
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capacity-dmips-mhz = <1024>;
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dynamic-power-coefficient = <436>;
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#cooling-cells = <2>; /* min followed by max */
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cpu-idle-states = <&CPU_SLEEP &CLUSTER_SLEEP>;
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thermal-idle {
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#cooling-cells = <2>;
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duration-us = <10000>;
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exit-latency-us = <500>;
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};
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};
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cpu_b1: cpu@101 {
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device_type = "cpu";
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compatible = "arm,cortex-a72";
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reg = <0x0 0x101>;
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enable-method = "psci";
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capacity-dmips-mhz = <1024>;
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dynamic-power-coefficient = <436>;
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#cooling-cells = <2>; /* min followed by max */
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cpu-idle-states = <&CPU_SLEEP &CLUSTER_SLEEP>;
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thermal-idle {
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#cooling-cells = <2>;
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duration-us = <10000>;
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exit-latency-us = <500>;
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};
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};
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/* ... */
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};
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/* ... */
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thermal_zones {
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cpu_thermal: cpu {
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polling-delay-passive = <100>;
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polling-delay = <1000>;
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/* ... */
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trips {
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cpu_alert0: cpu_alert0 {
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temperature = <65000>;
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hysteresis = <2000>;
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type = "passive";
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};
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cpu_alert1: cpu_alert1 {
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temperature = <70000>;
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hysteresis = <2000>;
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type = "passive";
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};
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cpu_alert2: cpu_alert2 {
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temperature = <75000>;
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hysteresis = <2000>;
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type = "passive";
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};
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cpu_crit: cpu_crit {
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temperature = <95000>;
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hysteresis = <2000>;
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type = "critical";
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};
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};
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cooling-maps {
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map0 {
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trip = <&cpu_alert1>;
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cooling-device = <&{/cpus/cpu@100/thermal-idle} 0 15 >,
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<&{/cpus/cpu@101/thermal-idle} 0 15>;
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};
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map1 {
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trip = <&cpu_alert2>;
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cooling-device =
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<&cpu_b0 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>,
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<&cpu_b1 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>;
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};
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};
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};
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};
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