Commit 3b25846f authored by Daniel Lezcano's avatar Daniel Lezcano
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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: default avatarRob Herring <robh@kernel.org>
Signed-off-by: default avatarDaniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: default avatarAmit Kucheria <amit.kucheria@linaro.org>
Link: https://lore.kernel.org/r/20200429103644.5492-2-daniel.lezcano@linaro.org
parent 333cff6c

Documentation/devicetree/bindings/thermal/thermal-idle.yaml

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# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)

# Copyright 2020 Linaro Ltd.

%YAML 1.2

---

$id: http://devicetree.org/schemas/thermal/thermal-idle.yaml#

$schema: http://devicetree.org/meta-schemas/core.yaml#



title: Thermal idle cooling device binding



maintainers:

  - Daniel Lezcano <daniel.lezcano@linaro.org>



description: |

  The thermal idle cooling device allows the system to passively

  mitigate the temperature on the device by injecting idle cycles,

  forcing it to cool down.



  This binding describes the thermal idle node.



properties:

   $nodename:

     const: thermal-idle

     description: |

        A thermal-idle node describes the idle cooling device properties to

        cool down efficiently the attached thermal zone.



   '#cooling-cells':

      const: 2

      description: |

         Must be 2, in order to specify minimum and maximum cooling state used in

         the cooling-maps reference. The first cell is the minimum cooling state

         and the second cell is the maximum cooling state requested.



   duration-us:

      description: |

         The idle duration in microsecond the device should cool down.



   exit-latency-us:

      description: |

         The exit latency constraint in microsecond for the injected

         idle state for the device. It is the latency constraint to

         apply when selecting an idle state from among all the present

         ones.



required:

  - '#cooling-cells'



examples:

  - |

    #include <dt-bindings/thermal/thermal.h>



    // Example: Combining idle cooling device on big CPUs with cpufreq cooling device

    cpus {

            #address-cells = <2>;

            #size-cells = <0>;



            /* ... */



                 cpu_b0: cpu@100 {

                         device_type = "cpu";

                         compatible = "arm,cortex-a72";

                         reg = <0x0 0x100>;

                         enable-method = "psci";

                         capacity-dmips-mhz = <1024>;

                         dynamic-power-coefficient = <436>;

                         #cooling-cells = <2>; /* min followed by max */

                         cpu-idle-states = <&CPU_SLEEP &CLUSTER_SLEEP>;

                         thermal-idle {

                                 #cooling-cells = <2>;

                                 duration-us = <10000>;

                                 exit-latency-us = <500>;

                         };

                };



                cpu_b1: cpu@101 {

                        device_type = "cpu";

                        compatible = "arm,cortex-a72";

                        reg = <0x0 0x101>;

                        enable-method = "psci";

                        capacity-dmips-mhz = <1024>;

                        dynamic-power-coefficient = <436>;

                        #cooling-cells = <2>; /* min followed by max */

                        cpu-idle-states = <&CPU_SLEEP &CLUSTER_SLEEP>;

                        thermal-idle {

                                #cooling-cells = <2>;

                                duration-us = <10000>;

                                exit-latency-us = <500>;

                        };

                 };



          /* ... */



    };



    /* ... */



    thermal_zones {

         cpu_thermal: cpu {

                polling-delay-passive = <100>;

                polling-delay = <1000>;



                /* ... */



                trips {

                        cpu_alert0: cpu_alert0 {

                                    temperature = <65000>;

                                    hysteresis = <2000>;

                                    type = "passive";

                        };



                        cpu_alert1: cpu_alert1 {

                                    temperature = <70000>;

                                    hysteresis = <2000>;

                                    type = "passive";

                        };



                        cpu_alert2: cpu_alert2 {

                                    temperature = <75000>;

                                    hysteresis = <2000>;

                                    type = "passive";

                        };



                        cpu_crit: cpu_crit {

                                    temperature = <95000>;

                                    hysteresis = <2000>;

                                    type = "critical";

                        };

                };



                cooling-maps {

                        map0 {

                             trip = <&cpu_alert1>;

                             cooling-device = <&{/cpus/cpu@100/thermal-idle} 0 15 >,

                                              <&{/cpus/cpu@101/thermal-idle} 0 15>;

                        };



                        map1 {

                             trip = <&cpu_alert2>;

                             cooling-device =

                                        <&cpu_b0 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>,

                                        <&cpu_b1 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>;

                       };

                };

          };

    };

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