From 0a1990a2d1f2217db20b30f915f48d9b8b976949 Mon Sep 17 00:00:00 2001 From: Daniel Lezcano Date: Thu, 19 Dec 2019 23:53:15 +0100 Subject: thermal/drivers/cpu_cooling: Add idle cooling device documentation Provide some documentation for the idle injection cooling effect in order to let people to understand the rational of the approach for the idle injection CPU cooling device. Signed-off-by: Daniel Lezcano Acked-by: Viresh Kumar Link: https://lore.kernel.org/r/20191219225317.17158-1-daniel.lezcano@linaro.org --- .../driver-api/thermal/cpu-idle-cooling.rst | 189 +++++++++++++++++++++ 1 file changed, 189 insertions(+) create mode 100644 Documentation/driver-api/thermal/cpu-idle-cooling.rst (limited to 'Documentation/driver-api') diff --git a/Documentation/driver-api/thermal/cpu-idle-cooling.rst b/Documentation/driver-api/thermal/cpu-idle-cooling.rst new file mode 100644 index 000000000000..e4f0859486c7 --- /dev/null +++ b/Documentation/driver-api/thermal/cpu-idle-cooling.rst @@ -0,0 +1,189 @@ + +Situation: +---------- + +Under certain circumstances a SoC can reach a critical temperature +limit and is unable to stabilize the temperature around a temperature +control. When the SoC has to stabilize the temperature, the kernel can +act on a cooling device to mitigate the dissipated power. When the +critical temperature is reached, a decision must be taken to reduce +the temperature, that, in turn impacts performance. + +Another situation is when the silicon temperature continues to +increase even after the dynamic leakage is reduced to its minimum by +clock gating the component. This runaway phenomenon can continue due +to the static leakage. The only solution is to power down the +component, thus dropping the dynamic and static leakage that will +allow the component to cool down. + +Last but not least, the system can ask for a specific power budget but +because of the OPP density, we can only choose an OPP with a power +budget lower than the requested one and under-utilize the CPU, thus +losing performance. In other words, one OPP under-utilizes the CPU +with a power less than the requested power budget and the next OPP +exceeds the power budget. An intermediate OPP could have been used if +it were present. + +Solutions: +---------- + +If we can remove the static and the dynamic leakage for a specific +duration in a controlled period, the SoC temperature will +decrease. Acting on the idle state duration or the idle cycle +injection period, we can mitigate the temperature by modulating the +power budget. + +The Operating Performance Point (OPP) density has a great influence on +the control precision of cpufreq, however different vendors have a +plethora of OPP density, and some have large power gap between OPPs, +that will result in loss of performance during thermal control and +loss of power in other scenarios. + +At a specific OPP, we can assume that injecting idle cycle on all CPUs +belong to the same cluster, with a duration greater than the cluster +idle state target residency, we lead to dropping the static and the +dynamic leakage for this period (modulo the energy needed to enter +this state). So the sustainable power with idle cycles has a linear +relation with the OPP’s sustainable power and can be computed with a +coefficient similar to: + + Power(IdleCycle) = Coef x Power(OPP) + +Idle Injection: +--------------- + +The base concept of the idle injection is to force the CPU to go to an +idle state for a specified time each control cycle, it provides +another way to control CPU power and heat in addition to +cpufreq. Ideally, if all CPUs belonging to the same cluster, inject +their idle cycles synchronously, the cluster can reach its power down +state with a minimum power consumption and reduce the static leakage +to almost zero. However, these idle cycles injection will add extra +latencies as the CPUs will have to wakeup from a deep sleep state. + +We use a fixed duration of idle injection that gives an acceptable +performance penalty and a fixed latency. Mitigation can be increased +or decreased by modulating the duty cycle of the idle injection. + + ^ + | + | + |------- ------- + |_______|_______________________|_______|___________ + + <------> + idle <----------------------> + running + + <-----------------------------> + duty cycle 25% + + +The implementation of the cooling device bases the number of states on +the duty cycle percentage. When no mitigation is happening the cooling +device state is zero, meaning the duty cycle is 0%. + +When the mitigation begins, depending on the governor's policy, a +starting state is selected. With a fixed idle duration and the duty +cycle (aka the cooling device state), the running duration can be +computed. + +The governor will change the cooling device state thus the duty cycle +and this variation will modulate the cooling effect. + + ^ + | + | + |------- ------- + |_______|_______________|_______|___________ + + <------> + idle <--------------> + running + + <-----------------------------> + duty cycle 33% + + + ^ + | + | + |------- ------- + |_______|_______|_______|___________ + + <------> + idle <------> + running + + <-------------> + duty cycle 50% + +The idle injection duration value must comply with the constraints: + +- It is less than or equal to the latency we tolerate when the + mitigation begins. It is platform dependent and will depend on the + user experience, reactivity vs performance trade off we want. This + value should be specified. + +- It is greater than the idle state’s target residency we want to go + for thermal mitigation, otherwise we end up consuming more energy. + +Power considerations +-------------------- + +When we reach the thermal trip point, we have to sustain a specified +power for a specific temperature but at this time we consume: + + Power = Capacitance x Voltage^2 x Frequency x Utilisation + +... which is more than the sustainable power (or there is something +wrong in the system setup). The ‘Capacitance’ and ‘Utilisation’ are a +fixed value, ‘Voltage’ and the ‘Frequency’ are fixed artificially +because we don’t want to change the OPP. We can group the +‘Capacitance’ and the ‘Utilisation’ into a single term which is the +‘Dynamic Power Coefficient (Cdyn)’ Simplifying the above, we have: + + Pdyn = Cdyn x Voltage^2 x Frequency + +The power allocator governor will ask us somehow to reduce our power +in order to target the sustainable power defined in the device +tree. So with the idle injection mechanism, we want an average power +(Ptarget) resulting in an amount of time running at full power on a +specific OPP and idle another amount of time. That could be put in a +equation: + + P(opp)target = ((Trunning x (P(opp)running) + (Tidle x P(opp)idle)) / + (Trunning + Tidle) + ... + + Tidle = Trunning x ((P(opp)running / P(opp)target) - 1) + +At this point if we know the running period for the CPU, that gives us +the idle injection we need. Alternatively if we have the idle +injection duration, we can compute the running duration with: + + Trunning = Tidle / ((P(opp)running / P(opp)target) - 1) + +Practically, if the running power is less than the targeted power, we +end up with a negative time value, so obviously the equation usage is +bound to a power reduction, hence a higher OPP is needed to have the +running power greater than the targeted power. + +However, in this demonstration we ignore three aspects: + + * The static leakage is not defined here, we can introduce it in the + equation but assuming it will be zero most of the time as it is + difficult to get the values from the SoC vendors + + * The idle state wake up latency (or entry + exit latency) is not + taken into account, it must be added in the equation in order to + rigorously compute the idle injection + + * The injected idle duration must be greater than the idle state + target residency, otherwise we end up consuming more energy and + potentially invert the mitigation effect + +So the final equation is: + + Trunning = (Tidle - Twakeup ) x + (((P(opp)dyn + P(opp)static ) - P(opp)target) / P(opp)target ) -- cgit From 23affa2e29c5faa8cb59778f71e3bce2c8b3aa5c Mon Sep 17 00:00:00 2001 From: Daniel Lezcano Date: Thu, 19 Dec 2019 23:53:17 +0100 Subject: thermal/drivers/cpu_cooling: Rename to cpufreq_cooling As we introduced the idle injection cooling device called cpuidle_cooling, let's be consistent and rename the cpu_cooling to cpufreq_cooling as this one mitigates with OPPs changes. Signed-off-by: Daniel Lezcano Acked-by: Viresh Kumar Reviewed-by: Amit Kucheria Link: https://lore.kernel.org/r/20191219225317.17158-3-daniel.lezcano@linaro.org --- Documentation/driver-api/thermal/exynos_thermal.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'Documentation/driver-api') diff --git a/Documentation/driver-api/thermal/exynos_thermal.rst b/Documentation/driver-api/thermal/exynos_thermal.rst index 5bd556566c70..d4e4a5b75805 100644 --- a/Documentation/driver-api/thermal/exynos_thermal.rst +++ b/Documentation/driver-api/thermal/exynos_thermal.rst @@ -67,7 +67,7 @@ TMU driver description: The exynos thermal driver is structured as:: Kernel Core thermal framework - (thermal_core.c, step_wise.c, cpu_cooling.c) + (thermal_core.c, step_wise.c, cpufreq_cooling.c) ^ | | -- cgit From ca07ee4e3de468582e9a370ccfad3e4cf83b5268 Mon Sep 17 00:00:00 2001 From: Krzysztof Kozlowski Date: Sat, 4 Jan 2020 16:20:53 +0100 Subject: thermal: exynos: Rename Samsung and Exynos to lowercase Fix up inconsistent usage of upper and lowercase letters in "Samsung" and "Exynos" names. "SAMSUNG" and "EXYNOS" are not abbreviations but regular trademarked names. Therefore they should be written with lowercase letters starting with capital letter. The lowercase "Exynos" name is promoted by its manufacturer Samsung Electronics Co., Ltd., in advertisement materials and on website. Although advertisement materials usually use uppercase "SAMSUNG", the lowercase version is used in all legal aspects (e.g. on Wikipedia and in privacy/legal statements on https://www.samsung.com/semiconductor/privacy-global/). Signed-off-by: Krzysztof Kozlowski Signed-off-by: Daniel Lezcano Link: https://lore.kernel.org/r/20200104152107.11407-7-krzk@kernel.org --- Documentation/driver-api/thermal/exynos_thermal.rst | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) (limited to 'Documentation/driver-api') diff --git a/Documentation/driver-api/thermal/exynos_thermal.rst b/Documentation/driver-api/thermal/exynos_thermal.rst index d4e4a5b75805..764df4ab584d 100644 --- a/Documentation/driver-api/thermal/exynos_thermal.rst +++ b/Documentation/driver-api/thermal/exynos_thermal.rst @@ -4,7 +4,7 @@ Kernel driver exynos_tmu Supported chips: -* ARM SAMSUNG EXYNOS4, EXYNOS5 series of SoC +* ARM Samsung Exynos4, Exynos5 series of SoC Datasheet: Not publicly available @@ -14,7 +14,7 @@ Authors: Amit Daniel TMU controller Description: --------------------------- -This driver allows to read temperature inside SAMSUNG EXYNOS4/5 series of SoC. +This driver allows to read temperature inside Samsung Exynos4/5 series of SoC. The chip only exposes the measured 8-bit temperature code value through a register. @@ -43,7 +43,7 @@ The three equations are: Trimming info for 85 degree Celsius (stored at TRIMINFO register) Temperature code measured at 85 degree Celsius which is unchanged -TMU(Thermal Management Unit) in EXYNOS4/5 generates interrupt +TMU(Thermal Management Unit) in Exynos4/5 generates interrupt when temperature exceeds pre-defined levels. The maximum number of configurable threshold is five. The threshold levels are defined as follows:: -- cgit