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CPUFREQ

This document describes the CPUFREQ functionality of the K3 SoC and its usage.

Overview​

The CPUFREQ subsystem dynamically adjusts CPU frequency and voltage while the CPUs are running. Its purpose is to minimize power consumption while maintaining the required level of performance.

Functionality​

  1. cpufreq core: the core module of the cpufreq framework. It mainly provides three categories of functionality:
    • abstracts the common control logic for dynamic frequency and voltage scaling
    • exposes a unified sysfs interface to user space and sends frequency-change notifications to other drivers through notifier callbacks
    • provides the driver framework for CPU frequency and voltage control
  2. cpufreq governor: implements the policies used for dynamic frequency and voltage scaling.
  3. cpufreq driver: implements the platform-specific frequency and voltage scaling mechanism.
  4. cpufreq stats: collects frequency transition information and residency statistics for each frequency point, and provides cpufreq-related statistics for each CPU.

Source Tree Overview​

The CPU frequency scaling driver source tree is organized as follows:

drivers/cpufreq/
├── cpufreq.c
├── cpufreq_conservative.c
├── cpufreq-dt.c
├── cpufreq-dt.h
├── cpufreq-dt-platdev.c
├── cpufreq_governor_attr_set.c
├── cpufreq_governor.c
├── cpufreq_governor.h
├── cpufreq_ondemand.c
├── cpufreq_ondemand.h
├── cpufreq_performance.c
├── cpufreq_powersave.c
├── cpufreq_stats.c
├── cpufreq_userspace.c
├── freq_table.c
├── spacemit-k3-cpufreq.c --> K3 platform driver

Key Features​

Feature Summary​

FeatureDescription
Dynamic frequency and voltage scalingFrequency and voltage can be adjusted in real time according to system load
Independent scaling for dual clustersThe X100 and A100 clusters can scale independently without affecting each other
X100 maximum frequency of 2.4 GHzThe highest OPP supported by the X100 cluster is 2.4 GHz

K3 CPU Topology​

The K3 SoC uses a 16-core heterogeneous design with two CPU clusters:

ClusterCoresCore TypeFrequency RangeVoltage Scaling
X100 Clustercpu_0 ~ cpu_7SpacemiT X100614.4 MHz ~ 2.4 GHzSupported
A100 Clustercpu_8 ~ cpu_15SpacemiT A100614.4 MHz ~ 2.0 GHzNot supported

Each cluster has an independent OPP (Operating Performance Point) table and clock source, allowing frequency scaling to be controlled independently.

Performance Parameters​

X100 Cluster (cpu_0 ~ cpu_7) OPP Table​

FrequencyVoltage
2400000000 Hz1.00 V
2300000000 Hz0.88 V
2200000000 Hz0.88 V
2150000000 Hz0.88 V
2100000000 Hz0.88 V
2000000000 Hz0.88 V
1900000000 Hz0.88 V
1850000000 Hz0.88 V
1800000000 Hz0.88 V
1700000000 Hz0.88 V
1600000000 Hz0.88 V
1500000000 Hz0.85 V
1400000000 Hz0.85 V
1300000000 Hz0.85 V
1200000000 Hz0.85 V
1100000000 Hz0.85 V
1000000000 Hz0.80 V
819200000 Hz0.80 V
614400000 Hz0.80 V

A100 Cluster (cpu_8 ~ cpu_15) OPP Table​

Frequency
2000000000 Hz
1900000000 Hz
1850000000 Hz
1800000000 Hz
1700000000 Hz
1600000000 Hz
1500000000 Hz
1400000000 Hz
1300000000 Hz
1200000000 Hz
1100000000 Hz
1000000000 Hz
819200000 Hz
614400000 Hz

Test Method​

  1. Change the governor to userspace mode.

    echo userspace > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor

  2. View the list of supported frequencies.

    cat /sys/devices/system/cpu/cpufreq/policy0/scaling_available_frequencies

  3. Set the CPU frequency.

    echo 1600000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeed

  4. Check whether the frequency was applied successfully.

    cat /sys/devices/system/cpu/cpufreq/policy0/scaling_cur_freq

  5. Repeat the same procedure for the A100 cluster.

    echo userspace > /sys/devices/system/cpu/cpufreq/policy8/scaling_governor
    cat /sys/devices/system/cpu/cpufreq/policy8/scaling_available_frequencies
    echo 1200000 > /sys/devices/system/cpu/cpufreq/policy8/scaling_setspeed
    cat /sys/devices/system/cpu/cpufreq/policy8/scaling_cur_freq

Configuration​

Configuration mainly includes driver enablement and Device Tree configuration.

Kconfig Configuration​

The CPUFREQ configuration is shown below:

CONFIG_SPACEMIT_K3_CPUFREQ:

  This adds the CPUFreq driver support for Spacemit K3 SoC
  which are capable of changing the CPU's frequency dynamically.

  Symbol: SPACEMIT_K3_CPUFREQ [=y]
  Type  : tristate
  Defined at drivers/cpufreq/Kconfig:256
  Prompt: CPU frequency scaling driver for Spacemit K1X
  Depends on: CPU_FREQ [=y] && OF [=y] && COMMON_CLK [=y]
  Location:
   -> CPU Power Management
    -> CPU Frequency scaling
     -> CPU Frequency scaling (CPU_FREQ [=y])
      -> CPU frequency scaling driver for Spacemit K1X (SPACEMIT_K3_CPUFREQ [=y])
  Selects: CPUFREQ_DT [=y] && CPUFREQ_DT_PLATDEV [=y]

DTS Configuration​

The complete OPP table definitions are located in arch/riscv/boot/dts/spacemit/k3_opp_table.dtsi.

This file defines two OPP tables:

  • clst_core_opp_table0_x100: OPP table for the X100 cluster, containing 19 frequency steps from 614.4 MHz to 2.4 GHz and including opp-microvolt voltage entries
  • clst_core_opp_table0_a100: OPP table for the A100 cluster, containing 14 frequency steps from 614.4 MHz to 2.0 GHz and not including voltage entries

The clock sources and OPP tables are bound to the CPU nodes in k3_opp_table.dtsi:

/* X100 Cluster: cpu_0 ~ cpu_7 */
&cpu_0 {
    clst-supply = <&edcdc1>;
    clocks = <&syscon_apmu CLK_APMU_CPU_C0_CORE>, <&syscon_apmu CLK_APMU_CPU_C1_CORE>;
    clock-names = "cls0", "cls1";
    operating-points-v2 = <&clst_core_opp_table0_x100>;
};

/* A100 Cluster: cpu_8 ~ cpu_15 */
&cpu_8 {
    clocks = <&syscon_apmu CLK_APMU_CPU_C2_CORE>, <&syscon_apmu CLK_APMU_CPU_C3_CORE>;
    clock-names = "cls0", "cls1";
    operating-points-v2 = <&clst_core_opp_table0_a100>;
};

The OPP table also defines the PLL clock sources used by the driver during frequency transitions:

opp_table0_x100 {
    compatible = "operating-points-v2";
    opp-shared;
    clocks = <&pll CLK_PLL3>, <&pll CLK_PLL4>, <&pll CLK_PLL3_D1>, <&syscon_apmu CLK_APMU_CPU_C1_PLL_SRC>;
    clock-names = "pll_clst0", "pll_clst1", "pll_src", "clt_pll_src";
    ...
};

Debugging​

sysfs​

The X100 cluster sysfs nodes are located at:

  • /sys/devices/system/cpu/cpufreq/policy0/

The A100 cluster sysfs nodes are located at:

  • /sys/devices/system/cpu/cpufreq/policy8/
NodeDescription
affected_cpus, related_cpusDisplays the CPU cores associated with the policy
scaling_governorShows the active frequency scaling governor
scaling_available_frequenciesLists the supported system frequency points
scaling_max_freqMaximum software frequency limit
cpuinfo_cur_freqReal-time hardware CPU frequency
scaling_available_governorsLists the governors supported by the system
scaling_min_freqMinimum software frequency limit
cpuinfo_max_freqMaximum hardware-supported frequency
scaling_setspeedInterface for setting CPU frequency in userspace mode
cpuinfo_min_freqMinimum hardware-supported frequency
scaling_cur_freqShows the current CPU operating frequency
scaling_driverName of the active CPU frequency scaling driver

Testing​

To validate dynamic frequency and voltage scaling, repeat the steps in Test Method and verify correct operation at different frequency settings.

FAQ​