Documentation/devicetree/bindings/arm/cpu-capacity.txt

   1 ==========================================
   2 ARM CPUs capacity bindings
   3 ==========================================
   4
   5 ==========================================
   6 1 - Introduction
   7 ==========================================
   8
   9 ARM systems may be configured to have cpus with different power/performance
  10 characteristics within the same chip. In this case, additional information has
  11 to be made available to the kernel for it to be aware of such differences and
  12 take decisions accordingly.
  13
  14 ==========================================
  15 2 - CPU capacity definition
  16 ==========================================
  17
  18 CPU capacity is a number that provides the scheduler information about CPUs
  19 heterogeneity. Such heterogeneity can come from micro-architectural differences
  20 (e.g., ARM big.LITTLE systems) or maximum frequency at which CPUs can run
  21 (e.g., SMP systems with multiple frequency domains). Heterogeneity in this
  22 context is about differing performance characteristics; this binding tries to
  23 capture a first-order approximation of the relative performance of CPUs.
  24
  25 CPU capacities are obtained by running a suitable benchmark. This binding makes
  26 no guarantees on the validity or suitability of any particular benchmark, the
  27 final capacity should, however, be:
  28
  29 * A "single-threaded" or CPU affine benchmark
  30 * Divided by the running frequency of the CPU executing the benchmark
  31 * Not subject to dynamic frequency scaling of the CPU
  32
  33 For the time being we however advise usage of the Dhrystone benchmark. What
  34 above thus becomes:
  35
  36 CPU capacities are obtained by running the Dhrystone benchmark on each CPU at
  37 max frequency (with caches enabled). The obtained DMIPS score is then divided
  38 by the frequency (in MHz) at which the benchmark has been run, so that
  39 DMIPS/MHz are obtained.  Such values are then normalized w.r.t. the highest
  40 score obtained in the system.
  41
  42 ==========================================
  43 3 - capacity-dmips-mhz
  44 ==========================================
  45
  46 capacity-dmips-mhz is an optional cpu node [1] property: u32 value
  47 representing CPU capacity expressed in normalized DMIPS/MHz. At boot time, the
  48 maximum frequency available to the cpu is then used to calculate the capacity
  49 value internally used by the kernel.
  50
  51 capacity-dmips-mhz property is all-or-nothing: if it is specified for a cpu
  52 node, it has to be specified for every other cpu nodes, or the system will
  53 fall back to the default capacity value for every CPU. If cpufreq is not
  54 available, final capacities are calculated by directly using capacity-dmips-
  55 mhz values (normalized w.r.t. the highest value found while parsing the DT).
  56
  57 ===========================================
  58 4 - Examples
  59 ===========================================
  60
  61 Example 1 (ARM 64-bit, 6-cpu system, two clusters):
  62 capacities-dmips-mhz are scaled w.r.t. 1024 (cpu@0 and cpu@1)
  63 supposing cluster0@max-freq=1100 and custer1@max-freq=850,
  64 final capacities are 1024 for cluster0 and 446 for cluster1
  65
  66 cpus {
  67         #address-cells = <2>;
  68         #size-cells = <0>;
  69
  70         cpu-map {
  71                 cluster0 {
  72                         core0 {
  73                                 cpu = <&A57_0>;
  74                         };
  75                         core1 {
  76                                 cpu = <&A57_1>;
  77                         };
  78                 };
  79
  80                 cluster1 {
  81                         core0 {
  82                                 cpu = <&A53_0>;
  83                         };
  84                         core1 {
  85                                 cpu = <&A53_1>;
  86                         };
  87                         core2 {
  88                                 cpu = <&A53_2>;
  89                         };
  90                         core3 {
  91                                 cpu = <&A53_3>;
  92                         };
  93                 };
  94         };
  95
  96         idle-states {
  97                 entry-method = "arm,psci";
  98
  99                 CPU_SLEEP_0: cpu-sleep-0 {
 100                         compatible = "arm,idle-state";
 101                         arm,psci-suspend-param = <0x0010000>;
 102                         local-timer-stop;
 103                         entry-latency-us = <100>;
 104                         exit-latency-us = <250>;
 105                         min-residency-us = <150>;
 106                 };
 107
 108                 CLUSTER_SLEEP_0: cluster-sleep-0 {
 109                         compatible = "arm,idle-state";
 110                         arm,psci-suspend-param = <0x1010000>;
 111                         local-timer-stop;
 112                         entry-latency-us = <800>;
 113                         exit-latency-us = <700>;
 114                         min-residency-us = <2500>;
 115                 };
 116         };
 117
 118         A57_0: cpu@0 {
 119                 compatible = "arm,cortex-a57","arm,armv8";
 120                 reg = <0x0 0x0>;
 121                 device_type = "cpu";
 122                 enable-method = "psci";
 123                 next-level-cache = <&A57_L2>;
 124                 clocks = <&scpi_dvfs 0>;
 125                 cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
 126                 capacity-dmips-mhz = <1024>;
 127         };
 128
 129         A57_1: cpu@1 {
 130                 compatible = "arm,cortex-a57","arm,armv8";
 131                 reg = <0x0 0x1>;
 132                 device_type = "cpu";
 133                 enable-method = "psci";
 134                 next-level-cache = <&A57_L2>;
 135                 clocks = <&scpi_dvfs 0>;
 136                 cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
 137                 capacity-dmips-mhz = <1024>;
 138         };
 139
 140         A53_0: cpu@100 {
 141                 compatible = "arm,cortex-a53","arm,armv8";
 142                 reg = <0x0 0x100>;
 143                 device_type = "cpu";
 144                 enable-method = "psci";
 145                 next-level-cache = <&A53_L2>;
 146                 clocks = <&scpi_dvfs 1>;
 147                 cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
 148                 capacity-dmips-mhz = <578>;
 149         };
 150
 151         A53_1: cpu@101 {
 152                 compatible = "arm,cortex-a53","arm,armv8";
 153                 reg = <0x0 0x101>;
 154                 device_type = "cpu";
 155                 enable-method = "psci";
 156                 next-level-cache = <&A53_L2>;
 157                 clocks = <&scpi_dvfs 1>;
 158                 cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
 159                 capacity-dmips-mhz = <578>;
 160         };
 161
 162         A53_2: cpu@102 {
 163                 compatible = "arm,cortex-a53","arm,armv8";
 164                 reg = <0x0 0x102>;
 165                 device_type = "cpu";
 166                 enable-method = "psci";
 167                 next-level-cache = <&A53_L2>;
 168                 clocks = <&scpi_dvfs 1>;
 169                 cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
 170                 capacity-dmips-mhz = <578>;
 171         };
 172
 173         A53_3: cpu@103 {
 174                 compatible = "arm,cortex-a53","arm,armv8";
 175                 reg = <0x0 0x103>;
 176                 device_type = "cpu";
 177                 enable-method = "psci";
 178                 next-level-cache = <&A53_L2>;
 179                 clocks = <&scpi_dvfs 1>;
 180                 cpu-idle-states = <&CPU_SLEEP_0 &CLUSTER_SLEEP_0>;
 181                 capacity-dmips-mhz = <578>;
 182         };
 183
 184         A57_L2: l2-cache0 {
 185                 compatible = "cache";
 186         };
 187
 188         A53_L2: l2-cache1 {
 189                 compatible = "cache";
 190         };
 191 };
 192
 193 Example 2 (ARM 32-bit, 4-cpu system, two clusters,
 194            cpus 0,1@1GHz, cpus 2,3@500MHz):
 195 capacities-dmips-mhz are scaled w.r.t. 2 (cpu@0 and cpu@1), this means that first
 196 cpu@0 and cpu@1 are twice fast than cpu@2 and cpu@3 (at the same frequency)
 197
 198 cpus {
 199         #address-cells = <1>;
 200         #size-cells = <0>;
 201
 202         cpu0: cpu@0 {
 203                 device_type = "cpu";
 204                 compatible = "arm,cortex-a15";
 205                 reg = <0>;
 206                 capacity-dmips-mhz = <2>;
 207         };
 208
 209         cpu1: cpu@1 {
 210                 device_type = "cpu";
 211                 compatible = "arm,cortex-a15";
 212                 reg = <1>;
 213                 capacity-dmips-mhz = <2>;
 214         };
 215
 216         cpu2: cpu@2 {
 217                 device_type = "cpu";
 218                 compatible = "arm,cortex-a15";
 219                 reg = <0x100>;
 220                 capacity-dmips-mhz = <1>;
 221         };
 222
 223         cpu3: cpu@3 {
 224                 device_type = "cpu";
 225                 compatible = "arm,cortex-a15";
 226                 reg = <0x101>;
 227                 capacity-dmips-mhz = <1>;
 228         };
 229 };
 230
 231 ===========================================
 232 5 - References
 233 ===========================================
 234
 235 [1] ARM Linux Kernel documentation - CPUs bindings
 236     Documentation/devicetree/bindings/arm/cpus.txt