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| 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 |
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