3.2.2.14.1. DFS¶
Overview
Dynamic frequency scaling (DFS) is the ability of a part to modify frequency it operates at based on need, user preference, or other factors. MPU DFS is supported in the kernel by the cpufreq driver. All supported SoCs use the generic cpufreq driver. The frequency at which the MPU operates is selected by a driver called a governor. Each governor has a different strategy for selecting the most appropriate frequency. The following governors are available within the kernel:
- ondemand: This governor samples the load of the cpu and scales it up aggressively in order to provide the proper amount of processing power.
- conservative: This governor is similar to ondemand but uses a less aggressive method of increasing the the OPP of the MPU.
- performance: This governor statically sets the OPP of the MPU to the highest possible frequency.
- powersave: This governor statically sets the OPP of the MPU to the lowest possible frequency.
- userspace: This governor allows the user to set the desired OPP using any value found within scaling_available_frequencies by echoing it into scaling_setspeed.
More in depth documentation about each governor can be found in the linux kernel documentation here: https://www.kernel.org/doc/Documentation/cpu-freq/governors.txt
By default, cpufreq, the cpufreq driver, and all of the standard governors are enabled with the ondemand governor selected as the default governor. To make changes, follow the instructions below.
Source Location
drivers/cpufreq/cpufreq-dt.c
The OPP data itself is used by the cpufreq DT driver to scale frequencies based on the governors.
Kernel Configuration Options
The driver can be built into the kernel as a static module, dynamic module, or both.
$ make menuconfig
Select CPU Power Management from the main menu.
...
...
Boot options --->
CPU Power Management --->
Floating point emulation --->
...
Select CPU Frequency Scaling as shown here:
...
...
CPU Frequency Scaling --->
[*] CPU idle PM support
...
All relevant options are listed below:
[*] CPU Frequency scaling
<*> CPU frequency translation statistics
[*] CPU frequency translation statistics details
Default CPUFreq governor (userspace) --->
<*> 'performance' governor
<*> 'powersave' governor
-*- 'userspace' governor for userspace frequency scaling
<*> 'ondemand' cpufreq policy governor
<*> 'conservative' cpufreq governor
*** CPU frequency scaling drivers ***
<M> Generic DT based cpufreq driver
<M> Generic DT based cpufreq driver using clk notifiers
<*> Texas Instruments CPUFreq support
...
DT Configuration
The clock information and the operating-points table need to be added as given in the example below.
/* From arch/arm64/boot/dts/ti/k3-j7200.dtsi */
cpus {
#address-cells = <1>;
#size-cells = <0>;
cpu-map {
cluster0: cluster0 {
core0 {
cpu = <&cpu0>;
};
core1 {
cpu = <&cpu1>;
};
};
};
cpu0: cpu@0 {
compatible = "arm,cortex-a72";
reg = <0x000>;
device_type = "cpu";
enable-method = "psci";
i-cache-size = <0xc000>;
i-cache-line-size = <64>;
i-cache-sets = <256>;
d-cache-size = <0x8000>;
d-cache-line-size = <64>;
d-cache-sets = <256>;
next-level-cache = <&L2_0>;
clocks = <&k3_clks 202 2>;
clock-names = "cpu";
operating-points-v2 = <&cpu0_opp_table>;
#cooling-cells = <2>; /* min followed by max */
};
cpu1: cpu@1 {
compatible = "arm,cortex-a72";
reg = <0x001>;
device_type = "cpu";
enable-method = "psci";
i-cache-size = <0xc000>;
i-cache-line-size = <64>;
i-cache-sets = <256>;
d-cache-size = <0x8000>;
d-cache-line-size = <64>;
d-cache-sets = <256>;
next-level-cache = <&L2_0>;
operating-points-v2 = <&cpu0_opp_table>;
#cooling-cells = <2>; /* min followed by max */
clocks = <&k3_clks 203 0>;
clock-names = "cpu";
};
};
Driver Usage
All of the standard governors are built-in to the kernel, and by default the performance governor is selected. Performance governor sets the CPU statically to the highest frequency within the borders of scaling_min_freq and scaling_max_freq.
To view available governors,
$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors
conservative userspace powersave ondemand performance
To view current governor,
$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
ondemand
To set a governor,
$ echo userspace > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
To view current OPP (frequency in kHz)
$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq
2000000000
To view supported OPP’s (frequency in kHz),
$ cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
750000000 1000000000 1500000000 2000000000
To change OPP (can be done only for userspace governor. If governors like ondemand is used, OPP change happens automatically based on the system load)
$ echo 1000000000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed
Operating Points
Design: OPP term is derived from the usage in DVFS where they are a pair of voltage frequency value. To keep compatibility with the DVFS system, DFS uses same OPP tables with the voltages removed from the table.
/* From arch/arm64/boot/dts/ti/k3-j7200.dtsi */
cpu0_opp_table: opp-table {
compatible = "operating-points-v2";
opp4-2000000000 {
opp-hz = /bits/ 64 <2000000000>;
};
opp3-1500000000 {
opp-hz = /bits/ 64 <1500000000>;
};
opp2-1000000000 {
opp-hz = /bits/ 64 <1000000000>;
};
opp1-750000000 {
opp-hz = /bits/ 64 <750000000>;
};
};