Changing CPU frequency in iMX93 processors

Changing the CPU frequency in iMX93 processors

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In iMX93 processors a number of core operating modes can be defined in order to select proper frequency and power consumption. This tutorial presents how to add additional modes to the default list and force the kernel driver to use the required one. It was prepared for the Yocto Scarthgap version with meta-somlabs layer.

There are four available operation modes:

Mode	Description
0	OD mode (1700 MHz) with DDR 3733 MTS
1	ND mode (1400 MHz) with DDR 1866 MTS
2	LD mode (900 MHz) with DDR 1866 MTS
3	LD mode (900 MHz) with DDR 625 MTS

The current mode can be changed by writing the mode number 0-4 to /sys/devices/platform/imx93-lpm/mode file. The same file can be read to obtain currently enabled mode.

root@visionsom-imx93:~# echo 1 > /sys/devices/platform/imx93-lpm/mode
root@visionsom-imx93:~# cat /sys/devices/platform/imx93-lpm/mode
System is in ND mode with DDR 1866 MTS!

The current core clock can be read from the /sys/kernel/debug/clk/a55_core/clk_rate file.

root@visionsom-imx93:~# cat /sys/kernel/debug/clk/a55_core/clk_rate
1400000000

Adding LD modes

By default only OD and ND modes are available in the kernel driver. To enable the LD modes the following change has to be made in the visionsom-imx93.dtsi file:

--- a/arch/arm64/boot/dts/freescale/visionsom-imx93.dtsi
+++ b/arch/arm64/boot/dts/freescale/visionsom-imx93.dtsi
@@ -163,6 +163,7 @@ &lcdif {
 
 &lpm {
        soc-supply = <&buck1>;
+        ld-mode-enabled;
        status = "okay";
 };

Verifying core frequency

Setting the core frequency can be easily tested using simple empty loop script:

echo 0 > /sys/devices/platform/imx93-lpm/mode
time bash -c "for i in {1..1000000}; do :; done"

real    0m8.158s
user    0m7.795s
sys     0m0.289s

echo 1 > /sys/devices/platform/imx93-lpm/mode
time bash -c "for i in {1..1000000}; do :; done"

real    0m10.100s
user    0m9.634s
sys     0m0.390s

echo 2 > /sys/devices/platform/imx93-lpm/mode
time bash -c "for i in {1..1000000}; do :; done"

real    0m15.358s
user    0m14.726s
sys     0m0.506s

echo 3 > /sys/devices/platform/imx93-lpm/mode
time bash -c "for i in {1..1000000}; do :; done"

real    0m16.292s
user    0m15.394s
sys     0m0.691s