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Linux Power Optimization Features
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SPRAC74A–February 2017–Revised March 2017
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Copyright © 2017, Texas Instruments Incorporated
AM335x Low Power Design Guide
3.1.4.5 Adding MPU OPP Table Entries
Starting in Processor SDK 3.01, OPP configuration information is contained in the device tree. For
AM335x, this information is defined in the file am33xx.dtsi. The first entry of the OPP table from
am33xx.dtsi is quoted below:
cpu0_opp_table: opp_table0 {
compatible = "operating-points-v2";
opp50@300000000 {
# OPP name and frequency
opp-hz = /bits/ 64 <300000000>;
# Target frequency in Hz
opp-microvolt = <950000 931000 969000>;
# PMIC voltage specification. < target ...
minimum maximum >
opp-supported-hw = <0x06 0x0010>;
# Specify which silicon revisions and device ...
efuse values support this OPP
opp-suspend;
# flag for lowest OPP setting -> ...
target OPP before processor
hits suspend
};
...
It is possible to modify the OPP table to include your own OPPs, however, improper frequency and
voltage settings can cause system instability, or potentially damage the system. Be sure to extensively
test frequency and voltage combinations to ensure that they are suitable for the use case and CPU load,
and there may be some OPP combinations that are just not stable. In order to have the best chances of
success implementing a new OPP, use the following recommendations to guide the process:
• Different OPPs can have the same target voltage, but cannot have the same frequency
• Lowering frequency while maintaining voltage should be safe
• When adding a new target voltage recommended min/max voltages are ±4% of the target voltage
• For safe maximum and minimum voltage levels for the MPU, see the device-specific data sheet.
• To enable the OPP for all platforms and silicon revisions, use the cell field <0xFF 0xFFFF > for the
opp-supported-hw property.
– This cell field matches whichever bits actually happen to be set on the device.
– If this is used, be sure to check that there are no frequency conflicts in the OPP table.
– For specific revision bits, check the DEVICE_ID register values in the TRM
To test new entries in the OPP table:
• Check the cpufreq interface in sysfs. If your OPP frequency is displayed, then the device tree table
was successfully updated.
• Export the CPU governor to userspace and select your new OPP as a target frequency. At this point, if
the board is designed for measurability, the target voltage should be observed on the VDD_MPU
voltage rail. In addtion, the BogoMIPS value in CPU info should be approximately your clock speed in
MHz.
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