Texas Instruments AM335 Series Design Manual page 27

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4.2.1 Device Tree Binary
4.2.1.1 Compiling the Device Tree Binary (DTB)
Using the included device tree source, it is simple to compile the device tree binary in Processor SDK.
Place the device tree source (DTS) file in the same directory as the other DTS files found here:
<sdk path>/board-support/linux-4.4.19*/arch/arm/boot/dts
Update your path as follows to include the arm gcc cross compilers:
export PATH=<sdk path>/linux-devkit/sysroots/x86_64-arago-linux/usr/bin:$PAT
In your sdk root directory, invoke make with the following command:
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- am335x-evm-powersave.dtb
Where the target is the name of the DTB to be generated from a DTS of the corresponding name.
Once this is complete copy the compiled DTB, which will be in the DTS source folder, to the boot directory
of the EVM.
4.2.1.2 Loading the DTB
U-Boot loads the device tree for the Linux kernel. The following instructions are for the default out-of-box
U-Boot environment that boots off of an SD card through the MMC0 interface. If your configuration differs
from this, further modifications may be necessary. The key is to ensure that U-Boot loads the powersave
device tree instead of another device tree, which may be present in the boot media.
First compile the device tree sources included (see
boot directory of the root file system partition on the SD card.
When powering on the evm, enter the U-Boot command line by pressing the space bar.
Use the following command sequence to select the desired device tree:
=> env default -f -a
=> printenv bootcmd
boodcmd=run findfdt;...
=> setenv bootcmd '....'
=> setenv fdtfile am335x-evm-powersave.dtb
=> saveenv
environment
=> boot
4.2.2 Ethernet
For test cases that do not require ethernet, use ifconfig to bring down the ethernet interface. Ethernet is
enabled by default to support network boot. The ethernet is not disabled so that a runtime optimization can
be demonstrated.
root@am335-evm:~#
4.2.3 Graphics (SGX)
Although SGX is disabled in device tree, its hardware default configuration is to be powered on. To realize
the lowest power state, it must be turned off using devmem. Note that if this is done when SGX is enabled
in device tree, it causes a kernel panic.
root@am335-evm:~# devmem2
root@am335-evm:~# devmem2
root@am335-evm:~# devmem2
4.3 Optimized Idle States
This is the amount of power consumed by the SoC at runtime, running the Linux Kernel, but no active
tasks. Since the MPU is clock gated in this state, power consumption does not increase significantly with
the scaling of CPU frequency.
SPRAC74A – February 2017 – Revised March 2017
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ifconfig eth0 down
0x44E01100 w 0x0
0x44E01104 w 0x1
0x44E01104 w 0x0
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Appendix A), and place the generated DTBs in the
# load default boot environment
# show the default boot command
# change the default boot command to not run findfdt
# manually set the fdtfile desired
# (Optional) save this as the default boot
# Boot as usual
# Poweroff state on next reset
# Assert reset
# De-assert reset
Example Low Power Use Cases
AM335x Low Power Design Guide
27