4.1.12. How to check some device tree information from user-space of Linux?¶
You might need to check value of some dtb entry, for debugging, checking that your dtb really got updated after you modfified, etc. Though you can’t have a .dts-file-like view of the device tree which was loaded, you can check values using the entries in /proc/device-tree.
In the command prompt of Linux, you should see something similar to the following in the /proc/device-tree directory:
root@j721e-evm:~# ls /proc/device-tree/
#address-cells firmware l3-cache0
#size-cells fixedregulator-evm12v0 main_r5fss_cpsw9g_virt_mac0
__symbols__ fixedregulator-sd memory@80000000
aliases fixedregulator-vsys3v3 model
chosen fixedregulator-vsys5v0 name
compatible gpio-keys pmu
connector interconnect@100000 reserved-memory
cpus interrupt-parent serial-number
dma_buf_phys ion sound@0
dummy-panel l2-cache0 timer-cl0-cpu0
Following are some typical usages you might need:
Let’s say you don’t know the exact path of the device tree entry you are trying to check. There is a __symbol__ directory in /proc/device-tree, which is very helpful in such cases. It has an entry for each symbol label in the device tree. You can find the exact path for that symbol by running
catcommand on that entry. Following is an example demonstrating the use:root@j721e-evm:~# cat /proc/device-tree/__symbols__/main_i2c0 /interconnect@100000/i2c@2000000 root@j721e-evm:~# ls /proc/device-tree/interconnect@100000/i2c@2000000/ #address-cells clock-names gpio@20 name pinctrl-names #size-cells clocks gpio@22 phandle power-domains clock-frequency compatible interrupts pinctrl-0 reg
You can check the value of a device tree entry using
catcommand if it is a string. But if the value is an integer or some numeric data, you will have to run thexxdcommand instead ofcat, to get output in a readable format. Following is an example demonstrating the use:# Example for a string value root@j721e-evm:~# cat /proc/device-tree/interconnect@100000/i2c@2000000/compatible ti,j721e-i2cti,omap4-i2c # Example for an integer value root@j721e-evm:~# xxd -g4 /proc/device-tree/interconnect@100000/i2c@2000000/clock-frequency 00000000: 00061a80 .... # The above value is in hexadecimal. You can calculate it's value in decimal by using following command root@j721e-evm:~# echo $((0x00061a80)) 400000
One common scenario of a device tree change is tweaking the memory for remoteproc processors like R5F. You can check if it got updated correctly, by running a command similar to following for the specific processor core.
# Finding symbols for R5Fs root@j721e-evm:~# ls /proc/device-tree/__symbols__/main_r5fss* /proc/device-tree/__symbols__/main_r5fss0 /proc/device-tree/__symbols__/main_r5fss0_core1_dma_memory_region /proc/device-tree/__symbols__/main_r5fss1_core0_memory_region /proc/device-tree/__symbols__/main_r5fss0_core0 /proc/device-tree/__symbols__/main_r5fss0_core1_memory_region /proc/device-tree/__symbols__/main_r5fss1_core1 /proc/device-tree/__symbols__/main_r5fss0_core0_dma_memory_region /proc/device-tree/__symbols__/main_r5fss1 /proc/device-tree/__symbols__/main_r5fss1_core1_dma_memory_region /proc/device-tree/__symbols__/main_r5fss0_core0_memory_region /proc/device-tree/__symbols__/main_r5fss1_core0 /proc/device-tree/__symbols__/main_r5fss1_core1_memory_region /proc/device-tree/__symbols__/main_r5fss0_core1 /proc/device-tree/__symbols__/main_r5fss1_core0_dma_memory_region # Finding location from the symbols root@j721e-evm:~# cat /proc/device-tree/__symbols__/main_r5fss0_core0_memory_region /reserved-memory/r5f-memory@a2100000 root@j721e-evm:~# cat /proc/device-tree/__symbols__/main_r5fss0_core0_dma_memory_region /reserved-memory/r5f-dma-memory@a2000000 # Checking the values root@j721e-evm:~# xxd -g4 /proc/device-tree/reserved-memory/r5f-memory@a2100000/reg 00000000: 00000000 a2100000 00000000 01f00000 ................ root@j721e-evm:~# xxd -g4 /proc/device-tree/reserved-memory/r5f-dma-memory@a2000000/reg 00000000: 00000000 a2000000 00000000 00100000 ................