4.1.15. 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 __symbols__ 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 cat command 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 cat command if it is a string. But if the value is an integer or some numeric data, you will have to run the hexdump command instead 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 where the first column represents input offsets
root@j721e-evm:~# hexdump -C /proc/device-tree/interconnect@100000/i2c@2000000/clock-frequency
00000000  00 06 1a 80                                       |....|
00000004

# 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:~# hexdump -C /proc/device-tree/reserved-memory/r5f-memory@a2100000/reg
00000000: 00000000 a2100000 00000000 01<address>0  ................

root@j721e-evm:~# hexdump -C /proc/device-tree/reserved-memory/r5f-dma-memory@a2000000/reg
00000000: 00000000 a2000000 00000000 00100000  ................

Warning

Before tweaking the memory for remote processors, please verify that the memory can be modified since some remote processors can be running firmware like DM firmware.