Access device tree information from User Space

Revision History

Introduction

The value of some dtb entry might have to be checked for debugging, or to verify that the dtb really got updated after it was modified, etc. Although a .dts-file-like view of the device tree which was loaded cannot be achieved, values can be verified using the entries on the in /proc/device-tree.

Something similar to the following can be seen on the command prompt in Linux in the /proc/device-tree directory:

root@j7-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

Typical usage examples

• If the exact path of the device tree entry to check is not known. 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. The exact path for a symbol can be found by running the cat command on an entry. Following is an example demonstrating the use:

  root@j7-evm:~# cat /proc/device-tree/__symbols__/main_i2c0
  /interconnect@100000/i2c@2000000
  
  root@j7-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
  

• The value of a device tree entry can be checked using the cat command if it is a string. If the value is an integer or some numeric data, however, the xxd command will have to be used instead of cat, to get output in a readable format. Following is an example demonstrating the use:

  # Example for a string value
  root@j7-evm:~# cat /proc/device-tree/interconnect@100000/i2c@2000000/compatible
  ti,j721e-i2cti,omap4-i2c
  
  # Example for an integer value
  root@j7-evm:~# xxd -g4 /proc/device-tree/interconnect@100000/i2c@2000000/clock-frequency
  00000000: 00061a80                             ....
  
  # The above value is in hexadecimal. You can calculate its value in decimal by using the following command
  root@j7-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 the following for the specific processor core.

  # Finding symbols for R5Fs
  root@j7-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 the location from the symbols
  root@j7-evm:~# cat /proc/device-tree/__symbols__/main_r5fss0_core0_memory_region
  /reserved-memory/r5f-memory@a2100000
  
  root@j7-evm:~# cat /proc/device-tree/__symbols__/main_r5fss0_core0_dma_memory_region
  /reserved-memory/r5f-dma-memory@a2000000
  
  # Checking the values
  root@j7-evm:~# xxd -g4 /proc/device-tree/reserved-memory/r5f-memory@a2100000/reg
  00000000: 00000000 a2100000 00000000 01f00000  ................
  
  root@j7-evm:~# xxd -g4 /proc/device-tree/reserved-memory/r5f-dma-memory@a2000000/reg
  00000000: 00000000 a2000000 00000000 00100000  ................