Insufficient permissions during ADB debugging

Administrator@aaron MINGW64 ~
$ adb shell
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
shell@firefly:/ $ cd sys/dvfs
cd sys/dvfs
shell@firefly:/sys/dvfs $ ls
ls
cpu_temp_enable
cpu_temp_target
dvfs_tree
shell@firefly:/sys/dvfs $ cat dvfs_tree
cat dvfs_tree
/system/bin/sh: cat: dvfs_tree: Permission denied

The reason for insufficient permissions is that you do not get root permissions. You need to use ADB root to get root permissions first, and then enter ADB shell

Administrator@aaron MINGW64 ~
$ adb root
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
restarting adbd as root

Administrator@aaron MINGW64 ~
$ adb shell
root@firefly:/ # cd sys/dvfs
cd sys/dvfs
root@firefly:/sys/dvfs # ls
ls
cpu_temp_enable
cpu_temp_target
dvfs_tree
root@firefly:/sys/dvfs # cat dvfs_tree
cat dvfs_tree