About the kernel configuration(CONFIG_CPU_FREQ and CONFIG_CPU_IDLE.)

Hi, list I am using Linux 5.4.78 with PREEMPT_RT patch. Hardware name: UNO-2372G-J021AE. It's strange that the frequency of the CPU is not constant(i.e. not fixed at a specific frequency) if I both disable CONFIG_CPU_FREQ and CONFIG_CPU_IDLE whereas the frequency of the CPU is constant if I only disable CONFIG_CPU_FREQ (leave CONFIG_CPU_IDLE enabled). Could somebody please shed some light on this matter?