I want to continuously increase /prco/sys/random/entropy_avail when it reduced.

I first check the rngd (https://wiki.archlinux.org/index.php/Rng-tools)

It says /dev/random is very slow since it only collects entropy from device drivers and other (slow) sources and I think that is why we use rngd.

And it says rngd mainly uses hardware random number generators (TRNG), present in modern hardware like recent AMD/Intel processors, VIA Nano or even Raspberry Pi.

However, when I start rngd it says

[root@localhost init.d]# rngd 
can't open entropy source(tpm or intel/amd rng)
Maybe RNG device modules are not loaded

But I don't have Intel RDRAND confirmed by cat /proc/cpuinfo | grep rdrand:

[root@localhost init.d]# cat /proc/cpuinfo | grep rdrand | wc -l
0

If there is any possible resources that I can use?

Alternatively, is it possible making script to increase /proc/sys/random/entropy_avail?