Investigate linux random number generator in random.c

I am studying the random number generator in linux kernel before version 3.16

I gain my knowledge from this blog post and by reading the linux source code. http://ift.tt/2l7sNnX

I can see the following four functions are contributing to the entropy pool

add_device_randomness() add_interrupt_randomness() add_disk_randomness() add_input_randomness()

According to the comment in the source code add_device_randomness() is only being used for initialise the entropy pool using hardware identifier, so it is not actually adding randomness to the entropy pool. From what I read, hwrng is not adding randomness to the pool in version before 3.16

By reading the source code, we can see the following two functions are controlling the number of entropy bits in the entropy pool.

credit_entropy_bits() // this is adding entropy bits to the pool extract_entropy() // This is removing entropy bits from the pool

I want to figure out what devices/actions is actually calling these functions and adding entropy to the kernel. Is there anyways to estimate or get a feeling about that?