No tpm_rng kernel module in current kernel versions.

[d0ntrash]

The tpm_rng kernel module was removed raspberrypi/linux@6e592a0.

To still use the TPM as hwrng device you need to change the value of '/sys/class/misc/hw_random/rng_current' to the desired value.
To do so:

1. Check if the TPM is available:
   # cat /sys/class/misc/hw_random/rng_available
   The output should look like this:
   3f104000.rng tpm-rng-0

2. Write the desired value to rng_current
   # echo tpm-rng-0 > /sys/class/misc/hw_random/rng_current

For more information see https://github.com/raspberrypi/linux/blob/rpi-4.19.y/Documentation/hw_random.txt

PS: Sorry for spamming Issues. This will be the last one. :)

[jeremyncc]

The PoC that shows how TPMGenie can control the output of the hwrng was really done only for fun. In reality the TPM's RNG isn't tied into anything useful within the kernel, and was exposed to userspace for unknown reasons. In practice, anyone that wants random bytes from the TPM will probably use the API (issue the GetRandom cmd directly), rather than using the hwrng chardev.

Anyhow, nice work tracking down a solution!