Re: random number generator missing after upgrade

[Anders Andersson <pipatron@gmail.com>]

On Sun, Aug 13, 2023 at 11:09 PM Björn Persson <Bjorn@rombobjörn.se> wrote:
>
> Jeffrey Walton wrote:
> > Maybe related to https://www.phoronix.com/news/Linux-Disables-RNG-AMD-fTPMs
>
> Not likely. That article is about a firmware TPM that comes with newer
> Ryzen processors. Older Ryzens supposedly don't have it. The processor
> in my APU2 is a GX-412TC, not a Ryzen at all, and my TPM is a discrete
> chip from Infineon. The change in question is supposed to disable the
> random number generator only if the TPM lists AMD as its manufacturer.

I agree that the patch looks ok, but I remember being hit by a kernel
change that inadvertently changed the behavior on other systems too
(ECC RAM background scrubbing), but nobody really noticed because it
was not in much use.

I suspect that the case of having an external TPM on an AMD system is
such an unusual case, and I couldn't trace exactly where that patch
checked the AMD string, so perhaps it's picking up the AMD string
earlier on, and decides to disable all TPM on the AMD system. At least
the timing of the problem and the patch is suspicious.