[PATCH v5 04/11] mips: use fallback for random_get_entropy() instead of just c0 random

[Maciej W. Rozycki]

On Tue, 19 Apr 2022, Jason A. Donenfeld wrote:

> For situations in which we don't have a c0 counter register available,
> we've been falling back to reading the c0 "random" register, which is
> usually bounded by the amount of TLB entries and changes every other
> cycle or so. This means it wraps extremely often. We can do better by
> combining this fast-changing counter with a potentially slower-changing
> counter from random_get_entropy_fallback() in the more significant bits.
> This commit combines the two, taking into account that the changing bits
> are in a different bit position depending on the CPU model. In addition,
> we previously were falling back to 0 for ancient CPUs that Linux does
> not support anyway; remove that dead path entirely.

Tested-by: Maciej W. Rozycki <macro at orcam.me.uk>

 I've pushed the algorithm through testing with a number of suitable 
systems:

- an R2000A and an R3000A with no timer of any kind, only jiffies,

- an R3400 with a chipset timer only,

- an R4400SC with a usable buggy CP0 counter and a chipset timer,

- a 5Kc with a good CP0 counter only,

with no obvious issues spotted.  Thank you for working on this!

  Maciej