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SubjectRe: [PATCH v6 09/17] mips: use fallback for random_get_entropy() instead of just c0 random
On Sat, 23 Apr 2022, Jason A. Donenfeld wrote:

> diff --git a/arch/mips/include/asm/timex.h b/arch/mips/include/asm/timex.h
> index b05bb70a2e46..8cfa485d19e6 100644
> --- a/arch/mips/include/asm/timex.h
> +++ b/arch/mips/include/asm/timex.h
[...]
> + if (cpu_has_3kex)
> + c0_random = (read_c0_random() >> 8) & 0x3f;

Hmm, I wonder whether we do need to mask the contents of the register out
here given that known implementations return zeros in reserved bits. Even
though R3000 documentation I have access to makes no guarantee as to the
values of the reserved bits here I think we can safely proceed according
to what systems we do actually support do (even though it only saves one
instruction).

> else
> - return 0; /* no usable register */
> + c0_random = read_c0_random() & 0x3f;

Here the architecture guarantees unused bits to be zero, but the number
of them varies between implementations. However we'll only ever use this
leg for the R4000/R4400 processors, which have 48 TLB entries, and for the
Ingenic XBurst cores, which I have now verified in documentation (which
user-reported dumps from /proc/cpuinfo are consistent with) that have 32
TLB entries. So I think this mask operation can go as well.

I guess these updates can be made with a follow-up change though.

Maciej

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