Re: [PATCH v4 01/11] timekeeping: add raw clock fallback for random_get_entropy()

Jason A. Donenfeld
2022-04-13T22:39:42Z

Hi Rob,

On Wed, Apr 13, 2022 at 4:32 PM Rob Herring <robh@kernel.org> wrote:
> 'does not have a usable get_cycles(), ...' as clearly some arches have
> get_cycles() and yet still need a fallback.
>
> Why not handle the 'if get_cycles() returns 0 do the fallback' within
> a weak random_get_entropy() function? Then more arches don't need any
> random_get_entropy() implementation.

No, this doesn't really work. Actually, most archs don't need a
random_get_entropy() function, because it exists in asm-generic doing
the thing we want. So that's taken care of. But weak functions as you
suggested would be quite suboptimal, because on, e.g. x86, what we
have now gets inlined into a single rdtsc instruction. Also, the
relation between get_cycles() and random_get_entropy() doesn't always
hold; some archs may not have a working get_cycles() function but do
have a path for a random_get_entropy(). Etc, etc. So I'm pretty sure
that this commit is really the most simple and optimal thing to do. I
really don't want to go the weak functions route.

Jason