On Thu, Apr 14, 2022 at 12:38:49AM +0200, Jason A. Donenfeld wrote: > Hi Rob, > > On Wed, Apr 13, 2022 at 4:32 PM Rob Herring <robh@xxxxxxxxxx> 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. Is random_get_entropy() a hot path? It doesn't have to be a weak function, but look at it this way. We have the following possibilities for what random_get_entropy() does: - get_cycles() - get_cycles() but returns 0 sometimes - returns 0 - something else You're handling the 3rd case. For the 2nd case, that's riscv, arm, nios2, and x86. That's not a lot, but is 2 or 3 of the most widely used architectures. Is it really too much to ask to support the 2nd case in the generic code/header? Rob
