On April 14, 2022 1:41:38 PM PDT, Rob Herring <robh@xxxxxxxxxx> wrote: >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 It goes into interrupts, which means it is latency critical.
