On Wed, Mar 29, 2023 at 07:20:43PM +0200, Johannes Berg wrote: > On Wed, 2023-03-29 at 10:05 -0700, Nathan Chancellor wrote: > > > > GCC has optimizations for division by a constant that clang does not > > implement, so this issue is not visible when building with GCC. > > Huh yeah, we did 32-bit builds with gcc ... > > > Using div_u64() would resolve this issue, but Arnd points out that this > > can be quite expensive and the timestamp is being read at nanosecond > > granularity. > > Doesn't matter though, all the calculations are based on just the > command response from the firmware, which (tries to) take it in a > synchronised fashion. Okay, that is good information, thanks for providing it! > So taking more time here would be fine, as far as I can tell. > > > Nick pointed out that the result of this division is being > > stored to a 32-bit type anyways, so truncate gp2_10ns first then do the > > division, which elides the need for libcalls. > > That loses ~7 top bits though, no? I'd be more worried about that, than > the time div_u64() takes. Right, I sent this version of the fix to spur discussion around whether or not this was an acceptable approach, rather than having the question sit unanswered in our issue tracker :) I have no problems sending a v2 to use div_u64() and be done with it, which I will do shortly. Thanks for the quick input, cheers! Nathan
