[PATCH] arm: spin one more cycle in timer-based delays

Doug Anderson dianders at chromium.org
Sun Nov 20 11:18:48 PST 2016


On Sat, Nov 19, 2016 at 10:29 AM, Mason <slash.tmp at free.fr> wrote:
>> Exactly - and the reason for that (as I've explained several times in
>> the past) the "standard" software delay loop calibrated against the
>> timer interrupt is _always_ going to be short.
> OK, so loop-based delays are known to be short. Would you or Linus
> accept a patch that adds a X% cushion *in the implementation* ?
> You are saying "people shouldn't expect udelay(10) to delay at least
> 10 µs, thus they should write udelay(10+N)".
> Why not hide that implementation detail inside the implementation,
> so as not to force the pessimization on every other implementation
> behind the udelay/ndelay wrapper?
> void loop_based_udelay(long us) {
>   spin_for_some_us(us + us/8);
> }

IMHO this would be an extremely sane thing to do.

Then, if you happened to be on a platform where udelay _was_ accurate
then you could eliminate the pointless overhead.  ...and also if you
were writing code to a datasheet and it said "delay for 50 us between
A and B" you could actually write udelay(50) instead of everyone
needing to know that they should write "udelay(53)".


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