Re: [PATCH] drm/i915: use udelay for very short delays

[Nicholas Mc Guire]

On Thu, Dec 15, 2016 at 12:20:01PM +0200, Jani Nikula wrote:
> On Thu, 15 Dec 2016, Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> wrote:
> > On Thu, Dec 15, 2016 at 11:52:34AM +0200, Jani Nikula wrote:
> >> On Thu, 15 Dec 2016, Nicholas Mc Guire <der.herr@xxxxxxx> wrote:
> >> > On Thu, Dec 15, 2016 at 11:08:49AM +0200, Jani Nikula wrote:
> >> >> On Thu, 15 Dec 2016, Nicholas Mc Guire <hofrat@xxxxxxxxx> wrote:
> >> >> > Even on fast systems a 2 microsecond delay is most likely more efficient
> >> >> > as a busy-wait loop. The overhead of a hrtimer does not seem warranted -
> >> >> > change this to a udelay(2).
> >> >> 
> >> >> Similar concerns as in [1]. We don't need the accuracy of udelay() here,
> >> >> so this boils down to which is the better use of CPU. We could probably
> >> >> relax the max delay more if that was helpful. But I'm not immediately
> >> >> sold on "is most likely more efficient" which sounds like a gut feeling.
> >> >> 
> >> >> I'm sorry it's not clear in my other reply that I do appreciate
> >> >> addressing incorrect/silly use of usleep_range(); I'm just not (yet)
> >> >> convinced udelay() is the answer.
> >> >
> >> > if the delay is not critical and all that is needed 
> >> > is an assurance that it is greater than X us then 
> >> > usleep_range is fine with a relaxed limit. 
> >> > So from what you wrote my patch proposal is wrong - the
> >> > udelay() is not the way to got.
> >> > My intent is to get rid of very small usleep_range() cases
> >> > so if usleep_range(20,50) causes no issues with this driver
> >> > and does not induce any performance penalty then that would 
> >> > be the way to go I think.
> >> 
> >> Okay, so I looked at the code, and I looked at our spec, and I looked at
> >> the MIPI D-PHY spec, and I cried a little.
> >> 
> >> Long story short, I think usleep_range(10, 50) will be fine.
> >
> > Note that I really want to see a comment next to every delay like this
> > documenting the actual hardware requirement, if the delay used by the
> > code doesn't 100% match it.
> 
> Our spec says, "Wait for 2us for ULPS to complete". That's a simplistic
> view wrt D-PHY, and our code doesn't even match the spec. Hence the
> tears. Want to propose a wording for the comment so we can apply this
> change, without going for a full rewrite of the sequence?
>
is that suitable or am I overdoing it ?

-               usleep_range(2, 3);
+               /* delay for at least 2us - relaxed to 10-50 to allow
+                * hrtimer subsystem to optimize uncritical timer handling
+                */
+               usleep_range(10, 50);

thx!
hofrat