On Thu, Dec 15, 2016 at 10:34:00AM +0000, Nicholas Mc Guire wrote: > 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 > + */ That's entirely too verbose IMO, and the reason for using usleep_range() is pretty obvious without spelling it out. All we really want to know is what the spec says is the minimum acceptable delay. > + usleep_range(10, 50); > > thx! > hofrat -- Ville Syrjälä Intel OTC _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
