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

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Message-ID: <20161215101036.GW31595@intel.com>
Date:   Thu, 15 Dec 2016 12:10:36 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Nicholas Mc Guire <der.herr@...r.at>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Nicholas Mc Guire <hofrat@...dl.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH] drm/i915: use udelay for very short delays

On Thu, Dec 15, 2016 at 11:52:34AM +0200, Jani Nikula wrote:
> On Thu, 15 Dec 2016, Nicholas Mc Guire <der.herr@...r.at> wrote:
> > On Thu, Dec 15, 2016 at 11:08:49AM +0200, Jani Nikula wrote:
> >> On Thu, 15 Dec 2016, Nicholas Mc Guire <hofrat@...dl.org> 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.

-- 
Ville Syrjälä
Intel OTC