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

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Date:   Thu, 15 Dec 2016 09:28:30 +0000
From:   Nicholas Mc Guire <der.herr@...r.at>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Nicholas Mc Guire <hofrat@...dl.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Shashank Sharma <shashank.sharma@...el.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915: use udelay for very short delays

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.

thx!
hofrat