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

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Date:   Thu, 15 Dec 2016 12:20:01 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Ville Syrjälä <ville.syrjala@...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, 15 Dec 2016, Ville Syrjälä <ville.syrjala@...ux.intel.com> wrote:
> 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.

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?


BR,
Jani.

-- 
Jani Nikula, Intel Open Source Technology Center