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

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Message-ID: <20161215092759.ud4enhbs2szf6ohl@phenom.ffwll.local>
Date:   Thu, 15 Dec 2016 10:27:59 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Nicholas Mc Guire <hofrat@...dl.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH] drm/i915: use udelay for very short delays

On Thu, Dec 15, 2016 at 10:25:19AM +0100, Daniel Vetter 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.
> 
> So one reason why we unconditionally use *sleep variants is the
> might_sleep check. Because in the past people have used udelay and mdelay,
> those delays had to be increased a lot because hw, and at the same time
> someone added users of these functions to our irq helper, resulting in irq
> handling times measures in multiple ms. That's not good.
> 
> So until someone can demonstrate that there's a real benefit (which let's
> be honest, for modeset code, will never be the case) I very highly prefer
> to use *sleep* functions. They prevent some silly things from happening by
> accident. Micro-optimizing modeset code and hampering maitainability in
> the process is not good.

Also, the entire premise seems backwards: usleep_range is inefficient for
certain parameter ranges and better replaced with udelay. That makes
sense.

But why exactly do we not fix udelay_range then, but instead do a cocci
job crawling through all the thousands of callers? Just fix usleep(_range)
to use udelay for very small values (and still keep the might_sleep check
ofc) if that's more efficient!
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch