Re: [PATCH 0/3] drm/i915: Reduce fence usage for scanout

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>, intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 0/3] drm/i915: Reduce fence usage for scanout
Date: Tue, 28 Jan 2014 15:51:06 +0200	[thread overview]
Message-ID: <20140128135106.GY9454@intel.com> (raw)
In-Reply-To: <20140128131244.GQ2508@nuc-i3427.alporthouse.com>

On Tue, Jan 28, 2014 at 01:12:44PM +0000, Chris Wilson wrote:
> On Tue, Jan 28, 2014 at 02:51:53PM +0200, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > After staring at the scanout vs. fences situation a bit, I decided
> > to reduce our scanout fence usage a bit.
> 
> I feel that is a glorious overreaction to a bug in another piece of
> code, but now that you've done it, I can not complain. ;-)
>  
> > Ville Syrjälä (3):
> >   drm/i915: Don't waste fences on sprite buffers
> 
> I suspect this is a little too generic and that there will be some
> freaky old piece of hardware that requires a fence.

I don't see a problem making such changes if and when needed. But if
you prefer I could adjust the code a bit to have one pin function for
primary planes (will check gen+FBC) and another for sprite/overlay
planes (will only check gen).

I suppose the current sprite code should work OKish for sprite B/C
on gen2/3, but for the video overlay the code would need more changes.
The hardware is more capable than the current code, and I'd hate not
taking full advantage of it.

> 
> However, for the gen that currently use sprites, it should just work I
> think. So which i-g-t hits tiled sprites?

There are none. I've been waiting for someone to write some sprite i-g-ts.
I do have my glplane thingy however which uses tiled buffers via gbm, and
it works equally well w/ and w/o fences.

> 
> >   drm/i915: Fix fence leaks if fbdev setup fails
> 
> If fbdev ever uses a tiled fb, I think we are in greater trouble. So
> other than the leak is only a theorectical one, the patch looks sane.

Hmm. Already forgot about that. Might be nice to make it tiled and
enable FBC for it :)

> >   drm/i915: Don't waste fences for scanout when not needed
> 
> But it's gen2/3 that is short of fences (and we've a... I feel this is a
> micro-optimisation, but now that it is done...

Yeah well, this will only "help" gen4 desktop parts and VLV since
everything else has FBC AFAIK.

-- 
Ville Syrjälä
Intel OTC