[Intel-gfx] [PATCH 2/2] drm/i915/fbc: Allow on unfenced surfaces, for recent gen

[Chris Wilson]

On Mon, Aug 22, 2016 at 09:39:17PM -0300, Paulo Zanoni wrote:
> 2016-08-18 5:21 GMT-03:00 Chris Wilson <chris at chris-wilson.co.uk>:
> > Only fbc1 is tied to using a fence. Later iterations of fbc are more
> > flexible and allow operation on unfenced frontbuffers.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Daniel Vetter <daniel.vetter at intel.com>
> > Cc: "Zanoni, Paulo R" <paulo.r.zanoni at intel.com>
> 
> Hi
> 
> I see this patch was applied. Now, on my Skylake machine, if I boot
> with i915.enable_fbc=1 I'll get FIFO underruns under fbcon. Just
> booting already gives me a FIFO underrun message, and then if I "sudo
> systemctl stop lightdm" I'll get a constantly-blinking screen.
> 
> Of course, applying the patch that disables FBC after a FIFO underrun
> will help stopping the ever-blinking fbcon, but I think we should try
> to avoid the underruns in the places we know we can, and leave the
> "disable FBC on FIFO underruns" just for the cases we're not expecting.
> 
> Also, please remember that I mentioned there are FBC workarounds for
> untiled that we still don't implement (although when I re-read my
> email it may sound like I suggested the workarounds are for non-GTT
> tracking). IMHO this argument alone should
> have prevented this patch from being merged...
> 
> Based on that, can we please revert this patch? I'm afraid some people
> would consider these underruns as blockers to enabling FBC, so it's
> probably better to enable FBC only on X tiled for now, and leave this
> for when we know how to prevent the underrun (possibly by implementing
> the missing WAs).

Sure you can disable FBC - just note that typically framebuffers will be
unfenced.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre