[Bug 55500] [sna gen4] corrupt rendering (and flickering on redraw)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=55500

--- Comment #100 from Zdenek Kabelac <zdenek.kabelac at gmail.com> ---
(In reply to comment #99)
> (In reply to comment #97)
> > (In reply to comment #96)
> > > (In reply to comment #93)
> > > > The bad part is - while before I've been getting 3.7Mchar/s in x11perf
> > > > -aa10text - now it's like 1.3Mchar/s  so significantly slower. 
> > But as I said before - if that would be plain hw defect - IMHO it would
> > simply always appear - but it seems like it's working for a while - then
> > 'something' happens - and flickering starts to appear - with (assumingly)
> > same amount
> > of texels/triangle/vertices - and than something again may happen,
> > and the problem is gone for a while.
> 
> It does. You do not have quite as much control over your tests as you
> presume.

Well I understand there are some weird things underneath  - but when I've
a page where the  scrolling up/down is showing heavy intel_gpu_top usage,
and flickering of pictures is visible -  then  I make nothing else
then page reload - I've just naive assumption, that this redrawn page
is just using different memory buffers - but otherwise the number of graphical
objects pushed to GPU should be approximately the same.

(I'm using plain good old xfce, so no fancy Gnome3 shell composite desktops...)

> No. I did not say the texel reads where the problem, just an indicator as to
> how long the EU would execute any particular shader for a fragment. Also
> there is only a single sampler and many EU running many more threads, so
> contention will also play a factor into how long each fragment takes to
> process, and so how long buffers will be active for. Look more closely at
> what it is going on, it is clearly that the hardware is not tracking
> lifetimes of its URB correctly.
> 
> > Also is there some explanation why  intel_gpu_top is showing so much higher
> > GPU usage when the flickering is visible ?
> 
> Other than the flickering correlates with GPU activity?

Well the same page could be scrolled with either i.e. 15% 'render busy' - and
no flickering -  or  with ~50% 'render busy' and visible problems.

I could make a debug build - but it would be probably needed to have some
kind of 'signal support' to dump needed data when necessary instead of logging
GB of data all the time.

> > i.e. I'd have expected if there would be a large press on GPU - but in this
> > case it just appear random pixel start to be drawn instead of some letter -
> > maybe some font cache corruption ?
> 
> It's still the same bug.

So maybe we could start at this - since  MAX 64 is giving the problem exposed
very easily - it's enough to run gnome-terminal on my desktop.

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