[Mesa-dev] [PATCH 0/2] i965: Back-port GPU hang workarounds (bug 50271)

[Paul Berry]

On 6 August 2012 21:20, Kenneth Graunke <kenneth at whitecape.org> wrote:

> On 08/05/2012 08:36 AM, Paul Berry wrote:
> > Just looking for a quick patch review to make sure I've backported
> > these patches to 8.0 correctly (and also confirmation that this
> > backport is a good idea).  These patches are intended to apply to the
> > 8.0 branch.
> >
> > The issue is that on Gen6 and Gen7, images rendered to depth and
> > stencil buffers must be aligned to a multiple of 8 pixels, otherwise
> > certain rendering operations produce a GPU hang.  Unfortunately, due
> > to the layout of miptrees (and the fact that we render to depth and
> > stencil miptrees using coordinate offsets), it would take a lot of
> > code rework to both produce correct rendering and guarantee no GPU
> > hangs.
> >
> > These patches avoid the GPU hangs by forcing the image offsets to be
> > multiples of 8.  As a result, correct rendering to depth and stencil
> > miptrees is sacrificed but the GPU doesn't hang.  The sacrifice is
> > minor, since it only affects miptree levels that would otherwise be
> > unaligned (in practice this is usually just the smallest miplevels),
> > and it only matters for (a) rendering to a depth/stencil buffer that
> > is later used as a texture, or (b) ping-ponging between miplevels
> > while rendering, both of which are fairly rare.  On the master branch,
> > we decided that this was a worthwhile compromise.
> >
> > As a result of this sacrifice, this will regress the following piglit
> > tests: fbo/fbo-depth-array (Gen6 only),
> > spec/ARB_depth_buffer_float/fbo-clear-formats,
> > spec/ARB_depth_texture/fbo-clear-formats, and
> > spec/EXT_packed_depth_stencil/fbo-clear-formats.  These tests
> > currently fail on master for the same reason.
> >
> > For more information see:
> > https://bugs.freedesktop.org/show_bug.cgi?id=50271
> >
> > [PATCH 1/2] i965/Gen6: Work around GPU hangs due to misaligned depth
> coordinate offsets.
> > [PATCH 2/2] i965/Gen7: Work around GPU hangs due to misaligned depth
> coordinate offsets.
>
> This trade-off makes sense to me.  Take small amounts of misrendering in
> a few piglit tests doing non-real-world cases but avoid nasty GPU hangs.
>  I hate to regress Piglit tests on stable, but it's for the best.
>

Ok, cool.  Thanks for the confirmation that this is a good idea.  I hate
doing cherry-picks to stable that have merge conflicts anyhow, and in this
case the fact that it would cause known regressions made it seem even less
certain--but I couldn't think of a better alternative.

I'll plan on pushing it to stable tomorrow.


>
> For some reason, I thought this needed the more invasive
> coarse/fine-grained tile offset patches.
>
> Have you tested this to confirm it fixes the cases?
>

Yes, I tested it on both Sandy Bridge and Ivy Bridge.  (Believe it or not,
I brought both with me to SIGGRAPH :))
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