https://bugs.freedesktop.org/show_bug.cgi?id=59187
--- Comment #8 from Paul Berry <stereotype...@gmail.com> ---
(In reply to comment #6)
> This isn't only an IronLake-issue.
>
> This issue is related to any chipsets older then SandyBridge.
Just a side note to anyone who decides to work on this bug;
If you would like to try to make chipsets prior to Gen5 work, please have a
look at section 6.2.3 ("Negative-W Clipping Errata") of the i965/G35 manual
(pp133-138), available from
https://01.org/linuxgraphics/documentation/intel%C2%AE-965-express-chipset-family-and-intel%C2%AE-g35-express-chipset-graphics-controller
Briefly, certain Gen4 chipsets contain a bug which causes vertices with a
negative gl_Position.w value to be clipped incorrectly. The workaround uses
some of the hardware that was intended to implement gl_ClipDistance[7]
(specifically, bit "UC7", which is output by the vertex shader to help the
fixed-function clipping hardware determine whether a vertex may be trivially
accepted or rejected). At present, applying the workaround is easy, since we
don't support gl_ClipDistance on those chipsets, so bit UC7 is otherwise
unused. If we want to make Gen4 chipsets implement gl_ClipDistance properly,
we'll need to carefully consider the interactions between the bug workaround
and gl_ClipDistance[7].
You can find the workaround code in Mesa by searching for the text
"has_negative_rhw_bug".
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