[Mesa-dev] XCOM: Enemy Unknown vs. NaN texture unit LOD bias

[Kenneth Graunke]

Hello,

Mesa master has been hitting assert failures when running "XCOM: Enemy
Unknown" since commit f8d69beed49c64f883bb8ffb28d4960306baf575, where we
started asserting that the SAMPLER_STATE LOD Bias value actually fits in
the correct number of bits.

Apparently, XCOM calls

   glTexEnv(GL_TEXTURE_FILTER_CONTROL_EXT, GL_TEXTURE_LOD_BIAS_EXT, val);

to set the texture unit LOD bias...but according to gdb, the value is:

   -nan(0x7ffff3)

In i965, we do CLAMP(lod bias, -16, 15)...but NaN ends up failing both
the < min and > max comparisons, so it slips through.  But, that raises
the question...what value *should* we be using?  0?  Min?  Max?

I couldn't find any immediately applicable GL spec text.  Anyone know of
any?  If not, does DirectX mandate something?

I wrote a hack to check isnan and replace it with 0, which gets the game
working again, but...it seems like we could have this problem in a lot of
other places too...and I'm not sure what the right answer is.

https://cgit.freedesktop.org/~kwg/mesa/commit/?h=xcom&id=6a1c0515b760c943eb547cced754b465aa3bd4ca

Thanks for any advice :)

--Ken
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