[Bug 97473] Memory corruption when uploading DXT5 cubemap faces

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97473

--- Comment #2 from Kenneth Graunke <kenneth at whitecape.org> ---
There are many solutions to this:

1. Fall back to untiled for compressed textures that are too big to blit.
   ("Better to fall off a performance cliff than hit a spike trap.")  Easy to
implement, and backport to stable releases, but awful.  Need to get the math
right to avoid sending more things than necessary off the performance cliff.

2. Take Chris Wilson's "unlimited GTT map!" patch, which apparently exploits a
Kernel 4.9 feature to eliminate the need for the check.  I'm not sure how this
works yet.  It would be nice to solve it without requiring Kernel 4.9.

3. Teach the BLT code about compressed textures.  Doable.  Probably
backportable.

4. Use BLORP for the map-blit path.  Jason has patches to make it able to do
dumb copies by lying about formats.  We could use this for MapTextureImage.

I'm thinking we want to do #3 (fix BLT, and backport), #2 (make CPU mapping
always work), and longer term #4 (stop using the BLT).

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