The cachelines are used for two things: XAA pixmap cache and XV memory.
Only XAA pixmap cache is referred to using an offset pointing at the
beginning of the front buffer in rendering, and XAA only uses the 2d BLT
engine, which actually has a vertical limit of 65536. So, pixmap cache is now
limited to that much vertical.
Additionally, the previous cachelines allocation was too small for our
advertised XV limits, so video at the limits would fail with BadAlloc. Now,
XAA allocates the same approximate amount of offscreen memory as EXA:
3 times the screen size, plus one packed HD video.
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