In the past, the GTT has always been sized just large enough to map the whole
graphics aperture. However, apparently on the G965 that isn't the case, and
it is actually 512KB on hardware with a 256MB aperture. This resulted in X
not bothering to allocate memory for 256KB that it thought was already mapped
into stolen memory, and thus garbage rendering (particularly visible in large
video modes that displayed this unallocated memory). The kernel happens to
get the right answer by hardwiring a 512KB GTT size already, but that may not
be true on future hardware.
Instead, we use a convenient field in PGETBL_CTL that's specifically for the
GTT size rather than the aperture size, which gets us the answer we want.
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