[Intel-gfx] [PATCH] drm/i915/region: fix order when adding blocks

[Chris Wilson]

Quoting Matthew Auld (2020-11-09 11:12:49)
> When performing an allocation we try split it down into the largest
> possible power-of-two blocks/pages-sizes, and for the common case we
> expect to allocate the blocks in descending order. This also naturally
> fits with our GTT alignment tricks(including the hugepages selftest),
> where we sometimes try to align to the largest possible GTT page-size
> for the allocation, in the hope that translates to bigger GTT
> page-sizes. Currently, we seem to incorrectly add the blocks in the
> opposite order, which is definitely not the intended behaviour.

Right, so currently we end up with small -> large blocks, and you want
to change it so that we use large -> small blocks, because we want to
keep things aligned for as long as possible (if we start small, we
immediately spoil the alignment provided for us).

How can we test this? Create a 2MiB + 4KiB object and ask if
page_sizes.gtt has a 2MiB? I don't think it can be directly observed
from userspace, right? Is it possible to generalise the characteristic
we expect and prevent future errors?
-Chris