[Bug 106848] [GM45] igt at gem_mmap_gtt@basic-small-bo-tiledx - fail - Failed assertion: memcmp(ptr , tiled_pattern, PAGE_SIZE) == 0 (edit)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=106848

--- Comment #9 from Chris Wilson <chris at chris-wilson.co.uk> ---
L-shaped memory!

In an uneven config (where the different channels have different number of
banks on them) the swizzling depends on where abouts in memory the page is (as
the system splits into a dual-channel portion and a single-channel portion).

For the uneven configs where you couldn't reproduce the issue, I expect it just
so happened that we didn't get memory placed in different regions (or the
unbalance was so much that the system didn't even try a dual/single setup.

In light of that, bisecting to a commit that changed the allocation pattern
still makes sense (albeit that it just changes the likelihood of receiving
differently swizzled pages).

Hmm. Oh, I see the problem. We are looking at a CPU mmap of the backing page to
check the tiling pattern across the GTT mmap. That doesn't work if the
swizzling varies (since the tiling pattern depends on the physical location of
the page in memory, which we cannot know in userspace).

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