[Mesa-dev] [PATCH] i965: Deny persistent mappings of incoherent GTT mmaps

Lionel Landwerlin lionel.g.landwerlin at intel.com
Fri Aug 31 11:32:23 UTC 2018

On 31/08/2018 12:22, Chris Wilson wrote:
> Quoting Lionel Landwerlin (2018-08-31 12:16:19)
>> We would need a fairly recent kernel (drm-tip?) to test this in CI.
> Unpatched mesa, assumes all is fine.
> Post-patch mesa, assumes all is broken.
> So we can quickly see if anything actually fails if a persistent GGTT
> mmap is rejected. Which is the important part for determining if such
> exclusion will harm anyone. The problem is then is the risk of
> corruption worth keeping it around.
>> I can't see any issue with this because we always have the meta path as
>> a fallback for tiled buffers.
> I'm worried if the mmap actually leaks through to glMapBufferRange with
> say GL_MAP_PERSISTENT_BIT. Hmm, maybe that's all ok so long at the
> client flushes are explicit.

As far as I can tell, buffers are linear, so brw_bo_map() wouldn't even 
try to map in gtt at first.

Then brw_bo_map_wc() would assert if it failed.

So we're fine? :)

> -Chris

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