Re: [PATCH] drm/i915: Only move to the CPU write domain if keeping the GTT pages

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On 8/9/2015 4:25 PM, Chris Wilson wrote:
On Sun, Aug 09, 2015 at 04:23:01PM +0530, Goel, Akash wrote:
On 8/7/2015 1:37 PM, Daniel Vetter wrote:
I presume though you only want to avoid clflush when actually purging an
object, so maybe we can keep this by purging the shmem backing node first
and checking here for __I915_MADV_PURGED instead?

An object marked as MADV_DONT_NEED, implies that it will be
purged/truncated right away after the call to put_pages_gtt
function.
So doing the other way round by purging first and then checking for
__I915_MADV_PURGED, might be equivalent.

But disregards a few nice sanity checks, which I would like to keep.
-Chris
Fine, just wanted to convey that doing the other way round may not be really beneficial.

About the other point of virtually indexed/physically tagged cache, would it be safe just rely on the MADV_DONT_NEED state of the object (which indicates that there are no active CPU mmappings) ? Due to an earlier CPU mmappings, there could be cachelines holding the stale data ?

Best regards
Akash


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