Quoting Ville Syrjälä (2018-08-22 13:35:52)
> On Fri, Aug 17, 2018 at 09:24:05AM +0100, Chris Wilson wrote:
> > The optimisation inherent in commit 6a2c4232ece1 ("drm/i915: Make the
> > physical object coherent with GTT") relies on that once we allocated a
> > cursor we would have coherent, zero overhead access to the scanout plane
> > holding the cursor. That is we could then do the very frequent cursor
> > updates X enjoys with no indirection or kernel involvement. However,
> > that all hinges on the GGTT mmap of the cursor being pinned and not
> > require refaulting on each access -- handling such a page fault likely
> > requires the busy GGTT to be rearranged causing a stall. A very simple
> > fix is then to handle the physical cursor exactly like other cursors and
> > keep its vma pinned while active.
> >
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107600
>
> I guess this wasn't the thing we wanted. But seems quite harmless to
> me anyway, so
It ties neatly in with the ggtt map being used for updates, so I think
it's harmless enough. If I could just get a contiguous page out of
shmemfs, I could follow up with the removal of phys_object. :|
>
> Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
>
> in case you still want to land it.
But without the bugzilla since that bug is occurring without cursor
updates, so is even more bizarre.
-Chris
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