[Intel-gfx] [PATCH 4/4] drm/i915: Force CPU synchronisation even if userspace requests ASYNC

[Chris Wilson]

Quoting Joonas Lahtinen (2017-07-26 11:19:26)
> On pe, 2017-07-21 at 15:50 +0100, Chris Wilson wrote:
> > The goal here was to minimise doing any thing or any check inside the
> > kernel that was not strictly required. For a userspace that assumes
> > complete control over the cache domains, the kernel is usually using
> > outdated information and may trigger clflushes where none were
> > required.
> > 
> > However, swapping is a situation where userspace has no knowledge of the
> > domain transfer, and will leave the object in the CPU cache. The kernel
> > must flush this out to the backing storage prior to use with the GPU. As
> > we use an asynchronous task tracked by an implicit fence for this, we
> > also need to cancel the ASYNC flag on the object so that the object will
> > wait for the clflush to complete before being executed. This also absolves
> > userspace of the responsibility imposed by commit 77ae9957897d ("drm/i915:
> > Enable userspace to opt-out of implicit fencing") that its needed to ensure
> > that the object was out of the CPU cache prior to use on the GPU.
> > 
> > Fixes: 77ae9957897d ("drm/i915: Enable userspace to opt-out of implicit fencing")
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101571
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > Cc: Jason Ekstrand <jason at jlekstrand.net>
> > Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
> 
> All explained out, makes perfect sense.
> 
> Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>

And finally pushed, hopefully making it to v4.12.y before too many people
hit the vk-on-(bsw,bxt,glk) bug.
-Chris