drm/i915/execlists: Apply a full mb before execution for Braswell

BugLink: https://bugs.launchpad.net/bugs/1837257
commit cf66b8a0ba142fbd1bf10ac8f3ae92d1b0cb7b8f upstream.

Braswell is really picky about having our writes posted to memory before
we execute or else the GPU may see stale values. A wmb() is insufficient
as it only ensures the writes are visible to other cores, we need a full
mb() to ensure the writes are in memory and visible to the GPU.

The most frequent failure in flushing before execution is that we see
stale PTE values and execute the wrong pages.

References: 987abd5c62f9 ("drm/i915/execlists: Force write serialisation into context image vs execution")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181206084431.9805-3-chris@chris-wilson.co.uk
(cherry picked from commit 490b8c65b9db45896769e1095e78725775f47b3e)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index abd1a51c1fe65fbe9b2711d8da61a83c733ebdf0..e696f5dd1f14561644b745f80469bad03e9524bc 100644 (file)
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -436,8 +436,13 @@ static u64 execlists_update_context(struct drm_i915_gem_request *rq)
         * may not be visible to the HW prior to the completion of the UC
         * register write and that we may begin execution from the context
         * before its image is complete leading to invalid PD chasing.
+        *
+        * Furthermore, Braswell, at least, wants a full mb to be sure that
+        * the writes are coherent in memory (visible to the GPU) prior to
+        * execution, and not just visible to other CPUs (as is the result of
+        * wmb).
         */
-       wmb();
+       mb();
        return ce->lrc_desc;
 }