Revert "drm/i915: Workaround incoherence between fences and LLC across multiple CPUs"

This reverts commit 25ff119 and the follow on for Valleyview commit 2dc8aae.

commit 25ff1195f8a0b3724541ae7bbe331b4296de9c06
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Apr 4 21:31:03 2013 +0100

    drm/i915: Workaround incoherence between fences and LLC across multiple CPUs

commit 2dc8aae06d53458dd3624dc0accd4f81100ee631
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed May 22 17:08:06 2013 +0100

    drm/i915: Workaround incoherence with fence updates on Valleyview

Jon Bloomfield came up with a plausible explanation and cheap fix
(drm/i915: Fix incoherence with fence updates on Sandybridge+) for the
race condition, so lets run with it.

This is a candidate for stable as the old workaround incurs a
significant cost (calling wbinvd on all CPUs before performing the
register write) for some workloads as noted by Carsten Emde.

Link: http://lists.freedesktop.org/archives/intel-gfx/2013-June/028819.html
References: https://www.osadl.org/?id=1543#c7602
References: https://bugs.freedesktop.org/show_bug.cgi?id=63825
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Carsten Emde <C.Emde@osadl.org>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 4 insertions, 43 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index a34e8e2ba98a..06d66e09da17 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2829,56 +2829,17 @@ static inline int fence_number(struct drm_i915_private *dev_priv,
         return fence - dev_priv->fence_regs;
 }
 
-struct write_fence {
-        struct drm_device *dev;
-        struct drm_i915_gem_object *obj;
-        int fence;
-};
-
-static void i915_gem_write_fence__ipi(void *data)
-{
-        struct write_fence *args = data;
-
-        /* Required for SNB+ with LLC */
-        wbinvd();
-
-        /* Required for VLV */
-        i915_gem_write_fence(args->dev, args->fence, args->obj);
-}
-
 static void i915_gem_object_update_fence(struct drm_i915_gem_object *obj,
                                          struct drm_i915_fence_reg *fence,
                                          bool enable)
 {
         struct drm_i915_private *dev_priv = obj->base.dev->dev_private;
-        struct write_fence args = {
-                .dev = obj->base.dev,
-                .fence = fence_number(dev_priv, fence),
-                .obj = enable ? obj : NULL,
-        };
-
-        /* In order to fully serialize access to the fenced region and
-         * the update to the fence register we need to take extreme
-         * measures on SNB+. In theory, the write to the fence register
-         * flushes all memory transactions before, and coupled with the
-         * mb() placed around the register write we serialise all memory
-         * operations with respect to the changes in the tiler. Yet, on
-         * SNB+ we need to take a step further and emit an explicit wbinvd()
-         * on each processor in order to manually flush all memory
-         * transactions before updating the fence register.
-         *
-         * However, Valleyview complicates matter. There the wbinvd is
-         * insufficient and unlike SNB/IVB requires the serialising
-         * register write. (Note that that register write by itself is
-         * conversely not sufficient for SNB+.) To compromise, we do both.
-         */
-        if (INTEL_INFO(args.dev)->gen >= 6)
-                on_each_cpu(i915_gem_write_fence__ipi, &args, 1);
-        else
-                i915_gem_write_fence(args.dev, args.fence, args.obj);
+        int reg = fence_number(dev_priv, fence);
+
+        i915_gem_write_fence(obj->base.dev, reg, enable ? obj : NULL);
 
         if (enable) {
-                obj->fence_reg = args.fence;
+                obj->fence_reg = reg;
                 fence->obj = obj;
                 list_move_tail(&fence->lru_list, &dev_priv->mm.fence_list);
         } else {