drm/i915: Serialise updates to GGTT with access through GGTT on Braswell

When accessing through the GTT from one CPU whilst concurrently updating
the GGTT PTEs in another thread, the hardware likes to return random
data. As we have strong serialisation prevent us from modifying the PTE
of an active GTT mmapping, we have to conclude that it whilst modifying
other PTE's that error occurs. (I have not looked for any pattern such
as modifying PTE within the same page or cacheline as active PTE -
though checking whether revoking neighbouring objects should be enough
to test that theory.) The corruption also seems restricted to Braswell
and disappears with maxcpus=0. This patch stops all access through the
GTT by other CPUs when we update any PTE by stopping the machine around
the GGTT update.

Note that splitting up the 64 bit write into two 32 bit writes was
tried and found to fail too.

Testcase: igt/gem_concurrent_blit
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89079
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add note about 2x 32bits failing too.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/Kconfig

@@ -10,6 +10,7 @@ config DRM_I915
 	# the shmem_readpage() which depends upon tmpfs
 	select SHMEM
 	select TMPFS
+	select STOP_MACHINE
 	select DRM_KMS_HELPER
 	select DRM_PANEL
 	select DRM_MIPI_DSI

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -24,6 +24,7 @@
  */
 
 #include <linux/seq_file.h>
+#include <linux/stop_machine.h>
 #include <drm/drmP.h>
 #include <drm/i915_drm.h>
 #include "i915_drv.h"
@@ -2533,6 +2534,26 @@ static int ggtt_bind_vma(struct i915_vma *vma,
 	return 0;
 }
 
+struct ggtt_bind_vma__cb {
+	struct i915_vma *vma;
+	enum i915_cache_level cache_level;
+	u32 flags;
+};
+
+static int ggtt_bind_vma__cb(void *_arg)
+{
+	struct ggtt_bind_vma__cb *arg = _arg;
+	return ggtt_bind_vma(arg->vma, arg->cache_level, arg->flags);
+}
+
+static int ggtt_bind_vma__BKL(struct i915_vma *vma,
+			      enum i915_cache_level cache_level,
+			      u32 flags)
+{
+	struct ggtt_bind_vma__cb arg = { vma, cache_level, flags };
+	return stop_machine(ggtt_bind_vma__cb, &arg, NULL);
+}
+
 static int aliasing_gtt_bind_vma(struct i915_vma *vma,
 				 enum i915_cache_level cache_level,
 				 u32 flags)
@@ -3000,6 +3021,9 @@ static int gen8_gmch_probe(struct drm_device *dev,
 	dev_priv->gtt.base.bind_vma = ggtt_bind_vma;
 	dev_priv->gtt.base.unbind_vma = ggtt_unbind_vma;
 
+	if (IS_CHERRYVIEW(dev))
+		dev_priv->gtt.base.bind_vma = ggtt_bind_vma__BKL;
+
 	return ret;
 }