drm/i915: flush system agent TLBs on SNB

This allows us to map the PTEs WC. I've not done thorough testing or
performance measurements with this patch, but it should be decent.

This is based on a patch from Jesse with the original commit message
> I've only lightly tested this so far, but the corruption seems to be
> gone if I write the GFX_FLSH_CNTL reg after binding an object.  This
> register should control the TLB for the system agent, which is what CPU
> mapped objects will go through.

It has been updated for the new AGP-less code by me, and included with
it is feedback from the original patch.

v2: Updated to reflect paranoia on pte updates/register posting reads.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by [v1]: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -379,6 +379,13 @@ static void gen6_ggtt_bind_object(struct drm_i915_gem_object *obj,
 	 */
 	if (i != 0)
 		WARN_ON(readl(&gtt_entries[i-1]) != pte_encode(dev, addr, level));
+
+	/* This next bit makes the above posting read even more important. We
+	 * want to flush the TLBs only after we're certain all the PTE updates
+	 * have finished.
+	 */
+	I915_WRITE(GFX_FLSH_CNTL_GEN6, GFX_FLSH_CNTL_EN);
+	POSTING_READ(GFX_FLSH_CNTL_GEN6);
 }
 
 void i915_gem_gtt_bind_object(struct drm_i915_gem_object *obj,
@@ -589,8 +596,8 @@ int i915_gem_gtt_init(struct drm_device *dev)
 		goto err_out;
 	}
 
-	dev_priv->mm.gtt->gtt = ioremap(gtt_bus_addr,
-					dev_priv->mm.gtt->gtt_total_entries * sizeof(gtt_pte_t));
+	dev_priv->mm.gtt->gtt = ioremap_wc(gtt_bus_addr,
+					   dev_priv->mm.gtt->gtt_total_entries * sizeof(gtt_pte_t));
 	if (!dev_priv->mm.gtt->gtt) {
 		DRM_ERROR("Failed to map the gtt page table\n");
 		teardown_scratch_page(dev);

drivers/gpu/drm/i915/i915_reg.h

@@ -694,6 +694,8 @@
 #define   CM0_RC_OP_FLUSH_DISABLE (1<<0)
 #define BB_ADDR		0x02140 /* 8 bytes */
 #define GFX_FLSH_CNTL	0x02170 /* 915+ only */
+#define GFX_FLSH_CNTL_GEN6	0x101008
+#define   GFX_FLSH_CNTL_EN	(1<<0)
 #define ECOSKPD		0x021d0
 #define   ECO_GATING_CX_ONLY	(1<<3)
 #define   ECO_FLIP_DONE		(1<<0)