drm/i915: use ORIGIN_CPU for frontbuffer invalidation on WC mmaps

... instead of the previous ORIGIN_GTT. This should actually
invalidate FBC once something is written on the frontbuffer using WC
mmaps. The problem with ORIGIN_GTT is that the automatic hardware
tracking is not able to detect the WC writes as it can detect the GTT
writes.

This should help fix the SKL bug where nothing happens when you type
your username/password on lightdm.

This patch was originally pasted on an email by Chris and converted to
an actual git patch by Paulo.

v2 (from Paulo):
 - Make it a full variable instead of a bit-field (Daniel)
 - Use WRITE_ONCE (Chris)
v3 (from Paulo):
 - Remove huge comment since now we have WRITE_ONCE (Chris)
 - Remove uneeded new line (Chris)
 - Add Chris' Signed-off-by, authorized via IRC

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466185599-26401-1-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -2214,6 +2214,7 @@ struct drm_i915_gem_object {
 
 	unsigned int frontbuffer_bits:INTEL_FRONTBUFFER_BITS;
 
+	unsigned int has_wc_mmap;
 	unsigned int pin_display;
 
 	struct sg_table *pages;

drivers/gpu/drm/i915/i915_gem.c

@@ -1810,6 +1810,13 @@ static struct intel_rps_client *to_rps_client(struct drm_file *file)
 	return &fpriv->rps;
 }
 
+static enum fb_op_origin
+write_origin(struct drm_i915_gem_object *obj, unsigned domain)
+{
+	return domain == I915_GEM_DOMAIN_GTT && !obj->has_wc_mmap ?
+	       ORIGIN_GTT : ORIGIN_CPU;
+}
+
 /**
  * Called when user space prepares to use an object with the CPU, either
  * through the mmap ioctl's mapping or a GTT mapping.
@@ -1866,9 +1873,7 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
 		ret = i915_gem_object_set_to_cpu_domain(obj, write_domain != 0);
 
 	if (write_domain != 0)
-		intel_fb_obj_invalidate(obj,
-					write_domain == I915_GEM_DOMAIN_GTT ?
-					ORIGIN_GTT : ORIGIN_CPU);
+		intel_fb_obj_invalidate(obj, write_origin(obj, write_domain));
 
 unref:
 	drm_gem_object_unreference(&obj->base);
@@ -1975,6 +1980,9 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
 		else
 			addr = -ENOMEM;
 		up_write(&mm->mmap_sem);
+
+		/* This may race, but that's ok, it only gets set */
+		WRITE_ONCE(to_intel_bo(obj)->has_wc_mmap, true);
 	}
 	drm_gem_object_unreference_unlocked(obj);
 	if (IS_ERR((void *)addr))