drm/i915: Drop ORIGIN_GTT for untracked GTT writes

If FBC is set on a framebuffer that is unmapped, all GTT faults will be
from a partial mapping. Writes by the user through the partial VMA are
then untracked by the FBC and so we must use the ORIGIN_CPU when flushing
the I915_GEM_DOMAIN_GTT.

v2: Keep ORIGIN_CPU for set-to-domain(.write=CPU)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: "Zanoni, Paulo R" <paulo.r.zanoni@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-25-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -2198,6 +2198,7 @@ struct drm_i915_gem_object {
 	unsigned int cache_dirty:1;
 
 	atomic_t frontbuffer_bits;
+	unsigned int frontbuffer_ggtt_origin; /* write once */
 
 	/** Current tiling stride for the object, if it's tiled. */
 	unsigned int tiling_and_stride;
@@ -2205,7 +2206,6 @@ struct drm_i915_gem_object {
 #define TILING_MASK (FENCE_MINIMUM_STRIDE-1)
 #define STRIDE_MASK (~TILING_MASK)
 
-	unsigned int has_wc_mmap;
 	/** Count of VMA actually bound by this object */
 	unsigned int bind_count;
 	unsigned int pin_display;

drivers/gpu/drm/i915/i915_gem.c

@@ -1497,8 +1497,8 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
 static inline 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;
+	return (domain == I915_GEM_DOMAIN_GTT ?
+		obj->frontbuffer_ggtt_origin : ORIGIN_CPU);
 }
 
 /**
@@ -1658,7 +1658,7 @@ i915_gem_mmap_ioctl(struct drm_device *dev, void *data,
 		up_write(&mm->mmap_sem);
 
 		/* This may race, but that's ok, it only gets set */
-		WRITE_ONCE(obj->has_wc_mmap, true);
+		WRITE_ONCE(obj->frontbuffer_ggtt_origin, ORIGIN_CPU);
 	}
 	i915_gem_object_put_unlocked(obj);
 	if (IS_ERR((void *)addr))
@@ -1761,6 +1761,11 @@ int i915_gem_fault(struct vm_area_struct *area, struct vm_fault *vmf)
 		if (chunk_size >= obj->base.size >> PAGE_SHIFT)
 			view.type = I915_GGTT_VIEW_NORMAL;
 
+		/* Userspace is now writing through an untracked VMA, abandon
+		 * all hope that the hardware is able to track future writes.
+		 */
+		obj->frontbuffer_ggtt_origin = ORIGIN_CPU;
+
 		vma = i915_gem_object_ggtt_pin(obj, &view, 0, 0, PIN_MAPPABLE);
 	}
 	if (IS_ERR(vma)) {
@@ -4093,6 +4098,7 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
 
 	obj->ops = ops;
 
+	obj->frontbuffer_ggtt_origin = ORIGIN_GTT;
 	obj->madv = I915_MADV_WILLNEED;
 
 	i915_gem_info_add_obj(to_i915(obj->base.dev), obj->base.size);