drm/i915: Simplify flush_cpu_write_domain

We can push down the decision whether to force flushing into the
implementation since in all places that matter obj->pin_display is
accurate already. The only place where the optimization really matters
is the sw_finish_ioctl, and that already checks for obj->pin_display
on its own.

I suspect that this was simply an artifact of how

commit 2c22569bba8af6c2976d5f9479fe54a53a39966b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Aug 9 12:26:45 2013 +0100

    drm/i915: Update rules for writing through the LLC with the cpu

evolved - only v2 added the pin_display tracking.

Note that we still retain the gist of this logic from the above commit
with the explicit force argument for the low-level clflush function.

Ville noted in his review that there's a slight behavioural change in
the set_to_gtt_domain function, which now also will flush display
plane data. This opens-open the potential for userspace to start doing
buggy things by omitting the sw_finish_ioctl, which is why I've
rejected a functional equivalent patch from Ville a while ago:

http://lists.freedesktop.org/archives/intel-gfx/2013-November/036421.html

But on second consideration it's not that evil, and in any case the
justification here is more clarity, not allowing crazy userspace.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -39,8 +39,7 @@
 #include <linux/dma-buf.h>
 
 static void i915_gem_object_flush_gtt_write_domain(struct drm_i915_gem_object *obj);
-static void i915_gem_object_flush_cpu_write_domain(struct drm_i915_gem_object *obj,
-						   bool force);
+static void i915_gem_object_flush_cpu_write_domain(struct drm_i915_gem_object *obj);
 static __must_check int
 i915_gem_object_wait_rendering(struct drm_i915_gem_object *obj,
 			       bool readonly);
@@ -1516,7 +1515,7 @@ i915_gem_sw_finish_ioctl(struct drm_device *dev, void *data,
 
 	/* Pinned buffers may be scanout, so flush the cache */
 	if (obj->pin_display)
-		i915_gem_object_flush_cpu_write_domain(obj, true);
+		i915_gem_object_flush_cpu_write_domain(obj);
 
 	drm_gem_object_unreference(&obj->base);
 unlock:
@@ -3680,15 +3679,14 @@ i915_gem_object_flush_gtt_write_domain(struct drm_i915_gem_object *obj)
 
 /** Flushes the CPU write domain for the object if it's dirty. */
 static void
-i915_gem_object_flush_cpu_write_domain(struct drm_i915_gem_object *obj,
-				       bool force)
+i915_gem_object_flush_cpu_write_domain(struct drm_i915_gem_object *obj)
 {
 	uint32_t old_write_domain;
 
 	if (obj->base.write_domain != I915_GEM_DOMAIN_CPU)
 		return;
 
-	if (i915_gem_clflush_object(obj, force))
+	if (i915_gem_clflush_object(obj, obj->pin_display))
 		i915_gem_chipset_flush(obj->base.dev);
 
 	old_write_domain = obj->base.write_domain;
@@ -3735,7 +3733,7 @@ i915_gem_object_set_to_gtt_domain(struct drm_i915_gem_object *obj, bool write)
 	if (ret)
 		return ret;
 
-	i915_gem_object_flush_cpu_write_domain(obj, false);
+	i915_gem_object_flush_cpu_write_domain(obj);
 
 	/* Serialise direct access to this object with the barriers for
 	 * coherent writes from the GPU, by effectively invalidating the
@@ -3981,7 +3979,7 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
 	if (ret)
 		goto err_unpin_display;
 
-	i915_gem_object_flush_cpu_write_domain(obj, true);
+	i915_gem_object_flush_cpu_write_domain(obj);
 
 	old_write_domain = obj->base.write_domain;
 	old_read_domains = obj->base.read_domains;