drm/i915: Fallback to using unmappable memory for scanout

The existing ABI says that scanouts are pinned into the mappable region
so that legacy clients (e.g. old Xorg or plymouthd) can write directly
into the scanout through a GTT mapping. However if the surface does not
fit into the mappable region, we are better off just trying to fit it
anywhere and hoping for the best. (Any userspace that is capable of
using ginormous scanouts is also likely not to rely on pure GTT
updates.) With the partial vma fault support, we are no longer
restricted to only using scanouts that we can pin (though it is still
preferred for performance reasons and for powersaving features like
FBC).

v2: Skip fence pinning when not mappable.
v3: Add a comment to explain the possible ramifications of not being
    able to use fences for unmappable scanouts.
v4: Rebase to skip over some local patches
v5: Rebase to defer until after we have unmappable GTT fault support
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Deepak S <deepak.s@linux.intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-27-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -3581,11 +3581,17 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
 
 	/* As the user may map the buffer once pinned in the display plane
 	 * (e.g. libkms for the bootup splash), we have to ensure that we
-	 * always use map_and_fenceable for all scanout buffers.
+	 * always use map_and_fenceable for all scanout buffers. However,
+	 * it may simply be too big to fit into mappable, in which case
+	 * put it anyway and hope that userspace can cope (but always first
+	 * try to preserve the existing ABI).
 	 */
+	vma = ERR_PTR(-ENOSPC);
+	if (view->type == I915_GGTT_VIEW_NORMAL)
 		vma = i915_gem_object_ggtt_pin(obj, view, 0, alignment,
-				       view->type == I915_GGTT_VIEW_NORMAL ?
-				       PIN_MAPPABLE : 0);
+					       PIN_MAPPABLE | PIN_NONBLOCK);
+	if (IS_ERR(vma))
+		vma = i915_gem_object_ggtt_pin(obj, view, 0, alignment, 0);
 	if (IS_ERR(vma))
 		goto err_unpin_display;
 

drivers/gpu/drm/i915/intel_fbc.c

@@ -776,6 +776,10 @@ static bool intel_fbc_can_activate(struct intel_crtc *crtc)
 
 	/* The use of a CPU fence is mandatory in order to detect writes
 	 * by the CPU to the scanout and trigger updates to the FBC.
+	 *
+	 * Note that is possible for a tiled surface to be unmappable (and
+	 * so have no fence associated with it) due to aperture constaints
+	 * at the time of pinning.
 	 */
 	if (cache->fb.tiling_mode != I915_TILING_X ||
 	    cache->fb.fence_reg == I915_FENCE_REG_NONE) {