drm/i915: Add a FIXME about FBC vs. fence. 90/270 degree rotation

Currently the FBC code doesn't handle the 90/270 degree rotated case
correctly. We would need the GTT tracking to monitor the fence on the
normal GTT view (the rotated view doesn't even have a fence). Not quite
sure how we should program the fence Y offset etc. in that case. For now
we'll end up disabling FBC with 90/270 degree rotation. Add a FIXME
to remind people about this fact.

v2: Reword the text (Chris)
    Move the FIXME to the fbc code

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221160235.11134-7-ville.syrjala@linux.intel.comReviewed-by: NChris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_fbc.c

@@ -810,6 +810,12 @@ static bool intel_fbc_can_activate(struct intel_crtc *crtc)
 	 * 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.
+	 *
+	 * FIXME with 90/270 degree rotation we should use the fence on
+	 * the normal GTT view (the rotated view doesn't even have a
+	 * fence). Would need changes to the FBC fence Y offset as well.
+	 * For now this will effecively disable FBC with 90/270 degree
+	 * rotation.
 	 */
 	if (!(cache->flags & PLANE_HAS_FENCE)) {
 		fbc->no_fbc_reason = "framebuffer not tiled or fenced";