drm/i915: wait for IPS_ENABLE when enabling IPS

At the end of haswell_crtc_enable we have an intel_wait_for_vblank
with a big comment, and the message suggests it's a workaround for
something we don't really understand. So I removed that wait and
started getting HW state readout error messages saying that the IPS
state is not what we expected.

I investigated and concluded that after you write IPS_ENABLE to
IPS_CTL, the bit will only actually become 1 on the next vblank. So
add code to wait for the IPS_ENABLE bit. We don't really need this
wait right now due to the wait I already mentioned, but at least this
one has a reason to be there, while the other one is just to
workaround some problem: we may remove it in the future.

The wait also acts as a POSTING_READ which we missed.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -3346,6 +3346,14 @@ void hsw_enable_ips(struct intel_crtc *crtc)
 	 * for a vblank, so all we need to do here is to enable the IPS bit. */
 	assert_plane_enabled(dev_priv, crtc->plane);
 	I915_WRITE(IPS_CTL, IPS_ENABLE);
+
+	/* The bit only becomes 1 in the next vblank, so this wait here is
+	 * essentially intel_wait_for_vblank. If we don't have this and don't
+	 * wait for vblanks until the end of crtc_enable, then the HW state
+	 * readout code will complain that the expected IPS_CTL value is not the
+	 * one we read. */
+	if (wait_for(I915_READ_NOTRACE(IPS_CTL) & IPS_ENABLE, 50))
+		DRM_ERROR("Timed out waiting for IPS enable\n");
 }
 
 void hsw_disable_ips(struct intel_crtc *crtc)