[PATCH 2/2] drm/i915: wait for IPS_ENABLE when enabling IPS

[Paulo Zanoni <przanoni@gmail.com>]

From: Paulo Zanoni <paulo.r.zanoni@intel.com>

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>
---
 drivers/gpu/drm/i915/intel_display.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 78ff5ed..8fd13ab 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -3374,6 +3374,14 @@ static 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");
 }
 
 static void hsw_disable_ips(struct intel_crtc *crtc)
-- 
1.8.3.1