[PATCH] drm: imx: Fix occasional screen corruption on modeset.

[Martin Fuzzey <martin.fuzzey@flowbird.group>]

When performing a modeset the atomic core calls
ipu_crtc_atomic_disable() which switches off the DC and DI.

When we immediately restart as in the modeset case this sometimes
leads to corruption at the bottom of the screen looking like a mirror
image of the top.

The exact reason isn't understood but it seems timing related.

This was observed on i.MX6DL on a system that does 2 mode
transitions on boot (fbcon->android boot animation->android homescreen)
then no more during normal operation resulting in corruption
about once every 10 boots that lasted for variable durations
from a few seconds to several hours.

Dumping the buffers confirmed that they were correct in memory,
just the display was wrong.

For tests the problem was reproduced systematically by forcing
a full modeset every 10 frames even if when not required.

Leaving the DC and DI on if the CRTC is staying on fixes this.

Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
---
 drivers/gpu/drm/imx/ipuv3-crtc.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/imx/ipuv3-crtc.c b/drivers/gpu/drm/imx/ipuv3-crtc.c
index 63c0284..9137b64 100644
--- a/drivers/gpu/drm/imx/ipuv3-crtc.c
+++ b/drivers/gpu/drm/imx/ipuv3-crtc.c
@@ -84,8 +84,15 @@ static void ipu_crtc_atomic_disable(struct drm_crtc *crtc,
 	struct ipu_crtc *ipu_crtc = to_ipu_crtc(crtc);
 	struct ipu_soc *ipu = dev_get_drvdata(ipu_crtc->dev->parent);
 
-	ipu_dc_disable_channel(ipu_crtc->dc);
-	ipu_di_disable(ipu_crtc->di);
+	/*
+	 * If we are just doing a modeset don't disable dc/di as that
+	 * sometimes leads to corruption at the bottom of the screen
+	 */
+	if (!crtc->state->active) {
+		ipu_dc_disable_channel(ipu_crtc->dc);
+		ipu_di_disable(ipu_crtc->di);
+	}
+
 	/*
 	 * Planes must be disabled before DC clock is removed, as otherwise the
 	 * attached IDMACs will be left in undefined state, possibly hanging
-- 
1.9.1