[Intel-gfx] [PATCH] drm/i915: Fix screen flickering on X

[Ismael Luceno]

A bisect showed that commit 32b7eeec4d1e861230b09d437e95d76c86ff4a68
introduced the issue.

The issue starts as soon as X takes control of the screen, even if just
a plain X doing nothing, so based on the code touched by the commit I
thought it had to be related to the so called "hardware cursor". I
confirmed it when hiding the cursor made the flickering go away.

The aforementioned commit removed some suspicious code, and the
Programmer's Reference Manual confirmed my suspicion:

"Incorrectly programmed watermark values can result in screen corruption.

The watermarks should be calculated and programmed when any of the
watermark calculation inputs change. This includes planes enabling or
disabling, plane source format or size changing, etc."

So I'm re-adding the few lines that update the watermarks after a cursor
size change.

Signed-off-by: Ismael Luceno <ismael at iodev.co.uk>
---
 drivers/gpu/drm/i915/intel_display.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index f75173c..e23f062 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -12258,6 +12258,7 @@ intel_commit_cursor_plane(struct drm_plane *plane,
 	struct intel_crtc *intel_crtc;
 	struct intel_plane *intel_plane = to_intel_plane(plane);
 	struct drm_i915_gem_object *obj = intel_fb_obj(state->base.fb);
+	unsigned old_width;
 	uint32_t addr;
 
 	crtc = crtc ? crtc : plane->crtc;
@@ -12282,11 +12283,15 @@ intel_commit_cursor_plane(struct drm_plane *plane,
 	intel_crtc->cursor_addr = addr;
 	intel_crtc->cursor_bo = obj;
 update:
+	old_width = intel_crtc->cursor_width;
 	intel_crtc->cursor_width = state->base.crtc_w;
 	intel_crtc->cursor_height = state->base.crtc_h;
 
-	if (intel_crtc->active)
+	if (intel_crtc->active) {
+		if (old_width != intel_crtc->cursor_width)
+			intel_update_watermarks(crtc);
 		intel_crtc_update_cursor(crtc, state->visible);
+	}
 }
 
 static struct drm_plane *intel_cursor_plane_create(struct drm_device *dev,
-- 
2.3.4