Flicker caused by "drm/i915: Eliminate usage of plane_wm_parameters from ILK-style WM code (v2)"

[Jan Niehusmann <jan@gondor.com>]

Hi,

On Thu, Sep 24, 2015 at 03:53:07PM -0700, Matt Roper wrote:
> Just pull the info out of the plane state structure rather than staging
> it in an additional structure.
> 
> v2: Add 'visible' condition to sprites_scaled so that we don't limit the
>     WM level when the sprite isn't enabled.  (Ville)

It looks like this patch - specifically the visible condition in
ilk_compute_cur_wm - causes screen flicker when moving the cursor from
one screen to another one in a multi-screen setup.

My hardware is a Thinkpad x201s
(http://www.thinkwiki.org/wiki/Category:X201s)
00:02.0 VGA compatible controller [0300]: Intel Corporation Core Processor Integrated Graphics Controller [8086:0046] (rev 02),
CPU is an i7-620LM.

The screen flickering is the one the coursor is leaving. In most cases,
parts of the screen just blank for a very short moment. In some rare
cases, the screen even stays blanked until I move the cursor back to
that screen. No stability issues were observed, this seems to be a
purely cosmetic issue.

I bisected the issue to 43d59eda1f69631c267e06ab6b94ed3c14f1f6d1. As I
knew the flickering was cursor-related, I tried the following minimal
patch, which actually fixed the symptom, when applied to 4.4-rc8:

diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index f091ad1..1ef0c54 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -1791,7 +1791,7 @@ static uint32_t ilk_compute_cur_wm(const struct intel_crtc_state *cstate,
 {
 	int bpp = pstate->base.fb ? pstate->base.fb->bits_per_pixel / 8 : 0;
 
-	if (!cstate->base.active || !pstate->visible)
+	if (!cstate->base.active)
 		return 0;
 
 	return ilk_wm_method2(ilk_pipe_pixel_rate(cstate),


I have no idea at all if this is actually fixing anything or if it's
just hiding the real bug. All I can say is that the flickering doesn't
occur any longer.

Jan

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