Bug 81647

Created attachment 103304 [details]
revert commit  c6930992948adf0f8fc1f6ff1da51c5002a2cf95

I found the problem on my computer Toshipa S70 
with VGA compatible controller: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller (rev 06) was caused by commit c6930992948adf0f8fc1f6ff1da51c5002a2cf95

file drivers/gpu/drm/i915/intel_dp.c

sequence around loop 

for (clock = min_clock; clock <= max_clock; clock++) {
for (lane_count = min_lane_count; lane_count <= max_lane_count; lane_count <<= 1) {

Reversing this commit solved the problem, I recompiled the 
the drim-intel-nightly (8734408c113bb38234ed03ec51c723b3deff579b)
and it works fine. 

The error is reproducible, ie. by swithcing the sequence of the loops I can reproduce the blury screen or flicker

Note this is a case where the earlier regression triumphs, and so this bug must persist until the root cause is known (i.e. we can fix this without breaking working machines).

(In reply to comment #3)
> Note this is a case where the earlier regression triumphs, and so this bug
> must persist until the root cause is known (i.e. we can fix this without
> breaking working machines).

Is there anything I can do to help?

(In reply to comment #4)
> (In reply to comment #3)
> > Note this is a case where the earlier regression triumphs, and so this bug
> > must persist until the root cause is known (i.e. we can fix this without
> > breaking working machines).
> 
> Is there anything I can do to help?

I presume this is the laptop panel. Please try this patch:

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index d7fc2c0e9ba8..f4248d7f64f9 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -889,23 +889,13 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 			bpp = dev_priv->vbt.edp_bpp;
 		}
 
-		if (IS_BROADWELL(dev)) {
-			/* Yes, it's an ugly hack. */
-			min_lane_count = max_lane_count;
-			DRM_DEBUG_KMS("forcing lane count to max (%u) on BDW\n",
-				      min_lane_count);
-		} else if (dev_priv->vbt.edp_lanes) {
-			min_lane_count = min(dev_priv->vbt.edp_lanes,
-					     max_lane_count);
-			DRM_DEBUG_KMS("using min %u lanes per VBT\n",
-				      min_lane_count);
-		}
-
-		if (dev_priv->vbt.edp_rate) {
-			min_clock = min(dev_priv->vbt.edp_rate >> 3, max_clock);
-			DRM_DEBUG_KMS("using min %02x link bw per VBT\n",
-				      bws[min_clock]);
-		}
+		/*
+		 * Use the maximum clock and number of lanes the eDP panel
+		 * advertizes being capable of. Typically these values
+		 * correspond to the native resolution of the panel.
+		 */
+		min_lane_count = max_lane_count;
+		min_clock = max_clock;
 	}
 
 	for (; bpp >= 6*3; bpp -= 2*3) {

(In reply to comment #5)
> (In reply to comment #4)
> > (In reply to comment #3)
> > > Note this is a case where the earlier regression triumphs, and so this bug
> > > must persist until the root cause is known (i.e. we can fix this without
> > > breaking working machines).
> > 
> > Is there anything I can do to help?
> 
> I presume this is the laptop panel. Please try this patch:
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c
> b/drivers/gpu/drm/i915/intel_dp.c
> index d7fc2c0e9ba8..f4248d7f64f9 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -889,23 +889,13 @@ intel_dp_compute_config(struct intel_encoder *encoder,
>  			bpp = dev_priv->vbt.edp_bpp;
>  		}
>  
> -		if (IS_BROADWELL(dev)) {
> -			/* Yes, it's an ugly hack. */
> -			min_lane_count = max_lane_count;
> -			DRM_DEBUG_KMS("forcing lane count to max (%u) on BDW\n",
> -				      min_lane_count);
> -		} else if (dev_priv->vbt.edp_lanes) {
> -			min_lane_count = min(dev_priv->vbt.edp_lanes,
> -					     max_lane_count);
> -			DRM_DEBUG_KMS("using min %u lanes per VBT\n",
> -				      min_lane_count);
> -		}
> -
> -		if (dev_priv->vbt.edp_rate) {
> -			min_clock = min(dev_priv->vbt.edp_rate >> 3, max_clock);
> -			DRM_DEBUG_KMS("using min %02x link bw per VBT\n",
> -				      bws[min_clock]);
> -		}
> +		/*
> +		 * Use the maximum clock and number of lanes the eDP panel
> +		 * advertizes being capable of. Typically these values
> +		 * correspond to the native resolution of the panel.
> +		 */
> +		min_lane_count = max_lane_count;
> +		min_clock = max_clock;
>  	}
>  
>  	for (; bpp >= 6*3; bpp -= 2*3) {


Yes, this is a laptop. 
Your patch is working, I used the drm-intel-nightly (2014y-09m-03d-15h-44m-26s   SHA1 ID: 4144c90b76dfe6eaa2205ac947090786b5091cff) for testing

Thanks

so it should have been fixed by https://bugs.freedesktop.org/show_bug.cgi?id=79386#c16

*** This bug has been marked as a duplicate of bug 79386 ***