Patch "drm/i915/dp: Revert "drm/i915/dp: fall back to 18 bpp when sink capability is unknown"" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

drm/i915/dp: Revert "drm/i915/dp: fall back to 18 bpp when sink capability is unknown"

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     drm-i915-dp-revert-drm-i915-dp-fall-back-to-18-bpp-when-sink-capability-is-unknown.patch

and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mario Kleiner <[email protected]>
Date: Wed, 6 Jul 2016 12:05:45 +0200
Subject: drm/i915/dp: Revert "drm/i915/dp: fall back to 18 bpp when sink capability is unknown"
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From: Mario Kleiner <[email protected]>

commit 196f954e250943df414efd3d632254c29be38e59 upstream.

This reverts commit 013dd9e03872
("drm/i915/dp: fall back to 18 bpp when sink capability is unknown")

This commit introduced a regression into stable kernels, as it reduces output color depth to 6 bpc for any video sink connected to a Displayport connector if that sink doesn't report a specific color depth via EDID, or if our EDID parser doesn't actually recognize the proper bpc from EDID.

Affected are active DisplayPort->VGA converters and active DisplayPort->DVI converters. Both should be able to handle 8 bpc, but are degraded to 6 bpc with this patch.

The reverted commit was meant to fix
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=105331

A followup patch implements a fix for that specific bug, which is caused by a faulty EDID of the affected DP panel by adding a new EDID quirk for that panel.

DP 18 bpp fallback handling and other improvements to DP sink bpc detection will be handled for future kernels in a separate series of patches.

Please backport to stable.

---
 drivers/gpu/drm/i915/intel_display.c |   20 +++++---------------
 1 file changed, 5 insertions(+), 15 deletions(-)

@@ -8841,21 +8841,11 @@ connected_sink_compute_bpp(struct intel_
 		pipe_config->pipe_bpp = connector->base.display_info.bpc*3;
 	}
 
-	/* Clamp bpp to default limit on screens without EDID 1.4 */
-	if (connector->base.display_info.bpc == 0) {
-		int type = connector->base.connector_type;
-		int clamp_bpp = 24;
-
-		/* Fall back to 18 bpp when DP sink capability is unknown. */
-		if (type == DRM_MODE_CONNECTOR_DisplayPort ||
-		    type == DRM_MODE_CONNECTOR_eDP)
-			clamp_bpp = 18;
-
-		if (bpp > clamp_bpp) {
-			DRM_DEBUG_KMS("clamping display bpp (was %d) to default limit of %d\n",
-				      bpp, clamp_bpp);
-			pipe_config->pipe_bpp = clamp_bpp;
-		}
+	/* Clamp bpp to 8 on screens without EDID 1.4 */
+	if (connector->base.display_info.bpc == 0 && bpp > 24) {
+		DRM_DEBUG_KMS("clamping display bpp (was %d) to default limit of 24\n",
+			      bpp);
+		pipe_config->pipe_bpp = 24;
 	}
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/drm-i915-dp-revert-drm-i915-dp-fall-back-to-18-bpp-when-sink-capability-is-unknown.patch