The patch below does not apply to the 3.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 951caa6acf7121f88a9e8e04ef75bd0ac323a033 Mon Sep 17 00:00:00 2001
From: Alex Deucher <[email protected]>
Date: Wed, 18 Feb 2015 00:59:45 -0500
Subject: [PATCH] drm/radeon: disable mclk switching with 120hz+ monitors

These tend to be problematic even if the vblank period is
long enough.  This needs more investigation across a wider
range of displays.  Disable for now.

bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=87796
https://bugs.freedesktop.org/show_bug.cgi?id=89198

Signed-off-by: Alex Deucher <[email protected]>
Cc: [email protected]

diff --git a/drivers/gpu/drm/radeon/radeon_pm.c 
b/drivers/gpu/drm/radeon/radeon_pm.c
index 9f758d39420d..33cf4108386d 100644
--- a/drivers/gpu/drm/radeon/radeon_pm.c
+++ b/drivers/gpu/drm/radeon/radeon_pm.c
@@ -852,6 +852,12 @@ static struct radeon_ps 
*radeon_dpm_pick_power_state(struct radeon_device *rdev,
                        single_display = false;
        }
 
+       /* 120hz tends to be problematic even if they are under the
+        * vblank limit.
+        */
+       if (single_display && (r600_dpm_get_vrefresh(rdev) >= 120))
+               single_display = false;
+
        /* certain older asics have a separare 3D performance state,
         * so try that first if the user selected performance
         */

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