drm/i915: Allow DBLSCAN user modes with eDP/LVDS/DSI

When encountering a connector with the scaling mode property both
intel and modesetting ddxs sometimes add tons of DBLSCAN modes
to the output's mode list. The idea presumably being that since the
output will be going through the panel fitter anyway we can pretend
to use any kind of mode.

Sadly that means we can't reject user modes with the DBLSCAN flag
until we know whether we're going to be using the panel's native
mode or the user mode directly. Doing otherwise means X clients using
xf86vidmode/xrandr will get a protocol error (and often self
terminate as a result) when the kernel refuses to use the requested
mode with the DBLSCAN flag.

To undo the regression we'll move the DBLSCAN checks into the
connector->mode_valid() and encoder->compute_config() hooks.

Cc: stable@vger.kernel.org
Cc: Vito Caputo <vcaputo@pengaru.com>
Reported-by: Vito Caputo <vcaputo@pengaru.com>
Fixes: e995ca0b8139 ("drm/i915: Provide a device level .mode_valid() hook")
References: https://lkml.org/lkml/2018/5/21/715
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180524125403.23445-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106804
Tested-by: Arkadiusz Miskiewicz <arekm@maven.pl>

1 files changed, 13 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 17c590b42fd7..3323470596ca 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -14529,12 +14529,22 @@ static enum drm_mode_status
 intel_mode_valid(struct drm_device *dev,
 		 const struct drm_display_mode *mode)
 {
+	/*
+	 * Can't reject DBLSCAN here because Xorg ddxen can add piles
+	 * of DBLSCAN modes to the output's mode list when they detect
+	 * the scaling mode property on the connector. And they don't
+	 * ask the kernel to validate those modes in any way until
+	 * modeset time at which point the client gets a protocol error.
+	 * So in order to not upset those clients we silently ignore the
+	 * DBLSCAN flag on such connectors. For other connectors we will
+	 * reject modes with the DBLSCAN flag in encoder->compute_config().
+	 * And we always reject DBLSCAN modes in connector->mode_valid()
+	 * as we never want such modes on the connector's mode list.
+	 */
+
 	if (mode->vscan > 1)
 		return MODE_NO_VSCAN;
 
-	if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
-		return MODE_NO_DBLESCAN;
-
 	if (mode->flags & DRM_MODE_FLAG_HSKEW)
 		return MODE_H_ILLEGAL;