drm/i915/mst: Do not retrain new links

The short pulse handler checks if channel equalization is okay and
goes onto retrain a link if there are active MST links. This retraining
path is not meant for new MST connections, but due to a bug elsewhere, if
active_mst_links is < 0 the boolean check for active_mst_links passes and
we proceed to retrain a new link. This results in a sequence of failed link
training attempts, most likely due to the hardware not setup for link
training at that point i.e., missing the DDI pre_enable sequence.

[   80.301272] [drm:intel_dp_check_mst_status] channel EQ not ok, retraining
[   80.301312] [drm:intel_ddi_prepare_link_retrain] *ERROR* Timeout waiting for DDI BUF C idle bit

The above error gives us a hint something went wrong before link
training started.

Check for a positive value of active_mst_links and throw in a warning for
invalid active_mst_links as debug aid.

Cc: Nathan Ciobanu <nathan.d.ciobanu@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Tested-by: Nathan Ciobanu <nathan.d.ciobanu@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180718171943.3246-1-dhinakaran.pandiyan@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -4253,12 +4253,14 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
 		int ret = 0;
 		int retry;
 		bool handled;
+
+		WARN_ON_ONCE(intel_dp->active_mst_links < 0);
 		bret = intel_dp_get_sink_irq_esi(intel_dp, esi);
 go_again:
 		if (bret == true) {
 
 			/* check link status - esi[10] = 0x200c */
-			if (intel_dp->active_mst_links &&
+			if (intel_dp->active_mst_links > 0 &&
 			    !drm_dp_channel_eq_ok(&esi[10], intel_dp->lane_count)) {
 				DRM_DEBUG_KMS("channel EQ not ok, retraining\n");
 				intel_dp_start_link_train(intel_dp);