drm/i915: Avoid a full port detection in the first eDP short pulse

Some eDP panels do not set a valid sink count value and even for the
ones that sets is should always be one for eDP, that is why it is not
cached in intel_edp_init_dpcd().

But intel_dp_short_pulse() compares the old count with the read one
if there is a mistmatch a full port detection will be executed, what
was happening in the first short pulse interruption of eDP panels
that sets sink count.

Instead of just skip the compasison for eDP panels, lets not read
the sink count at all for eDP.

v2: the previous version of this patch it was caching the sink count
in intel_edp_init_dpcd() but I was pointed out by Ville a patch that
handled a case of a eDP panel that do not set sink count and as sink
count is not used to eDP certification was choosed to just not read
it at all.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121225441.18785-1-jose.souza@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -3936,8 +3936,6 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
 static bool
 intel_dp_get_dpcd(struct intel_dp *intel_dp)
 {
-	u8 sink_count;
-
 	if (!intel_dp_read_dpcd(intel_dp))
 		return false;
 
@@ -3947,25 +3945,35 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
 		intel_dp_set_common_rates(intel_dp);
 	}
 
-	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_SINK_COUNT, &sink_count) <= 0)
-		return false;
-
 	/*
-	 * Sink count can change between short pulse hpd hence
-	 * a member variable in intel_dp will track any changes
-	 * between short pulse interrupts.
+	 * Some eDP panels do not set a valid value for sink count, that is why
+	 * it don't care about read it here and in intel_edp_init_dpcd().
 	 */
-	intel_dp->sink_count = DP_GET_SINK_COUNT(sink_count);
+	if (!intel_dp_is_edp(intel_dp)) {
+		u8 count;
+		ssize_t r;
 
-	/*
-	 * SINK_COUNT == 0 and DOWNSTREAM_PORT_PRESENT == 1 implies that
-	 * a dongle is present but no display. Unless we require to know
-	 * if a dongle is present or not, we don't need to update
-	 * downstream port information. So, an early return here saves
-	 * time from performing other operations which are not required.
-	 */
-	if (!intel_dp_is_edp(intel_dp) && !intel_dp->sink_count)
-		return false;
+		r = drm_dp_dpcd_readb(&intel_dp->aux, DP_SINK_COUNT, &count);
+		if (r < 1)
+			return false;
+
+		/*
+		 * Sink count can change between short pulse hpd hence
+		 * a member variable in intel_dp will track any changes
+		 * between short pulse interrupts.
+		 */
+		intel_dp->sink_count = DP_GET_SINK_COUNT(count);
+
+		/*
+		 * SINK_COUNT == 0 and DOWNSTREAM_PORT_PRESENT == 1 implies that
+		 * a dongle is present but no display. Unless we require to know
+		 * if a dongle is present or not, we don't need to update
+		 * downstream port information. So, an early return here saves
+		 * time from performing other operations which are not required.
+		 */
+		if (!intel_dp->sink_count)
+			return false;
+	}
 
 	if (!drm_dp_is_branch(intel_dp->dpcd))
 		return true; /* native DP sink */