drm/i915: Read sink_count dpcd always

Sink count can change between short pulse hpd hence this patch
adds a member variable to intel_dp so we can track any changes
between short pulse interrupts.

This patch reads sink_count dpcd always and removes its
read operation based on values in downstream port dpcd.

SINK_COUNT dpcd is not dependent on DOWNSTREAM_PORT_PRESENT dpcd.
SINK_COUNT denotes if a display is attached, while
DOWNSTREAM_PORT_PRESET indicates how many ports are available
in the dongle where display can be attached. so it is possible
for sink count to change irrespective of value in downstream
port dpcd.

Here is a table of possible values and scenarios

sink_count      downstream_port
                present
0               0               no display is attached
0               1               dongle is connected without display
1               0               display connected directly
1               1               display connected through dongle

v2: Storing value of intel_dp->sink_count that is ready
    for consumption. (Ander)
    Squashing two commits into one. (Ander)

v3: Added comment to explain the need of early return when
    sink count is 0. (Ander)
Tested-by: Nathan D Ciobanu <nathan.d.ciobanu@intel.com>
Signed-off-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Shubhangi Shrivastava <shubhangi.shrivastava@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459341326-13142-4-git-send-email-shubhangi.shrivastava@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -3788,6 +3788,27 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
 	if (intel_dp->dpcd[DP_DPCD_REV] == 0)
 		return false; /* DPCD not present */
 
+	if (intel_dp_dpcd_read_wake(&intel_dp->aux, DP_SINK_COUNT,
+				    &intel_dp->sink_count, 1) < 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.
+	 */
+	intel_dp->sink_count = DP_GET_SINK_COUNT(intel_dp->sink_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;
+
 	/* Check if the panel supports PSR */
 	memset(intel_dp->psr_dpcd, 0, sizeof(intel_dp->psr_dpcd));
 	if (is_edp(intel_dp)) {
@@ -4308,14 +4329,9 @@ intel_dp_detect_dpcd(struct intel_dp *intel_dp)
 	/* If we're HPD-aware, SINK_COUNT changes dynamically */
 	if (intel_dp->dpcd[DP_DPCD_REV] >= 0x11 &&
 	    intel_dp->downstream_ports[0] & DP_DS_PORT_HPD) {
-		uint8_t reg;
-
-		if (intel_dp_dpcd_read_wake(&intel_dp->aux, DP_SINK_COUNT,
-					    &reg, 1) < 0)
-			return connector_status_unknown;
 
-		return DP_GET_SINK_COUNT(reg) ? connector_status_connected
-					      : connector_status_disconnected;
+		return intel_dp->sink_count ?
+		connector_status_connected : connector_status_disconnected;
 	}
 
 	/* If no HPD, poke DDC gently */

drivers/gpu/drm/i915/intel_drv.h

@@ -796,6 +796,7 @@ struct intel_dp {
 	uint32_t DP;
 	int link_rate;
 	uint8_t lane_count;
+	uint8_t sink_count;
 	bool has_audio;
 	bool detect_done;
 	enum hdmi_force_audio force_audio;