drm/i915: Increase the response time for slow SDVO devices

Some devices may respond very slowly and only flag that the reply is
pending within the first 15us response window. Be kind to such devices
and wait a further 15ms, before checking for the pending reply. This
moves the existing special case delay of 30ms down from the detection
routine into the common path and pretends to explain it...

v2: Simplify the loop constructs as suggested by Jani Nikula.

References: https://bugs.freedesktop.org/show_bug.cgi?id=36997
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_sdvo.c

@@ -509,7 +509,7 @@ out:
 static bool intel_sdvo_read_response(struct intel_sdvo *intel_sdvo,
 				     void *response, int response_len)
 {
-	u8 retry = 5;
+	u8 retry = 15; /* 5 quick checks, followed by 10 long checks */
 	u8 status;
 	int i;
 
@@ -522,14 +522,27 @@ static bool intel_sdvo_read_response(struct intel_sdvo *intel_sdvo,
 	 * command to be complete.
 	 *
 	 * Check 5 times in case the hardware failed to read the docs.
+	 *
+	 * Also beware that the first response by many devices is to
+	 * reply PENDING and stall for time. TVs are notorious for
+	 * requiring longer than specified to complete their replies.
+	 * Originally (in the DDX long ago), the delay was only ever 15ms
+	 * with an additional delay of 30ms applied for TVs added later after
+	 * many experiments. To accommodate both sets of delays, we do a
+	 * sequence of slow checks if the device is falling behind and fails
+	 * to reply within 5*15µs.
 	 */
 	if (!intel_sdvo_read_byte(intel_sdvo,
 				  SDVO_I2C_CMD_STATUS,
 				  &status))
 		goto log_fail;
 
-	while (status == SDVO_CMD_STATUS_PENDING && retry--) {
-		udelay(15);
+	while (status == SDVO_CMD_STATUS_PENDING && --retry) {
+		if (retry < 10)
+			msleep(15);
+		else
+			udelay(15);
+
 		if (!intel_sdvo_read_byte(intel_sdvo,
 					  SDVO_I2C_CMD_STATUS,
 					  &status))
@@ -1535,15 +1548,9 @@ intel_sdvo_detect(struct drm_connector *connector, bool force)
 	struct intel_sdvo_connector *intel_sdvo_connector = to_intel_sdvo_connector(connector);
 	enum drm_connector_status ret;
 
-	if (!intel_sdvo_write_cmd(intel_sdvo,
-				  SDVO_CMD_GET_ATTACHED_DISPLAYS, NULL, 0))
-		return connector_status_unknown;
-
-	/* add 30ms delay when the output type might be TV */
-	if (intel_sdvo->caps.output_flags & SDVO_TV_MASK)
-		msleep(30);
-
-	if (!intel_sdvo_read_response(intel_sdvo, &response, 2))
+	if (!intel_sdvo_get_value(intel_sdvo,
+				  SDVO_CMD_GET_ATTACHED_DISPLAYS,
+				  &response, 2))
 		return connector_status_unknown;
 
 	DRM_DEBUG_KMS("SDVO response %d %d [%x]\n",