drm/dp_helper: Always wait before retrying native aux transactions

This is part of a patch series to migrate all of the workarounds for
commonly seen behavior from bad sinks in intel_dp_dpcd_read_wake() to
drm's DP helper.

Some sinks need some time during the process of resuming the system from
sleep before they're ready to handle transactions. While it would be
nice if they responded with NACKs in these scenarios, this isn't always
the case as a few sinks will just timeout on all of the transactions
they receive until they're ready.
Signed-off-by: Lyude <cpaul@redhat.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460559513-32280-2-git-send-email-cpaul@redhat.com

drivers/gpu/drm/drm_dp_helper.c

@@ -196,6 +196,10 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 	 * sufficient, bump to 32 which makes Dell 4k monitors happier.
 	 */
 	for (retry = 0; retry < 32; retry++) {
+		if (err != 0 && err != -ETIMEDOUT) {
+			usleep_range(AUX_RETRY_INTERVAL,
+				     AUX_RETRY_INTERVAL + 100);
+		}
 
 		err = aux->transfer(aux, &msg);
 		if (err < 0) {
@@ -205,7 +209,6 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 			goto unlock;
 		}
 
-
 		switch (msg.reply & DP_AUX_NATIVE_REPLY_MASK) {
 		case DP_AUX_NATIVE_REPLY_ACK:
 			if (err < size)
@@ -215,10 +218,6 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 		case DP_AUX_NATIVE_REPLY_NACK:
 			err = -EIO;
 			goto unlock;
-
-		case DP_AUX_NATIVE_REPLY_DEFER:
-			usleep_range(AUX_RETRY_INTERVAL, AUX_RETRY_INTERVAL + 100);
-			break;
 		}
 	}