drm/dp_helper: Rate limit timeout errors from drm_dp_i2c_do_msg()

Timeouts can be errors, but timeouts are also usually normal behavior
and happen a lot. Since the kernel already lets us know when we're
suppressing messages due to rate limiting, rate limit timeout errors so
we don't make too much noise in the kernel log.
Signed-off-by: Lyude <cpaul@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470443443-27252-8-git-send-email-cpaul@redhat.com

drivers/gpu/drm/drm_dp_helper.c

@@ -574,7 +574,17 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 			if (ret == -EBUSY)
 				continue;
 
+			/*
+			 * While timeouts can be errors, they're usually normal
+			 * behavior (for instance, when a driver tries to
+			 * communicate with a non-existant DisplayPort device).
+			 * Avoid spamming the kernel log with timeout errors.
+			 */
+			if (ret == -ETIMEDOUT)
+				DRM_DEBUG_KMS_RATELIMITED("transaction timed out\n");
+			else
 				DRM_DEBUG_KMS("transaction failed: %d\n", ret);
+
 			return ret;
 		}