drm/dp: Define AUX_RETRY_INTERVAL as 500 us

Currently we react to native and i2c defers by waiting either 400-500 us
or 500-600 us, depending on which code path we take. Consolidate them
all to one define AUX_RETRY_INTERVAL which defines the minimum interval.
Since we've been using two different intervals pick the longer of them
and define AUX_RETRY_INTERVAL as 500 us. For the maximum just use
AUX_RETRY_INTERVAL+100 us.

I want to have a define for this so that I can use it when calculating
the estimated duration of i2c-over-aux transfers. Without a define it
would be very easy to change the sleep duration and neglect to update
the i2c-over-aux estimates.

Cc: Simon Farnsworth <simon.farnsworth@onelan.com>
Cc: moosotc@gmail.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Simon Farnsworth <simon.farnsworth@onelan.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/drm_dp_helper.c

@@ -159,6 +159,8 @@ int drm_dp_bw_code_to_link_rate(u8 link_bw)
 }
 EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);
 
+#define AUX_RETRY_INTERVAL 500 /* us */
+
 /**
  * DOC: dp helpers
  *
@@ -213,7 +215,7 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 			return -EIO;
 
 		case DP_AUX_NATIVE_REPLY_DEFER:
-			usleep_range(400, 500);
+			usleep_range(AUX_RETRY_INTERVAL, AUX_RETRY_INTERVAL + 100);
 			break;
 		}
 	}
@@ -476,7 +478,7 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 			 * For now just defer for long enough to hopefully be
 			 * safe for all use-cases.
 			 */
-			usleep_range(500, 600);
+			usleep_range(AUX_RETRY_INTERVAL, AUX_RETRY_INTERVAL + 100);
 			continue;
 
 		default:
@@ -506,7 +508,7 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 			aux->i2c_defer_count++;
 			if (defer_i2c < 7)
 				defer_i2c++;
-			usleep_range(400, 500);
+			usleep_range(AUX_RETRY_INTERVAL, AUX_RETRY_INTERVAL + 100);
 			continue;
 
 		default: