drm/i915: Add a delay in Displayport AUX transactions for compliance testing

The Displayport Link Layer Compliance Testing Specification 1.2 rev 1.1
specifies that repeated AUX transactions after a failure (no response /
invalid response) must have a minimum delay of 400us before the resend can
occur. Tests 4.2.1.1 and 4.2.1.2 are two tests that require this specifically.

Also, the check for DP_AUX_CH_CTL_TIME_OUT_ERROR has been moved out into a
separate case. This case just continues with the next iteration of the loop
as the HW has already waited the required amount of time.

V2:
- Changed udelay() to usleep_range()
V3:
- Removed extraneous check for timeout
- Updated comment to reflect this change
V4:
- Reformatted a comment
V5:
- Added separate check for HW timeout on AUX transactions. A message
  is logged upon detection of this case.
V6:
- Add continue statement to HW timeout detect case
- Remove the log message indicating a timeout has been
  detected (review feedback)
V7:
- Updated the commit message to remove verbage about the HW timeout
  case that is no longer valid.

Signed-off-by: Todd Previte <tprevite@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -874,9 +874,18 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
 				   DP_AUX_CH_CTL_TIME_OUT_ERROR |
 				   DP_AUX_CH_CTL_RECEIVE_ERROR);
 
-			if (status & (DP_AUX_CH_CTL_TIME_OUT_ERROR |
-				      DP_AUX_CH_CTL_RECEIVE_ERROR))
+			if (status & DP_AUX_CH_CTL_TIME_OUT_ERROR)
 				continue;
+
+			/* DP CTS 1.2 Core Rev 1.1, 4.2.1.1 & 4.2.1.2
+			 *   400us delay required for errors and timeouts
+			 *   Timeout errors from the HW already meet this
+			 *   requirement so skip to next iteration
+			 */
+			if (status & DP_AUX_CH_CTL_RECEIVE_ERROR) {
+				usleep_range(400, 500);
+				continue;
+			}
 			if (status & DP_AUX_CH_CTL_DONE)
 				break;
 		}