[Intel-gfx] [PATCH 03/11] drm/i915: Add a delay in Displayport AUX transactions for compliance testing

[Todd Previte]

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. As of now, the only action taken is to log the error, since
the HW will have already waited the required amount of time for the
transaction to complete.

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)

Signed-off-by: Todd Previte <tprevite at gmail.com>
---
 drivers/gpu/drm/i915/intel_dp.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index ed2f60c..8b59458 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -877,9 +877,18 @@ 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;
 		}
-- 
1.9.1