[Intel-gfx] [PATCH] drm/i915/dp: wait on timeout before retry

[Arun R Murthy]

On linktraining error/timeout before retry need to wait for 400usec as
per the DP CTS spec1.2
Under section 2.7.2 AUX Transaction Response/Reply Time-outs
AUX Replier (the uPacket RX) must start sending the reply back to the AUX
requester (the uPacket TX) within the response period of 300μs. The timer
for Response Time-out starts ticking after the uPacket RX has finished
receiving the AUX STOP condition which ends the AUX Request transaction.
The timer is reset either when the Response Time-out period has elapsed or
when the uPacket RX has started to send the AUX Sync pattern (which follows
10 to 16 active pre-charge pulses) for the Reply transaction. If the
uPacket TX does not receive a reply from the uPacket RX it must wait for a
Reply Time-out period of 400us before initiating the next AUX Request
transaction. The timer for the Reply Time-out starts ticking after the
uPacket TX has finished sending the AUX STOP condition.

The patch with commit 74ebf294a1dd ("drm/i915: Add a delay in Displayport
AUX transactions for compliance testing")
removes this delay mentioning the hardware already meets this requirement,
but as per the spec the delay mentioned in the spec specifies how long to
wait for the receiver response before timeout. So the delay here to wait
for timeout and not a delay after timeout. The DP spec specifies a delay
after timeout and hence adding this delay.

v2: fixed checkpatch warning and error
v3: used proper indentation
v4: added DP CEA 1.2 spec details in patch commit msg

Signed-off-by: Arun R Murthy <arun.r.murthy at intel.com>
Acked-by: Jani Nikula <jani.nikula at intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp_aux.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
index 2bc119374555..722c9f210690 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
@@ -286,13 +286,9 @@ intel_dp_aux_xfer(struct intel_dp *intel_dp,
 			/*
 			 * 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_TIME_OUT_ERROR)
-				continue;
-
-			if (status & DP_AUX_CH_CTL_RECEIVE_ERROR) {
+			if (status & (DP_AUX_CH_CTL_RECEIVE_ERROR |
+				      DP_AUX_CH_CTL_TIME_OUT_ERROR)) {
 				usleep_range(400, 500);
 				continue;
 			}
-- 
2.25.1