[PATCH v3] drm/amd/display: Fix AppleDongle can't be detected

[Louis Li]

[Why]
External monitor cannot be displayed consistently, if connecting
via this Apple dongle (A1621, USB Type-C to HDMI).
Experiments prove that the dongle needs 200ms at least to be ready
for communication, after it drives HPDsignal high, and DPCD cannot
be read correctly during the period, even reading it repeatedly.
In such a case, driver does not perform link training bcz of no DPCD.

[How]
When driver is run to the modified point, EDID is read correctly
and dpcd_sink_count of link is not zero. Therefore, link training
should be successfully performed. Which implies parameters should
be updated, e.g. lane count, link rate, etc. Checking parameters,
if values of those parameters are zero, link training is not
performed. So, do link-training to have detection completed.

With this patch applied, the problem cannot be reproduced.
Testing other dongles, results are PASS.
Patch(v3) is verified PASS by both AMD internal lab and customer.


Signed-off-by: Louis Li <Ching-shih.Li at amd.com>
---
 drivers/gpu/drm/amd/display/dc/core/dc_link.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_link.c b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
index 7372dedd2f48..6188edc92d0f 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc_link.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
@@ -725,7 +725,9 @@ bool dc_link_detect(struct dc_link *link, enum dc_detect_reason reason)
 
 		if (link->connector_signal == SIGNAL_TYPE_DISPLAY_PORT &&
 			sink_caps.transaction_type == DDC_TRANSACTION_TYPE_I2C_OVER_AUX &&
-			reason != DETECT_REASON_HPDRX) {
+			link->verified_link_cap.lane_count == 0 &&
+			link->verified_link_cap.link_rate == 0 &&
+			link->verified_link_cap.link_spread == 0) {
 			/*
 			 * TODO debug why Dell 2413 doesn't like
 			 *  two link trainings
-- 
2.21.0