[PATCH 5.19 30/48] drm/amd/display: Assume an LTTPR is always present on fixed_vs links

Greg Kroah-Hartman
2022-10-10T07:13:18Z

From: Michael Strauss <michael.strauss@amd.com>

[ Upstream commit 29956d0fded036a570bd8e7d4ea4b1a1730307d2 ]

[WHY]
LTTPRs can in very rare instsances fail to increment DPCD LTTPR count.
This results in aux-i LTTPR requests to be sent to the wrong DPCD
address, which causes link training failure.

[HOW]
Override internal repeater count if fixed_vs flag is set for a given link

Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c b/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
index a4fc9a6c850e..0c52506b367d 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c
@@ -5211,6 +5211,14 @@ bool dp_retrieve_lttpr_cap(struct dc_link *link)
lttpr_dpcd_data[DP_PHY_REPEATER_128B132B_RATES -
DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV];

+ /* If this chip cap is set, at least one retimer must exist in the chain
+ * Override count to 1 if we receive a known bad count (0 or an invalid value) */
+ if (link->chip_caps & EXT_DISPLAY_PATH_CAPS__DP_FIXED_VS_EN &&
+ (dp_convert_to_count(link->dpcd_caps.lttpr_caps.phy_repeater_cnt) == 0)) {
+ ASSERT(0);
+ link->dpcd_caps.lttpr_caps.phy_repeater_cnt = 0x80;
+ }
+
/* Attempt to train in LTTPR transparent mode if repeater count exceeds 8. */
is_lttpr_present = (link->dpcd_caps.lttpr_caps.max_lane_count > 0 &&
link->dpcd_caps.lttpr_caps.max_lane_count <= 4 &&
--
2.35.1