This is a note to let you know that I've just added the patch titled drm/amd/display: Assume an LTTPR is always present on fixed_vs links to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: drm-amd-display-assume-an-lttpr-is-always-present-on.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 6582572ba6f1deb934c8abb0db68a31e1926de7f Author: Michael Strauss <michael.strauss@xxxxxxx> Date: Wed Aug 31 15:10:43 2022 -0400 drm/amd/display: Assume an LTTPR is always present on fixed_vs links [ 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@xxxxxxx> Acked-by: Wayne Lin <wayne.lin@xxxxxxx> Signed-off-by: Michael Strauss <michael.strauss@xxxxxxx> Tested-by: Daniel Wheeler <daniel.wheeler@xxxxxxx> Signed-off-by: Alex Deucher <alexander.deucher@xxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> 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 6d5dc5ab3d8c..a6ff1b17fd22 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 @@ -3703,6 +3703,14 @@ bool dp_retrieve_lttpr_cap(struct dc_link *link) lttpr_dpcd_data[DP_PHY_REPEATER_EXTENDED_WAIT_TIMEOUT - 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 = (dp_convert_to_count(link->dpcd_caps.lttpr_caps.phy_repeater_cnt) != 0 && link->dpcd_caps.lttpr_caps.max_lane_count > 0 &&