From: Khaled Almahallawy khaled.almahallawy@intel.com
[ Upstream commit 7b4d8db657192066bc6f1f6635d348413dac1e18 ]
The sequence for Source DP PHY CTS automation is [2][1]: 1- Emulate successful Link Training(LT) 2- Short HPD and change link rates and number of lanes by LT. (This is same flow for Link Layer CTS) 3- Short HPD and change PHY test pattern and swing/pre-emphasis levels (This step should not trigger LT)
The problem is with DP PHY compliance setup as follow:
[DPTX + on board LTTPR]------Main Link--->[Scope] ^ | | | | | ----------Aux Ch------>[Aux Emulator]
At step 3, before writing TRAINING_LANEx_SET/LINK_QUAL_PATTERN_SET to declare the pattern/swing requested by scope, we write link config in LINK_BW_SET/LANE_COUNT_SET on a port that has LTTPR. As LTTPR snoops aux transaction, LINK_BW_SET/LANE_COUNT_SET writes indicate a LT will start [Check DP 2.0 E11 -Sec 3.6.8.2 & 3.6.8.6.3], and LTTPR will reset the link and stop sending DP signals to DPTX/Scope causing the measurements to fail. Note that step 3 will not trigger LT and DP link will never recovered by the Aux Emulator/Scope.
The reset of link can be tested with a monitor connected to LTTPR port simply by writing to LINK_BW_SET or LANE_COUNT_SET as follow
igt/tools/dpcd_reg write --offset=0x100 --value 0x14 --device=2
OR
printf '\x14' | sudo dd of=/dev/drm_dp_aux2 bs=1 count=1 conv=notrunc seek=$((0x100))
This single aux write causes the screen to blank, sending short HPD to DPTX, setting LINK_STATUS_UPDATE = 1 in DPCD 0x204, and triggering LT.
As stated in [1]: "Before any TX electrical testing can be performed, the link between a DPTX and DPRX (in this case, a piece of test equipment), including all LTTPRs within the path, shall be trained as defined in this Standard."
In addition, changing Phy pattern/Swing/Pre-emphasis (Step 3) uses the same link rate and lane count applied on step 2, so no need to redo LT.
The fix is to not rewrite link config in step 3, and just writes TRAINING_LANEx_SET and LINK_QUAL_PATTERN_SET
[1]: DP 2.0 E11 - 3.6.11.1 LTTPR DPTX_PHY Electrical Compliance
[2]: Configuring UnigrafDPTC Controller - Automation Test Sequence https://www.keysight.com/us/en/assets/9922-01244/help-files/ D9040DPPC-DisplayPort-Test-Software-Online-Help-latest.chm
Cc: Imre Deak imre.deak@intel.com Cc: Jani Nikula jani.nikula@intel.com Cc: Or Cochvi or.cochvi@intel.com Signed-off-by: Khaled Almahallawy khaled.almahallawy@intel.com Signed-off-by: Jani Nikula jani.nikula@intel.com Link: https://patchwork.freedesktop.org/patch/msgid/20220916054900.415804-1-khaled... Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/gpu/drm/display/drm_dp_helper.c | 9 --------- 1 file changed, 9 deletions(-)
diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c index e5bab236b3ae..4c0c4e3d1e20 100644 --- a/drivers/gpu/drm/display/drm_dp_helper.c +++ b/drivers/gpu/drm/display/drm_dp_helper.c @@ -2638,17 +2638,8 @@ int drm_dp_set_phy_test_pattern(struct drm_dp_aux *aux, struct drm_dp_phy_test_params *data, u8 dp_rev) { int err, i; - u8 link_config[2]; u8 test_pattern;
- link_config[0] = drm_dp_link_rate_to_bw_code(data->link_rate); - link_config[1] = data->num_lanes; - if (data->enhanced_frame_cap) - link_config[1] |= DP_LANE_COUNT_ENHANCED_FRAME_EN; - err = drm_dp_dpcd_write(aux, DP_LINK_BW_SET, link_config, 2); - if (err < 0) - return err; - test_pattern = data->phy_pattern; if (dp_rev < 0x12) { test_pattern = (test_pattern << 2) &