drm/i915/dp: Fix error handling during 128b/132b link training

At the end of a 128b/132b link training sequence, the HW expects the
transcoder training pattern to be set to TPS2 and from that to normal
mode (disabling the training pattern). Transitioning from TPS1 directly
to normal mode leaves the transcoder in a stuck state, resulting in
page-flip timeouts later in the modeset sequence.

Atm, in case of a failure during link training, the transcoder may be
still set to output the TPS1 pattern. Later the transcoder is then set
from TPS1 directly to normal mode in intel_dp_stop_link_train(), leading
to modeset failures later as described above. Fix this by setting the
training patter to TPS2, if the link training failed at any point.

The clue in the specification about the above HW behavior is the
explicit mention that TPS2 must be set after the link training sequence
(and there isn't a similar requirement specified for the 8b/10b link
training), see the Bspec links below.

v2: Add bspec aspect/link to the commit log. (Jani)

Bspec: 54128, 65448, 68849
Cc: stable@vger.kernel.org # v5.18+
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250217223828.1166093-2-imre.deak@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Author: ideak

drivers/gpu/drm/i915/display/intel_dp_link_training.c

@@ -1563,7 +1563,7 @@ intel_dp_128b132b_link_train(struct intel_dp *intel_dp,
 
 	if (wait_for(intel_dp_128b132b_intra_hop(intel_dp, crtc_state) == 0, 500)) {
 		lt_err(intel_dp, DP_PHY_DPRX, "128b/132b intra-hop not clear\n");
-		return false;
+		goto out;
 	}
 
 	if (intel_dp_128b132b_lane_eq(intel_dp, crtc_state) &&
@@ -1575,6 +1575,19 @@ intel_dp_128b132b_link_train(struct intel_dp *intel_dp,
 	       passed ? "passed" : "failed",
 	       crtc_state->port_clock, crtc_state->lane_count);
 
+out:
+	/*
+	 * Ensure that the training pattern does get set to TPS2 even in case
+	 * of a failure, as is the case at the end of a passing link training
+	 * and what is expected by the transcoder. Leaving TPS1 set (and
+	 * disabling the link train mode in DP_TP_CTL later from TPS1 directly)
+	 * would result in a stuck transcoder HW state and flip-done timeouts
+	 * later in the modeset sequence.
+	 */
+	if (!passed)
+		intel_dp_program_link_training_pattern(intel_dp, crtc_state,
+						       DP_PHY_DPRX, DP_TRAINING_PATTERN_2);
+
 	return passed;
 }