FAILED: patch "[PATCH] drm/i915: Fix modesetting in case of unexpected AUX timeouts" failed to apply to 5.11-stable tree

The patch below does not apply to the 5.11-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d2b9935d65dab6e92beb33c150c1a6ded14ab670 Mon Sep 17 00:00:00 2001
From: Imre Deak <[email protected]>
Date: Tue, 13 Apr 2021 02:24:12 +0300
Subject: [PATCH] drm/i915: Fix modesetting in case of unexpected AUX timeouts
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

In case AUX failures happen unexpectedly during a modeset, the driver should still complete the modeset. In particular the driver should perform the link training sequence steps even in case of an AUX failure, as this sequence also includes port initialization steps. Not doing that can leave the port/pipe in a broken state and lead for instance to a flip done timeout.

Fix this by continuing with link training (in a no-LTTPR mode) if the DPRX DPCD readout failed for some reason at the beginning of link training. After a successful connector detection we already have the DPCD read out and cached, so the failed repeated read for it should not cause a problem. Note that a partial AUX read could in theory partly overwrite the cached DPCD (and return error) but this overwrite should not happen if the returned values are corrupted (due to a timeout or some other IO error).

Kudos to Ville to root cause the problem.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/3308

index be6ac0dd846e..2ed309534e97 100644
@@ -848,7 +848,8 @@ void intel_dp_start_link_train(struct intel_dp *intel_dp,
 	int lttpr_count = intel_dp_init_lttpr_and_dprx_caps(intel_dp);
 
 	if (lttpr_count < 0)
-		return;
+		/* Still continue with enabling the port and link training. */
+		lttpr_count = 0;
 
 	if (!intel_dp_link_train_all_phys(intel_dp, crtc_state, lttpr_count))
 		intel_dp_schedule_fallback_link_training(intel_dp, crtc_state);