linux-kernel - [PATCH 4.19 190/205] drm/i915/dp: Fix link retraining comment in intel_dp_long

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Date:   Mon, 19 Nov 2018 17:28:17 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Lyude Paul <lyude@...hat.com>,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Jan-Marek Glogowski <glogow@...home.de>,
        Dhinakaran Pandiyan <dhinakaran.pandiyan@...el.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Subject: [PATCH 4.19 190/205] drm/i915/dp: Fix link retraining comment in intel_dp_long_pulse()

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dhinakaran Pandiyan <dhinakaran.pandiyan@...el.com>

commit 49af5d95b9b3c21a84ad115a9db9acbc036d849a upstream.

Comment claims link needs to be retrained because the connected sink raised
a long pulse to indicate link loss. If the sink did so,
intel_dp_hotplug() would have handled link retraining. Looking at the
logs in Bugzilla referenced in commit '3cf71bc9904d ("drm/i915: Re-apply
Perform link quality check, unconditionally during long pulse"")', the
issue is that the sink does not trigger an interrupt. What we want is
->detect() from user space to check link status and retrain. Ville's
review for the original patch also indicates the same root cause. So,
rewrite the comment.

v2: Patch split and rewrote comment.

Cc: Lyude Paul <lyude@...hat.com>
Cc: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc: Jani Nikula <jani.nikula@...ux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc: Jan-Marek Glogowski <glogow@...home.de>
References: 3cf71bc9904d ("drm/i915: Re-apply "Perform link quality check, unconditionally during long pulse"")
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@...el.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180927205735.16651-1-dhinakaran.pandiyan@intel.com
(cherry picked from commit 9ebd8202393dde9d3678c9ec162c1aa63ba17eac)
Fixes: 399334708b4f ("drm/i915: Re-apply "Perform link quality check, unconditionally during long pulse"")
Cc: stable@...r.kernel.org
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/gpu/drm/i915/intel_dp.c |   13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -4741,16 +4741,9 @@ intel_dp_long_pulse(struct intel_connect
 		goto out;
 	} else {
 		/*
-		 * If display is now connected check links status,
-		 * there has been known issues of link loss triggering
-		 * long pulse.
-		 *
-		 * Some sinks (eg. ASUS PB287Q) seem to perform some
-		 * weird HPD ping pong during modesets. So we can apparently
-		 * end up with HPD going low during a modeset, and then
-		 * going back up soon after. And once that happens we must
-		 * retrain the link to get a picture. That's in case no
-		 * userspace component reacted to intermittent HPD dip.
+		 * Some external monitors do not signal loss of link
+		 * synchronization with an IRQ_HPD, so force a link status
+		 * check.
 		 */
 		struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;