linux-kernel - [PATCH] drm/dp: Avoid Reading DPCD

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Date:   Wed, 31 Aug 2022 18:13:21 -0400
From:   Fangzhi Zuo <Jerry.Zuo@....com>
To:     Daniel Vetter <daniel@...ll.ch>,
        Ville Syrjala <ville.syrjala@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>, Lyude Paul <lyude@...hat.com>,
        Alex Deucher <alexander.deucher@....com>,
        Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>
CC:     <linux-kernel@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <amd-gfx@...ts.freedesktop.org>, Fangzhi Zuo <Jerry.Zuo@....com>
Subject: [PATCH] drm/dp: Avoid Reading DPCD_REV Before Native Aux Read

The attempt to read DPCD_REV before any native aux read breaks
majority of DP2 compliance.

The spec. requires DP_SINK_STATUS to be polled for the reset status
DP_INTRA_HOP_AUX_REPLY_INDICATION during the clear training stage.

Polling DP_SINK_STATUS each time gets DPCD_REV read first
that makes non link training regsiter DPCD_REV get read
during UHBR link training. It violates DP2 compliance.

Cc: Ville Syrjala <ville.syrjala@...ux.intel.com>
Cc: Lyude Paul <lyude@...hat.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@....com>
---
 drivers/gpu/drm/display/drm_dp_helper.c | 18 ------------------
 1 file changed, 18 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index e7c22c2ca90c..c7aa5bafa667 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -571,24 +571,6 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 {
 	int ret;
 
-	/*
-	 * HP ZR24w corrupts the first DPCD access after entering power save
-	 * mode. Eg. on a read, the entire buffer will be filled with the same
-	 * byte. Do a throw away read to avoid corrupting anything we care
-	 * about. Afterwards things will work correctly until the monitor
-	 * gets woken up and subsequently re-enters power save mode.
-	 *
-	 * The user pressing any button on the monitor is enough to wake it
-	 * up, so there is no particularly good place to do the workaround.
-	 * We just have to do it before any DPCD access and hope that the
-	 * monitor doesn't power down exactly after the throw away read.
-	 */
-	if (!aux->is_remote) {
-		ret = drm_dp_dpcd_probe(aux, DP_DPCD_REV);
-		if (ret < 0)
-			return ret;
-	}
-
 	if (aux->is_remote)
 		ret = drm_dp_mst_dpcd_read(aux, offset, buffer, size);
 	else
-- 
2.34.1