linux-kernel - [PATCH] drm/dp: Fix bogus DPCD version check in drm_dp_read_downstream

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Date:   Fri,  7 May 2021 17:42:09 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] drm/dp: Fix bogus DPCD version check in drm_dp_read_downstream_info()

Ville pointed this out to me when fixing some issues in
drm_dp_read_downstream_info() - the DPCD version check here is bogus as
there's no DisplayPort versions prior to 1.0. The original code from i915
that this was extracted from actually did:

  dpcd[DP_DPCD_REV] == DP_DPCD_REV_10

Which is correct, and somehow got missed when extracting this function. So
let's fix this. Note that as far as I'm aware, I don't think this fixes any
actual issues users are hitting.

Signed-off-by: Lyude Paul <lyude@...hat.com>
Cc: Ville Syrjälä <ville.syrjala@...ux.intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 0f84df8798ab..55b53df6ce34 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -677,7 +677,7 @@ int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
 	memset(downstream_ports, 0, DP_MAX_DOWNSTREAM_PORTS);
 
 	/* No downstream info to read */
-	if (!drm_dp_is_branch(dpcd) || dpcd[DP_DPCD_REV] < DP_DPCD_REV_10)
+	if (!drm_dp_is_branch(dpcd) || dpcd[DP_DPCD_REV] == DP_DPCD_REV_10)
 		return 0;
 
 	/* Some branches advertise having 0 downstream ports, despite also advertising they have a
-- 
2.30.2