linux-kernel - [PATCH 1/2] drm/dp: Handle zeroed port counts in drm_dp_read_downstream

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Message-Id: <20210430223428.10514-1-lyude@redhat.com>
Date:   Fri, 30 Apr 2021 18:34:27 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Cc:     Jérôme de Bretagne 
        <jerome.debretagne@...il.com>, stable@...r.kernel.org,
        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>, Sean Paul <sean@...rly.run>,
        Jani Nikula <jani.nikula@...el.com>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH 1/2] drm/dp: Handle zeroed port counts in drm_dp_read_downstream_info()

While the DP specification isn't entirely clear on if this should be
allowed or not, some branch devices report having downstream ports present
while also reporting a downstream port count of 0. So to avoid breaking
those devices, we need to handle this in drm_dp_read_downstream_info().

So, to do this we assume there's no downstream port info when the
downstream port count is 0.

Signed-off-by: Lyude Paul <lyude@...hat.com>
Tested-by: Jérôme de Bretagne <jerome.debretagne@...il.com>
Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3416
Fixes: 3d3721ccb18a ("drm/i915/dp: Extract drm_dp_read_downstream_info()")
Cc: <stable@...r.kernel.org> # v5.10+
---
 drivers/gpu/drm/drm_dp_helper.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index cb56d74e9d38..27c8c5bdf7d9 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -682,7 +682,14 @@ int drm_dp_read_downstream_info(struct drm_dp_aux *aux,
 	    !(dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT))
 		return 0;
 
+	/* Some branches advertise having 0 downstream ports, despite also advertising they have a
+	 * downstream port present. The DP spec isn't clear on if this is allowed or not, but since
+	 * some branches do it we need to handle it regardless.
+	 */
 	len = drm_dp_downstream_port_count(dpcd);
+	if (!len)
+		return 0;
+
 	if (dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE)
 		len *= 4;
 
-- 
2.30.2