[PATCH 5.12 171/242] drm/dp: Handle zeroed port counts in drm_dp_read_downstream_info()

[Greg Kroah-Hartman]

From: Lyude Paul <lyude@xxxxxxxxxx>

commit 205bb69a90363541a634a662a599fddb95956524 upstream.

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@xxxxxxxxxx>
Tested-by: Jérôme de Bretagne <jerome.debretagne@xxxxxxxxx>
Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3416
Fixes: 3d3721ccb18a ("drm/i915/dp: Extract drm_dp_read_downstream_info()")
Cc: <stable@xxxxxxxxxxxxxxx> # v5.10+
Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430223428.10514-1-lyude@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/gpu/drm/drm_dp_helper.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -679,7 +679,14 @@ int drm_dp_read_downstream_info(struct d
 	    !(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;