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@redhat.com>
Tested-by: Jérôme de Bretagne <redacted>
Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3416
Fixes: 3d3721ccb18a ("drm/i915/dp: Extract drm_dp_read_downstream_info()")
Cc: <redacted> # 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;
+