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;
