From 205bb69a90363541a634a662a599fddb95956524 Mon Sep 17 00:00:00 2001 From: Lyude Paul Date: Fri, 30 Apr 2021 18:34:27 -0400 Subject: [PATCH] drm/dp: Handle zeroed port counts in drm_dp_read_downstream_info() MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Tested-by: Jérôme de Bretagne Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3416 Fixes: 3d3721ccb18a ("drm/i915/dp: Extract drm_dp_read_downstream_info()") Cc: # v5.10+ Reviewed-by: Ville Syrjälä Link: https://patchwork.freedesktop.org/patch/msgid/20210430223428.10514-1-lyude@redhat.com --- 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; -- 2.11.0