[Intel-gfx] [PATCH v2] drm/dp/mst: fix kernel oops when turning off secondary monitor

[Pierre-Louis Bossart]

100% reproducible issue found on SKL SkullCanyon NUC with two external
DP daisy-chained monitors in DP/MST mode. When turning off or changing
the input of the second monitor the machine stops with a kernel
oops. This issue happened with 4.8.8 as well as drm/drm-intel-nightly.

This issue is traced to an inconsistent control flow in
drm_dp_update_payload_part1(): the 'port' pointer is set to NULL at
the same time as 'req_payload.num_slots' is set to zero, but the pointer
is dereferenced even when req_payload.num_slot is zero.

The problematic dereference was introduced in commit dfda0df34
("drm/mst: rework payload table allocation to conform better")
and may impact all versions since v3.18

The fix suggested by Chris Wilson removes the kernel oops and was found to
work well after 10mn of monkey-testing with the second monitor power and
input buttons

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98990
Cc: Dave Airlie <airlied at redhat.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index aa64448..f59771d 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -1817,7 +1817,7 @@ int drm_dp_update_payload_part1(struct drm_dp_mst_topology_mgr *mgr)
 				mgr->payloads[i].vcpi = req_payload.vcpi;
 			} else if (mgr->payloads[i].num_slots) {
 				mgr->payloads[i].num_slots = 0;
-				drm_dp_destroy_payload_step1(mgr, port, port->vcpi.vcpi, &mgr->payloads[i]);
+				drm_dp_destroy_payload_step1(mgr, port, mgr->payloads[i].vcpi, &mgr->payloads[i]);
 				req_payload.payload_state = mgr->payloads[i].payload_state;
 				mgr->payloads[i].start_slot = 0;
 			}
-- 
2.7.4