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

[Jani Nikula]

From: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>

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
Fixes: dfda0df34264 ("drm/mst: rework payload table allocation to conform better.")
Cc: Dave Airlie <airlied at redhat.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Nathan D Ciobanu <nathan.d.ciobanu at linux.intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
Cc: Sean Paul <seanpaul at chromium.org>
Cc: <stable at vger.kernel.org> # v3.18+
Tested-by: Nathan D Ciobanu <nathan.d.ciobanu at linux.intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula at 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 122a1b04bebc..f2cc375907d0 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;
 			}
-- 
2.1.4