drm/dp/mst: fix kernel oops when turning off secondary monitor

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 <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Nathan D Ciobanu <[email protected]>
Cc: Dhinakaran Pandiyan <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: <[email protected]> # v3.18+
Tested-by: Nathan D Ciobanu <[email protected]>
Reviewed-by: Dhinakaran Pandiyan <[email protected]>
Signed-off-by: Pierre-Louis Bossart <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

0 files changed, 0 insertions, 0 deletions