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

[Nathan Ciobanu]

I tested this patch in dinq on a KBL system and it fixed the bug. The 
system doesn't crash on disconnecting or powering off the second monitor 
in the DP-MST chain. I also replied to the Bugzilla issue.

Tested-by: Nathan D Ciobanu <nathan.d.ciobanu at linux.intel.com>

On 12/05/2016 01:49 PM, Pierre-Louis Bossart wrote:
> 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;
>   			}