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

[Pandiyan, Dhinakaran]

On Sun, 2016-12-04 at 19:31 -0600, 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.
> 
> Fix by adding test condition to make sure both variables
> are used consistently. This removes the kernel oops.
> 
> There are still annoying cases where the primary display goes black
> when the secondary display is turned off but it can be recovered from
> by playing with the monitor inputs and power buttons.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98990
> 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..5481fde 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -1815,7 +1815,7 @@ int drm_dp_update_payload_part1(struct drm_dp_mst_topology_mgr *mgr)
>  				drm_dp_create_payload_step1(mgr, mgr->proposed_vcpis[i]->vcpi, &req_payload);
>  				mgr->payloads[i].num_slots = req_payload.num_slots;
>  				mgr->payloads[i].vcpi = req_payload.vcpi;
> -			} else if (mgr->payloads[i].num_slots) {
> +			} else if (mgr->payloads[i].num_slots && port != NULL) {
>  				mgr->payloads[i].num_slots = 0;
>  				drm_dp_destroy_payload_step1(mgr, port, port->vcpi.vcpi, &mgr->payloads[i]);
>  				req_payload.payload_state = mgr->payloads[i].payload_state;


Or port shouldn't have been set to NULL in the first place. By adding,
(port != NULL), you are not updating the payload state.

-DK