Re: [PATCH] drm/display/dp_mst: Add protection against 0 vcpi

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 13 Nov 2025, Suraj Kandpal <suraj.kandpal@intel.com> wrote:
> When releasing a timeslot there is a slight chance we may end up
> with the wrong payload mask due to overflow if the delayed_destroy_work
> ends up coming into play after a DP 2.1 monitor gets disconnected
> which causes vcpi to become 0 then we try to make the payload =
> ~BIT(vcpi - 1) which is a negative shift.

Is that a real scenario or a hypothetical one?

>
> Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
> ---
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index 64e5c176d5cc..3cf1eafcfcb5 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -4531,6 +4531,7 @@ int drm_dp_atomic_release_time_slots(struct drm_atomic_state *state,
>  	struct drm_dp_mst_atomic_payload *payload;
>  	struct drm_connector_state *old_conn_state, *new_conn_state;
>  	bool update_payload = true;
> +	int bit;
>  
>  	old_conn_state = drm_atomic_get_old_connector_state(state, port->connector);
>  	if (!old_conn_state->crtc)
> @@ -4572,7 +4573,8 @@ int drm_dp_atomic_release_time_slots(struct drm_atomic_state *state,
>  	if (!payload->delete) {
>  		payload->pbn = 0;
>  		payload->delete = true;
> -		topology_state->payload_mask &= ~BIT(payload->vcpi - 1);
> +		bit = payload->vcpi ? payload->vcpi - 1 : 0;
> +		topology_state->payload_mask &= ~BIT(bit);
>  	}
>  
>  	return 0;

-- 
Jani Nikula, Intel