[RFC PATCH] drm/msm/dp: move link_ready out of HPD event thread

[Kuogee Hsieh]

On 3/6/2024 11:50 AM, Abhinav Kumar wrote:
> There are cases where the userspace might still send another
> frame after the HPD disconnect causing a modeset cycle after
> a disconnect. This messes the internal state machine of MSM DP driver
> and can lead to a crash as there can be an imbalance between
> bridge_disable() and bridge_enable().
>
> This was also previously reported on [1] for which [2] was posted
> and helped resolve the issue by rejecting commits if the DP is not
> in connected state.
>
> The change resolved the bug but there can also be another race condition.
> If hpd_event_thread does not pick up the EV_USER_NOTIFICATION and process it
> link_ready will also not be set to false allowing the frame to sneak in.
>
> Lets move setting link_ready outside of hpd_event_thread() processing to
> eliminate a window of race condition.
>
> [1] : https://gitlab.freedesktop.org/drm/msm/-/issues/17
> [2] : https://lore.kernel.org/all/1664408211-25314-1-git-send-email-quic_khsieh@quicinc.com/
>
> Signed-off-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
Tested-by: Kuogee Hsieh <quic_khsieh at quicinc.com>
Reviewed-by: Kuogee Hsieh <quic_khsieh at quicinc.com>
> ---
>   drivers/gpu/drm/msm/dp/dp_display.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index 068d44eeaa07..e00092904ccc 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -345,8 +345,6 @@ static int dp_display_send_hpd_notification(struct dp_display_private *dp,
>   							 dp->panel->downstream_ports);
>   	}
>   
> -	dp->dp_display.link_ready = hpd;
> -
>   	drm_dbg_dp(dp->drm_dev, "type=%d hpd=%d\n",
>   			dp->dp_display.connector_type, hpd);
>   	drm_bridge_hpd_notify(bridge, dp->dp_display.link_ready);
> @@ -399,6 +397,8 @@ static int dp_display_process_hpd_high(struct dp_display_private *dp)
>   		goto end;
>   	}
>   
> +	dp->dp_display.link_ready = true;
> +
>   	dp_add_event(dp, EV_USER_NOTIFICATION, true, 0);
>   
>   end:
> @@ -466,6 +466,8 @@ static int dp_display_notify_disconnect(struct device *dev)
>   {
>   	struct dp_display_private *dp = dev_get_dp_display_private(dev);
>   
> +	dp->dp_display.link_ready = false;
> +
>   	dp_add_event(dp, EV_USER_NOTIFICATION, false, 0);
>   
>   	return 0;
> @@ -487,6 +489,7 @@ static int dp_display_handle_port_status_changed(struct dp_display_private *dp)
>   		drm_dbg_dp(dp->drm_dev, "sink count is zero, nothing to do\n");
>   		if (dp->hpd_state != ST_DISCONNECTED) {
>   			dp->hpd_state = ST_DISCONNECT_PENDING;
> +			dp->dp_display.link_ready = false;
>   			dp_add_event(dp, EV_USER_NOTIFICATION, false, 0);
>   		}
>   	} else {