Re: [Intel-gfx] [PATCH] drm: Allow modeset on unregisted connectors unconditionally

[Imre Deak <imre.deak@intel.com>]

On Mon, May 20, 2019 at 08:41:09PM +0300, Imre Deak wrote:
> We allowed modesetting an unregistered connector only in the case the
> mode is getting disabled on the connector.
> 
> The reason for this check was the lack of proper refcounting for the
> backing memory objects. That problem has been solved meanwhile so there
> is no reason any more to reject the modesetting in general. The check
> for that also makes driver internal modesets more cumbersome where we
> need to add exemptions for the cases where we do need to allow the
> modeset even for unregistered connectors. One such case is the
> restoration of the mode during resume.
> 
> Simplify things by removing the unneeded check. I can't see how
> modesetting an unregistered connector can cause any problem and the race
> (described in the code comment) can anyway result in such a modeset (if
> the connector is unregistered right after the check).
> 
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Imre Deak <imre.deak@intel.com>

Lyude, could you test this change against the DDX fail scenario that the
check was added for originally (based on our IRC discussion)?

Thanks,
Imre

> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 29 ++---------------------------
>  1 file changed, 2 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 2e0cb4246cbd..e94e69483498 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -319,33 +319,6 @@ update_connector_routing(struct drm_atomic_state *state,
>  		return 0;
>  	}
>  
> -	crtc_state = drm_atomic_get_new_crtc_state(state,
> -						   new_connector_state->crtc);
> -	/*
> -	 * For compatibility with legacy users, we want to make sure that
> -	 * we allow DPMS On->Off modesets on unregistered connectors. Modesets
> -	 * which would result in anything else must be considered invalid, to
> -	 * avoid turning on new displays on dead connectors.
> -	 *
> -	 * Since the connector can be unregistered at any point during an
> -	 * atomic check or commit, this is racy. But that's OK: all we care
> -	 * about is ensuring that userspace can't do anything but shut off the
> -	 * display on a connector that was destroyed after it's been notified,
> -	 * not before.
> -	 *
> -	 * Additionally, we also want to ignore connector registration when
> -	 * we're trying to restore an atomic state during system resume since
> -	 * there's a chance the connector may have been destroyed during the
> -	 * process, but it's better to ignore that then cause
> -	 * drm_atomic_helper_resume() to fail.
> -	 */
> -	if (!state->duplicated && drm_connector_is_unregistered(connector) &&
> -	    crtc_state->active) {
> -		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] is not registered\n",
> -				 connector->base.id, connector->name);
> -		return -EINVAL;
> -	}
> -
>  	funcs = connector->helper_private;
>  
>  	if (funcs->atomic_best_encoder)
> @@ -390,6 +363,8 @@ update_connector_routing(struct drm_atomic_state *state,
>  
>  	set_best_encoder(state, new_connector_state, new_encoder);
>  
> +	crtc_state = drm_atomic_get_new_crtc_state(state,
> +						   new_connector_state->crtc);
>  	crtc_state->connectors_changed = true;
>  
>  	DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] using [ENCODER:%d:%s] on [CRTC:%d:%s]\n",
> -- 
> 2.17.1
> 
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