[PATCH] drm: Allow modeset on unregisted connectors unconditionally

[Daniel Vetter]

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.

I'm not parsing this at all ... maybe references to the commits that fix
this? Or do you mean the refcounting work for all the things hanging of
connectors, including the entire mst tree?

> 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.

Yeah this one actually makes sense to me. We could still keep this check
here, but for the atomic ioctl only when called from userspace. But iirc
Lyude also said she has some plans here, so no idea whether that all fits.

> 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).

Not saying we don't need this, but there's fairly enormous amounts of
history behind all this stuff, and lots of discussions. Would be good to
at least reference those, so we have a good story for when this then all
goes wrong again.
-Daniel

> 
> Cc: Lyude Paul <lyude at redhat.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Signed-off-by: Imre Deak <imre.deak at intel.com>
> ---
>  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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch