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

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

On Mon, May 20, 2019 at 08:37:46PM +0200, Daniel Vetter wrote:
> 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?

Yes the check was added to solve the issue related to the removal of MST
connectors that could happen asynchronously wrt. a modeset referring to
that MST connector.  That could happen since the MST core doesn't hold
any locks (for instance the connection_mutex) during removing an MST
connector that would prevent doing a modeset at the same time.

Adding the refcounting for such MST connectors (via the
drm_connector_get()/drm_connector_put()) got rid of the above problem.

> 
> > 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.

I still don't see why this check is needed. There is no justification
for it - besides the original reason for it as discussed above about the
refcounting problem, which is solved now - so I think we should remove
it, instead of just making it a special case for the user space modeset.

As I wrote a user space modeset can end up anyway doing a modeset on an
unregistered connector when the unregistering - by MST core - happens just
right after the check.

> -Daniel
> 
> > 
> > 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>
> > ---
> >  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
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