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

Imre Deak imre.deak at intel.com
Mon May 20 20:50:57 UTC 2019 

On Mon, May 20, 2019 at 10:15:20PM +0200, Daniel Vetter wrote:
> On Mon, May 20, 2019 at 10:06 PM Imre Deak <imre.deak at intel.com> wrote:
> > On Mon, May 20, 2019 at 09:23:00PM +0200, Daniel Vetter wrote:
> > > On Mon, May 20, 2019 at 10:09:24PM +0300, Imre Deak wrote:
> > > > 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.
> > >
> > > We added the check way after that stuff landed. Before all the connector
> > > reworking connectors were forcefully disabled by the kernel. The idea
> > > behind this check is to make sure that that userspace notices a connector
> > > is gone (only thing that's not allowed is enabling it, you can keep
> > > pageflipping). I think we've always had behaviour like ever since mst (all
> > > userspace has some "oops mst connector probably gone" failure catching
> > > around modesets).
> >
> > Right, pageflipping works.
> >
> > > So no idea what all blows up if we stop catching userspace this way.
> > >
> > > Now very much possible I'm getting all this wrong again or missing
> > > something, this stuff is often way over my head. But I'm really vary of
> > > breaking userspace here. E.g. just the drm_connector_get/put lifetime
> > > changes results in some userspace breaking if you unplug/replug fast
> > > enough, because then it doesn't notice the connector change anymore. I
> > > couldn't figure out a way to paper over that regression without
> > > reintroduce the rather broken and oops-prone old connector lifetime
> > > management.
> >
> > Yes, but what is the the actual description of the failing scenario? I
> > can't see how anything can go wrong without this check. The resume time
> > restoration modeset may have to act in the same way on an old connector.
> 
> That's what the !state->duplicated thing is meant to check for btw.
> 
> > I don't understand how userspace would not notice the connector change.
> > It will get a hotplug uevent in response to which it would have to do a
> > detect which returns to it the updated information about the new MST
> > connector tree.
> 
> uevent handling can take positively forever, at which point the new
> connector could already be plugged in, and then userspace makes a mess
> aliasing the two since the path property matches. Just needs a wobbly
> cable. This is the issue with the "fixed" lifetime management, and I'm
> wary of breaking more stuff.

Yea, processing can be delayed arbitrarily, the corresponding
GETRESOURCES/GETCONNECTOR should still return to the userspace the
correct information to do right thing, even if the path property
matches: the connector ID for the old and new connector will be
different and the GETCONNECTOR for the old ID will return properly that
this old connector is already disconnected (see intel_dp_mst_detect()).
So I don't see how userspace could mess up things. Obviously if user
space is buggy, then well, it is just buggy and then that user space bug
would need to be fixed instead of papering over such problems in the
kernel.

> Other way round: What do we gain if userspace is allowed to turn on a
> connector again which doesn't exist and will not ever show any pixels?
> I'm not seeing a benefit here in allowing that. And history says we
> change something around mst handling, it'll break something somewhere.

Let's then describe the actual reason for this check, that description
is missing.

We should remove this check to simplify things if there isn't an actual
need for it. Userspace can do already a modeset on connectors in general
that are not in a connected state. So this special casing - for MST
connectors only - is a bad idea if there is no reason for special casing
it. If it's there to paper over some user space bug that user space bug
should be fixed instead of adding this special casing which adds
complexity to the driver.

The only justification for a check in the driver if something could blow
up in the kernel without that check. Nothing can blow up in the kernel
without this check. Every other failure scenario should be handled/fixed
in userspace.

> -Daniel
> 
> >
> > > > > > 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.
> > >
> > > Yup. Always been like that.
> > > -Daniel
> > >
> > > >
> > > > > -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
> > >
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> 
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

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