On Mon, May 20, 2019 at 10:06 PM Imre Deak <imre.deak@xxxxxxxxx> 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.
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.
-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@xxxxxxxxxx>
> > > > > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > > > > Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > > > > Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
> > > > > ---
> > > > > 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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