[PATCH v7 09/11] drm: uevent for connector status change

Wed May 15 07:49:54 UTC 2019

Hi,

On Wed, 2019-05-15 at 10:37 +0300, Pekka Paalanen wrote:
> On Tue, 14 May 2019 16:34:01 +0200
> Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> 
> > On Tue, May 14, 2019 at 3:36 PM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > > On Tue, 14 May 2019 13:02:09 +0200
> > > Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > >  
> > > > On Tue, May 14, 2019 at 10:18 AM Ser, Simon <simon.ser at intel.com> wrote:  
> > > > > On Tue, 2019-05-14 at 11:02 +0300, Pekka Paalanen wrote:  
> 
> ...
> 
> > > > > > Hi Daniel,
> > > > > > 
> > > > > > just to clarify the first case, specific to one very particular
> > > > > > property:
> > > > > > 
> > > > > > With HDCP, there is a property that may change dynamically at runtime
> > > > > > (the undesired/desired/enabled tristate). Userspace must be notified
> > > > > > when it changes, I do not want userspace have to poll that property
> > > > > > with a timer.
> > > > > > 
> > > > > > When that property alone changes, and userspace is prepared to handle
> > > > > > that property changing alone, it must not trigger a reprobe of the
> > > > > > connector. There is no reason to reprobe at that point AFAIU.
> > > > > > 
> > > > > > How do you ensure that userspace can avoid triggering a reprobe with the
> > > > > > epoch approach or with any alternate uevent design?
> > > > > > 
> > > > > > We need an event to userspace that indicates that re-reading the
> > > > > > properties is enough and reprobe of the connector is not necessary.
> > > > > > This is complementary to indicating to userspace that only some
> > > > > > connectors need to be reprobed instead of everything.  
> > > > > 
> > > > > Can't you use the PROPERTY hint? If PROPERTY is the HDCP one, skip the
> > > > > reprobing. Would that work?  
> > > 
> > > Hi,
> > > 
> > > yes, that would work, if it was acceptable to DRM upstream. The replies
> > > to Paul seemed to be going south so fast that I thought we wouldn't get
> > > any new uevent fields in favour of "epoch counters".
> > >  
> > > > Yes that's the idea, depending upon which property you get you know
> > > > it's a sink change (needs full reprobe) or something else like hdcp
> > > > state machinery update.  
> > > 
> > > Right.
> > >  
> > > > Wrt avoiding the full reprobe for sink changes: I think we should
> > > > indeed decouple that from the per-connector event for sink changes.
> > > > That along is a good win already, since you know for which connector
> > > > you need to call drmGetConnector (which forces the reprobe). It would
> > > > be nice to only call drmGetConnectorCurrent (avoids the reprobe), but
> > > > historically speaking every time we tried to rely on this we ended up
> > > > regretting things.  
> > > 
> > > What changed? This sounds very much what Paul suggested. Looking at it
> > > from userspace side:  
> > 
> > This sounds solid, some refinements below:
> > 
> > > HOTPLUG=1 CONNECTOR=xx PROPERTY=yy
> > > 
> > > - If yy is "Content Protection", no need to drmModeGetConnector(), just
> > >   re-get the connector properties.
> > > 
> > > - Kernel probably shouldn't bother sending this for properties where
> > >   re-probe could be necessary, and send the below instead.  
> > 
> > I think we should assert that the kernel can get the new property
> > values using drmModeGetConnectorCurrent for this case, i.e. the kernel
> > does not expect a full reprobe. I.e. upgrade your idea from "should"
> > to "must"
> 
> Hi Daniel,
> 
> ok, that's good.
> 
> > Furthermore different property can indicate different kind of updates,
> > e.g. hdcp vs general sink change vs. whatever else might come in the
> > future.
> 
> What do you mean by different kinds of updates?
> 
> Btw. I started thinking, maybe we should completely leave out the "If
> yy is "Content Protection"" and require the kernel to guarantee, that
> if PROPERTY is set, then drmModeGetConnector() (probing) must not be
> necessary based on this event alone.

I agree, this is precisely what I had in mind.

> Writing it down again:
> 
> HOTPLUG=1 CONNECTOR=xx PROPERTY=yy
> 
> - yy denotes which connector xx property changed.
> 
> - Userspace does not need to do drmModeGetConnector(), it only needs to
>   drmModeObjectGetProperties() on the connector to receive the new
>   updated property values.
> 
> - Kernel must not send this event for changes that may require probing
>   for correct results, exceptional conditions (buggy hardware, etc.)
>   included. Instead, the kernel must send one of the below events.

Agreed, and leave this up to the driver in the end, not the core.

> Is there actually anything interesting that
> drmModeGetConnectorCurrent() could guaranteed correctly return that is
> not a property already? I'd probably leave this consideration out
> completely, and just say do one of the needs-probing events if anything
> there changed.

In the end, I think this should help move to a situation where
userspace would not have to do a reprobe at any point and the kernel
side just does it.

I see no justification for asking userspace to probe anyway, we can
manage all that in-kernel with each driver keeping its connector props
up to date and properly notifying userspace.

On the other hand, it's definitely good to keep the ability for
userspace to voluntarily reprobe at times, but those two ideas can work
fine together.

> > > HOTPLUG=1 CONNECTOR=xx
> > > 
> > > - Needs to drmModeGetConnector() on the one connector, no need to probe
> > >   others. Implies that one needs to re-get the connector properties as
> > >   well.  
> > 
> > Sounds good.
> > 
> > > HOTPLUG=1
> > > 
> > > - Need to do drmModeGetResouces() to discover new/disappeared
> > >   connectors, and need to drmModeGetConnector to re-probe every
> > >   connector. (As always.)  
> > 
> > Maybe we should clarify that this is also what you get when an entire
> > connector appears/disappears (for dp mst hotplug).
> 
> Yes, that's what I wrote. :-)
> 
> Weston implements the discovery of appearing/disappearing connectors
> (as opposed to connecting/disconnecting connectors). Not sure anyone
> has ever tested it though...
> 
> > Maybe we could make an additional rule that if a connector has the
> > EPOCH property, then it does _not_ need to be reprobe for the global
> > events. For that case userspace should only check whether there's
> > new/removed connectors, and then probe the new ones (and disable the
> > removed ones as needed). We can also use some other flag to indicate
> > this if we don't add the epoch proprty.
> 
> Sounds fine to me, though I'm not too clear what the epoch property
> is designed to achieve. Is it about avoiding re-probing when re-gaining
> DRM master after having let it go, e.g. VT-switching back from another
> VT? That would be nice.
> 
> > > That should be also backwards-compatible: any userspace that doesn't
> > > understand CONNECTOR will see HOTPLUG=1 and re-probe everything. Any
> > > userspace that doesn't understand PROPERTY or the property it refers to
> > > will fall back to probing either the connector or everything.  
> > 
> > Agreed, that should work.
> 
> Cool. The epoch exception you worded seems to fit backward-compatible
> as well.
> 
> > > I would be happy to get that behaviour into Weston, particularly as the
> > > HDCP feature is brewing for Weston too.
> > > 
> > > --------
> > > 
> > > When discussing this in IRC, I had the concern about how uevents are
> > > delivered in userspace. Is there a possibility that they might be
> > > overwritten, contain stale attributes, or get squashed together?
> > > 
> > > Particularly if a display server is current on the VT and active and
> > > monitoring udev, but stuck doing something and cannot service uevents
> > > very fast, and the kernel sends more than one event before the process
> > > gets back to dispatching. The terminology in libudev API confused me as
> > > an event is a device. Squashing together would make sense if the
> > > uevent were just updating a device attribute list. Previously when we
> > > had just a single kind of uevent, that would not have made a
> > > difference, but if we gain different kinds of uevents like here, it
> > > starts to matter.
> > > 
> > > However, Paul came to the conclusion that we will be ok as long as the
> > > events come via netlink.  
> > 
> > Yeah netlink shouldn't drop events on the floor I think. It might
> > still happen, but then I think you should get an indication of that
> > error, and you just treat it as a general hotplug event like on older
> > kernels.
> 
> Alright, although reading Paul it sounds like there is another
> (fallback?) method as well that wouldn't work. Should userspace worry
> about that?

It's unclear to me when that non-netlink thing is actually used by
systemd. IMO we can just leave this out. I think overall uevent/udev is
reliable enough for us to just expect it to work.

Cheers,

Paul

> Hmm, get an indication of an error... I don't know how that would be
> presented in libudev API and I can't point to any code in Weston that
> would deal with it. Does anyone have a clue about that?
> 
> Userspace cannot really start taking advantage of any new fine-grained
> hotplug events until it can rely on the event delivery. Granted, this
> seems purely a userspace issue, but I bet it could be formulated as a
> kernel regression: things stop working after upgrading the kernel while
> having always used new userspace which was ready for detailed hotplug
> events but didn't ensure the delivery in userspace.
> 
> Thanks,
> pq
-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com