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

Tue May 14 13:58:21 UTC 2019

Hi,

On Tue, 2019-05-14 at 16:36 +0300, Pekka Paalanen 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:  
> > > > On Mon, 13 May 2019 11:34:58 +0200
> > > > Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > > >  
> > > > > On Mon, May 13, 2019 at 11:02 AM Paul Kocialkowski
> > > > > <paul.kocialkowski at bootlin.com> wrote:  
> > > > > > Hi,
> > > > > > 
> > > > > > On Fri, 2019-05-10 at 16:54 +0200, Daniel Vetter wrote:  
> > > > > > > On Fri, May 10, 2019 at 2:12 PM Paul Kocialkowski
> > > > > > > <paul.kocialkowski at bootlin.com> wrote:  
> > > > > > > > Hi,
> > > > > > > > 
> > > > > > > > On Tue, 2019-05-07 at 21:57 +0530, Ramalingam C wrote:  
> > > > > > > > > DRM API for generating uevent for a status changes of connector's
> > > > > > > > > property.
> > > > > > > > > 
> > > > > > > > > This uevent will have following details related to the status change:
> > > > > > > > > 
> > > > > > > > >   HOTPLUG=1, CONNECTOR=<connector_id> and PROPERTY=<property_id>
> > > > > > > > > 
> > > > > > > > > Need ACK from this uevent from userspace consumer.  
> > > > > > > > 
> > > > > > > > So we just had some discussions over on IRC and at about the hotplug
> > > > > > > > issue and came up with similar ideas:
> > > > > > > > https://lists.freedesktop.org/archives/dri-devel/2019-May/217408.html
> > > > > > > > 
> > > > > > > > The conclusions of these discussions so far would be to have a more or
> > > > > > > > less fine grain of uevent reporting depending on what happened. The
> > > > > > > > point is that we need to cover different cases:
> > > > > > > > - one or more properties changed;
> > > > > > > > - the connector status changed;
> > > > > > > > - something else about the connector changed (e.g. EDID/modes)
> > > > > > > > 
> > > > > > > > For the first case, we can send out:
> > > > > > > > HOTPLUG=1
> > > > > > > > CONNECTOR=<id>
> > > > > > > > PROPERTY=<id>
> > > > > > > > 
> > > > > > > > and no reprobe is required.
> > > > > > > > 
> > > > > > > > For the second one, something like:
> > > > > > > > HOTPLUG=1
> > > > > > > > CONNECTOR=<id>
> > > > > > > > STATUS=Connected/Disconnected
> > > > > > > > 
> > > > > > > > and a connector probe is needed for connected, but not for
> > > > > > > > disconnected;
> > > > > > > > 
> > > > > > > > For the third one, we can only indicate the connector:
> > > > > > > > HOTPLUG=1
> > > > > > > > CONNECTOR=<id>
> > > > > > > > 
> > > > > > > > and a reprobe of the connector is always needed  
> > > > > > > 
> > > > > > > There's no material difference between this one and the previous one.
> > > > > > > Plus there's no beenfit in supplying the actual value of the property,
> > > > > > > i.e. we can reuse the same PROPERTY=<id-of-status-property> trick.  
> > > > > > 
> > > > > > That's the idea, but we need to handle status changes differently than
> > > > > > properties, since as far as I know, connected/unconnected status is not
> > > > > > exposed as a prop for the connector.  
> > > > > 
> > > > > Oops, totally missed that. "Everything is a property" is kinda
> > > > > new-ish, at least compared to kms. Kinda tempted to just make status
> > > > > into a property. Or another excuse why we should expose the epoch
> > > > > property :-)  
> > > > 
> > > > 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:
> 
> 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.
> 
> 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.
> 
> HOTPLUG=1
> 
> - Need to do drmModeGetResouces() to discover new/disappeared
>   connectors, and need to drmModeGetConnector to re-probe every
>   connector. (As always.)
> 
> 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.
> 
> 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.

Yes, basically netlink is fine and uevent file polling is not. If the
application is too slow to service new udev devices (events), I'm
pretty sure they get queued if they come from netlink. It's only when
not using netlink with udev (not sure when that really happens, and the
info doesn't seem to be exposed to the application) that the latest
contents are read from the uevent file.

The way I see it, we should put the assumption that userspace will get
all our events or just figure it out on its own with the epoch counter.
It's a notification mechanism, and I think we should count on the fact
that userspace can reliably get notified that way (not to say it has to
be the only way, the epoch counter can provide a polling-ish
alternative).

Cheers and thanks for the recap,

Paul

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com