[systemd-devel] DisplayPort display non-persistent device naming

[Simon Ser]

On Friday, October 30, 2020 9:09 AM, Pekka Paalanen <ppaalanen at gmail.com> wrote:

> On Thu, 29 Oct 2020 21:53:34 +0100
> Marcin Kocur marcin2006 at gmail.com wrote:
>
> > Hello,
> > this is the output of turning off and on my display (using power button):
>
> ...
>
> > The monitor was visible in xrandr as DP-2, after power off and on it's
> > visible as DP-3 (DP-2 is still there "disconnected").
> > It's troublesome for:
> >
> > -   GUI display configurators
> >
> > -   scripting
> >
> > -   for Xorg configuration which stops to work:
> >
> >
> > Section "Monitor"
> >     Identifier  "DP-2"
> >     Option      "Primary" "true"
> > EndSection
> > Is this a bug or a feature?
>
> Hi,
>
> this is how the kernel DRM sub-system works. The connector names are
> not persistent in general, just like you can't expect that the same gfx
> card appears always at the same /dev/dri/cardN node if you have
> several. By default, cardN nodes are assigned in the order of which
> driver instance happens to initialize first and it can be random.
>
> Usually hard-wired (in hardware) connectors just happen to always be
> discovered in the same order, and if you only have a single gfx card in
> your machine, the connector naming is practically persistent. This is
> an accident. It is not guaranteed if you have multiple cards or you
> have MST connectors.
>
> MST connectors can appear and disappear dynamically. There is a KMS
> property that attempts to reflect something about the MST topology so
> that you might have some hope to match a "connector", but this is not
> in the connector name. The concept of a persistent connector is
> problematic if the connector is in a MST monitor for daisy-chaining
> more monitors - you can always unplug the first monitor making the
> connector disappear (not just become disconnected).
>
> If you want reliable monitor matching, monitor serial number (if
> present) would be a more reliable method. I'm not sure Xorg config has
> matching rules for that though, but I suppose RandR based configuration
> utilities could do it.
>
> If you want to discuss this further, dri-devel mailing list is the
> place - Cc'd. Nothing to do with systemd here, nor even with udev.

If you really want to reliably match the physical connector, this
proposal from Ville may help:

https://lists.freedesktop.org/archives/dri-devel/2019-June/221902.html

In fact, MST connectors already all have a PATH prop. What we're
missing is a PATH prop for other connectors as well.