[systemd-devel] Some questions regarding seat preperty CanGraphical

Laércio de Sousa laerciosousa at sme-mogidascruzes.sp.gov.br
Mon Nov 30 04:33:32 PST 2015

I'm contributing with some DM projects (LightDM, SDDM) in order to provide
full systemd-logind multi-seat support. IMHO the most challenging part is
monitoring seat's property CanGraphical an reacting properly when this
property changes.

If I understand systemd-logind code correctly, a seat will only have
CanGraphical=yes if it has a master device (which used to be the graphics
device if it uses KMS drivers), right? If so, than this property is only
relevant for seat0, because other seats doesn't exist at all if they don't
have a master device, right?

Since system release 198, we can tag any suitable device as a seat master
device, so shouldn't it be more clear if this property was renamed to e.g.
HasMasterDevice? Or introduce another check to ensure a seat can really
handle a graphical display?

I imagine this property was introduced for the case where system boots so
fast that a given DM service is started before seat0 master (graphics)
device is ready. However, if seat0 graphics device doesn't have KMS drivers
(e.g. a VirtualBox guest without Guest Additions installed), then seat0
will never get CanGraphical=yes. That said, how do you suggest this to be
handled by DM? It seems GDM currently doesn't support it, and LightDM was
forced to make this support optional and disabled by default (after some
VirtualBox guests without Guest Additions failed to boot to graphical
display). SDDM implementation is currently in progress, and there may be
some discussion whether CanGraphical monitoring should be supported or not.

The last (but not least): is this property really needed? I mean, do you
have any use case where a DM was started before main graphics device is
*Laércio de Sousa*
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