tile property contents

[Dave Airlie]

>> > > There are also configurations where users configure multiple heads to
>> > > drive power walls that they want to be treated as one logical monitor,
>> > > similar to the DP MST tiled display case.  Normally, those powerwall
>> > > configurations don't have any layout information from the monitors
>> > > themselves, and the layout is configured by the user.
>> > >
>> > > Would it be appropriate for users to be able to set the TILE property
>> > > in that sort of scenario?
>> > >
>> > > For the sake of generality, I wonder if max[hv]tiles and [hv]_tile_loc
>> > > should be expressed in pixels rather than tiles?  Sometimes, the tiles
>> > > in those powerwalls may not all have the same resolution, or may be
>> > > configured with overlap.  I suppose that would make the TILE configuration
>> > > specific to the current modetimings on each tile...
>> >
>> > Why can't users just set that mode?
>>
>> Sure, users can set the mode, but:
>>
>> * Part of what the TILE property conveys is how monitors should be grouped
>>   for purposes of window maximization.  Users don't have a great way to
>>   express that today, that I'm aware of.
>
> My understanding for why we want the TILE property is to avoid to
> duplicate displayId parsing over every bit of userspace (and the fbcon
> stuff in the kernel) interested in it. Imo the proper way for userspace is
> to always just inherit whatever modeset config is already there.

Andy's idea is good, I'd considered it before, the problem being how
to expose it nicely,

not sure if you'd want persistent via /sys or dynamic setting of the
property by user for that session, so we could do it like xrandr
modes.

Daniel you are missing the nice case of using TILE for non-displayid
monitors once the infrastructure is in place.

Having it so you can create user defined tile groups to allow users to
configure arbitrary walls is a useful thing, that you can't do any
other way.

>
>> * Users might configure the mode they want, but then gnome-settings-daemon
>>   may come along later and decide it knows better than the user how things
>>   should be configured.  One scenario where this comes up is:
>>     (a) user meticulously configures his power wall
>>     (b) user hotplugs another monitor
>>   I've definitely seen cases where window managers will try to be clever
>>   in response to a hotplug, and clobber the user's current configuration.
>>   If the TILE property conveyed how some set of monitors was supposed
>>   to be grouped, that would hopefully give window managers additional
>>   information, such that they would know to keep that group intact.
>
> Well, imnsho gnome display control center is a bit too opiniated about
> automatic modeset changes. If their assumption is that they always know
> perfectly what the user wants upon hotplug I really don't want to work
> around this in the kernel. Since for everything else than a laptop +
> beamer gnome panel always pisses me off ;-)
>
> I think gnome should just ask the user what it wants if there's more than
> 2 physical displays (treating a tiled 4k screen as one ofc), since there's
> really no way at all to tell.

Well its not just a GNOME problem either, once things like SDL respect
tIle properrty,
we can create arbitary tile walls that the whole stack will respect,
instead of hacks
like fake xinerama.

Dave.