How do we want to deal with 4k tiled displays?

Pierre-Loup A. Griffais pgriffais at
Mon Jan 27 19:06:18 PST 2014

I see lots of discussion has happened on that front already; sorry for 
being late to the party.

I'm currently wrestling against a Dell UP3214Q, which seems to exhibit 
all of the problems you refer to. There are many existing binary games 
out there that most likely will never get touched again that are RandR 
clients and try to either maximize themselves to one of the displays or 
ask the window manager to do it for them, after discovering the monitor 
topology. Some of these games ship their own libXRandR, some of them 
statically link against it, some of them inline the protocol code to 
take bug fixes that weren't widespread in distros at the time they were 
packaged. It's pretty bad. Given that the only viable solution seems to 
lie completely on the X server side of things.

It seems the X server should be responsible for both making sure the two 
halves are laid out properly (right now they're in reverse order) and 
hidden from clients. This database of quirks would ideally live in the 
server itself where it can be easily leveraged by everyone shipping DDXen.

  - Pierre-Loup

On 01/16/2014 11:11 AM, Aaron Plattner wrote:
> So, monitor manufacturers are starting to make high-resolution displays that
> consist of one LCD panel that appears to the PC as two.  The one I've got is a
> Dell UP2414Q.  It shows up to the PC as two DisplayPort 1.2 multistream devices
> that have the same GUID but different EDIDs.  There's an extension block in the
> EDID that's supposed to indicate which side is the left tile and which is the
> right, though I haven't tried to decode it yet.
> The problem, obviously, is that applications (including some games) treat the
> two tiles as if they were completely separate monitors.  Windows maximize to
> only half of the screen.  My question is, how do we want to deal with these
> monitors?
> As far as I see it, we have four options:
>   1. Hide the presence of the second tile in the X server.
>      Somehow combine the two tiles into a single logical output at the RandR
>      protocol level.  The X server would be responsible for setting up the right
>      configuration to drive the logical output using the correct physical
>      resources.
>   2. Hide the presence of the second tile in libXrandr.
>      This would allow interested applications to query the real state of the
>      hardware while also making it easier to do modesets on a per-monitor level
>      rather than per-output.
>      This could be exposed either as a new "simple" modeset API in libXrandr or
>      similar, or by modifying the existing interface and having a new interface
>      to punch through the façade and get at the real configuration, for clients
>      that care.
>   3. Update every application that uses RandR 1.2.
>      Applications can detect the presence of these monitors and deal with them
>      themselves, but this might have poor adoption because programmers are a lazy
>      bunch in general.
>   4. Do nothing and hope the problem goes away.
>      Hopefully, the situation with current 4k monitors is temporary and we'll
>      start seeing single-tile 4k displays soon, fixing the problem "forever".
>      Until we get 8k tiled displays.
> If the real output devices are still exposed through the protocol, it might make
> sense to add new properties describing their relative positions to make it
> easier for clients to lay them out in the right order.  This might be useful for
> power-walls too.
> The problem with the first two options is that driving these monitors consumes
> two crtcs.  If we present them as a single output to applications, they'll make
> the assumption that they can just assign a single crtc to that output and use
> the remaining crtcs for something else.  I suspect that deleting crtcs or
> otherwise marking them as used as a side effect of setting a mode on a different
> crtc is going to explode a lot of existing applications.
> ~~
> Regardless of what we do about the current crop of 4k monitors, one feature I
> would like to add is a standardized OutputGroup property.  Multiple outputs with
> the same value of OutputGroup should be considered (both by clients and the
> server) as a single logical monitor.  This would affect the Xinerama information
> presented by rrxinerama.c, and window managers that use RandR 1.2 directly would
> be encouraged to consider output groups in their UI behavior.
> The X server could configure OutputGroups automatically when setting up the
> initial configuration based on the presence of tiled displays, and clients could
> reconfigure the groups at runtime to get different behavior if desired.
> Does this sound like a reasonable extension to RandR?

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