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

Jasper St. Pierre jstpierre at
Fri Jan 17 05:36:01 PST 2014

Is there a reason we can't somehow "merge" the two CRTCs together into one
virtual CRTC? Too difficult to modeset?

On Thu, Jan 16, 2014 at 2:11 PM, Aaron Plattner <aplattner at>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?
> --
> Aaron
> _______________________________________________
> xorg-devel at X.Org development
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