Wayland / Weston Multi Display Client-Side Functionality

Pekka Paalanen ppaalanen at gmail.com
Wed Feb 27 09:23:40 UTC 2019

On Tue, 26 Feb 2019 00:11:45 +1300
Scott Anderson <scott.anderson at collabora.com> wrote:

> On 25/02/19 8:21 pm, AMAR SEN wrote:
> > Hi,
> > 
> > 1. Is there any client side functionality to handle multiple displays? 
> > It can be seen as like a particular client has some options that can 
> > direct the server to show frames on either of the displays or both 
> > (multiple displays) of them at the same time. If it has such 
> > functionality, can somebody point me to an example implementation of 
> > such client?  
> A normal Wayland client has no control regarding how it is placed by the 
> compositor, which is an intentional design decision.
> The most control I can think of (which may fit your use case), is that a 
> client can ask to be placed fullscreen on a particular output using the 
> fullscreen-shell protocol


that is almost true, I just want to clarify a certain difference.

For desktop systems, the shell protocol extension used (wl_shell,
xdg_wm_base, ...) may offer a method of fullscreening to a specific
output. Aside from fullscreening, clients usually cannot affect where
they will positioned, by design. Proper positioning is the compositor's

Fullscreen shell extension is another beast completely, and mutually
exclusive to the desktop shell extensions. The fullscreen shell is
intended for a single client to take over all outputs, and does not
support multiple simultaneous clients. It also requires explicit
support from clients, and application toolkits usually do not have that.

> > 2. As I have read at multiple places that weston supports multi-display 
> > for drm based devices, is it also possible for fbdev backend, or it has 
> > some functionality constraint that do not let that to work on fbdev?  
> I _think_ the fbdev backend does support multiple outputs. I've never 
> actually used the fbdev backend before, but the code implies that it 
> does. Either way, I would strongly recommend that you use the DRM 
> backend instead.

Right, please do not even consider using the fbdev backend for any new
projects. The only reason it has not been deleted yet is because there
may still be few lingering users. fbdev is in all ways inferior to the
DRM backend:

Let fbdev die in peace, it has been almost dead for so many
years by now.

> > 3. I am actually using weston-3.0.0 version for my project, does it have 
> > multi-display support for any backend? If yes, can someone please help 
> > me findĀ  the commit to the wayland weston project which brought in the 
> > multi-display support?  
> Just to be absolutely sure, are you just referring to having multiple 
> computer monitors ("heads") connected? I believe weston has always had 
> support for that.
> Later versions of weston are capable of cloning a monitor's contents 
> efficiently across multiple monitors, which may potentially be useful 
> for you.

Yes, Weston supports multiple outputs on the same graphics card.
Support for using multiple graphics cards is not there yet.

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