Dual display in clone mode or extended mode with Weston

[Konstantin Kharlamov]

On 21.06.2018 21:44, Matheus Santana wrote:
> 
> 
> On Thu, Jun 21, 2018 at 5:11 AM, Pekka Paalanen <ppaalanen at gmail.com 
> <mailto:ppaalanen at gmail.com>> wrote:
> 
>     On Wed, 13 Jun 2018 17:40:59 +0530
>     Ashvini Deshmukh <ashvini2781 at gmail.com
>     <mailto:ashvini2781 at gmail.com>> wrote:
> 
>     > Hello All,
>     > 
>     > I have read queries for dual display on freedesktop.org <http://freedesktop.org>
>     > 
>     > I need your help in the same context.
>     > 
>     > Currently we are creating one application to support multiple displays with
>     > Wayland.
> 
>     Hi,
> 
>     you are developing an application, ok. I assume that means a Wayland
>     client specifically.
> 
>     > We are unaware that one compositor will be sufficient for dual display.
> 
>     Sorry, I don't understand. Are you asking whether one Wayland
>     compositor could driver multiple displays? Yes, they can in general.
>     Capabilities will vary between different compositor implementations.
> 
>     > We need to know about how virtual framebuffer is created per display.
> 
>     As an application developer, why would you care about that? That is a
>     compositor internal implementation detail.
> 
>     Why virtual? Real outputs do not have virtual framebuffers, they have
>     real framebuffers as far as the compositor is concerned.
> 
>     > As DRM supports only one compositor,
>     > How to display same content on second display OR can we have different user
>     > events on second display monitor.
> 
>     What do you mean?
> 
>     If a Wayland compositor supports and has been configured to show the
>     same content on multiple displays, then it will do that. From a Wayland
>     client perspective, there is nothing you need to do to have your
>     window show up on cloned displays compared to a single display case.
> 
>     By user events, do you mean input events?
> 
>     It is certainly possible to write a compositor that dedicates one set
>     of input devices for one display and another set of input devices for
>     the other display.
> 
>     Applications are expected to support multiple wl_seat globals (similar
>     to multi-pointer X in essence). There is nothing else they would need
>     to specifically support for a compositor that had multiple outputs,
>     cloned outputs, or divided input devices in any arbitrary way.
> 
>     I did not understand your requirements well enough to say how well
>     Weston would work for you.
> 
>     For example, Weston currently does not support multiple KMS devices,
>     but it does support multiple displays on a single KMS device. Support
>     for multiple KMS devices is desired in Weston though, so maybe it will
>     in the future.
> 
> 
> I'm curious about what you specifically mean by KMS device. Each 
> graphics card?
> 
> I've just tried Weston with two displays and it worked out on the fly 
> ("extended mode").
> 
> 
>     Weston's clone mode is currently limited to sharing a CRTC between all
>     displays, assuming someone reviews the final patch needed to configure
>     it. Support for this configuration does not seem to be common among PC
>     graphics hardware, embedded boards may have better chances.
> 
> 
> What does CRTC stand for?

In this context, AFAIR, "CRTC" means a device that scans out an image 
from a buffer to a display. The acronym is historical, stands for 
"cathode ray tube controller".