Friday, 14 June 2013

Re: non-Unity flavours and Mir

I think most of the points have been covered in this thread but I'll just emphasise a few points:

- The use of Mir in Ubuntu should have no effect on using any alternative display systems. Some things will gain Mir backends but everything will continue to support X and Wayland if the upstreams support it and developers package and support it in Ubuntu.

- There are a number of different levels of co-existence with Unity/Mir that are possible:

a) Unity on X and KDE on X - the current case. You can install both and choose a session from a greeter

b) Unity on XMir and KDE on XMir using Mir as a system compositor [1] - this is the case you can test from the PPA [2] today. Looks and behaves much the same as a).

c) Unity on Mir and KDE on XMir using Mir as a system compositor - this is the case we're working towards. For KDE it will be the same as b).

d) Unity on Mir and KDE on Mir using Mir as a system compositor - IF KWM was able (have not seen any technical reason this can't be done) and wanted to create a Mir backend this would work like b) (see discussion later in this thread).

e) Unity on Mir and KDE on Wayland using Mir as a system compositor - IF Wayland is able (have not seen any technical reason this can't be done) and gets a Mir backend this will work IF LightDM support is added like b) (see below).

f) Unity on Mir using Mir as a system compositor and KDE on Wayland using Wayland as a system compositor - Would not be able to switch between sessions. Would either need to switch LightDM configuration or display managers.

g) Unity on Mir and KDE on Wayland using Wayland as a system compositor - Would be technically possible but the Mir team has no plans to add a Wayland backend. Would not be shipped as a default so would not be well tested by Canonical.

- Canonical will need to support XMir for the foreseeable future. XMir is just with a Mir backend. This means supporting XMir will mean supporting most of the case of running in the traditional sense.

- LightDM will continue to support the traditional VT switched X display. We have extensive regression tests for this case and it is used by a number of other desktops.

- While Canonical is not going to add Wayland support to LightDM the work we are doing to support Mir in LightDM is much the same as would be required to support Wayland in LightDM. Anyone is welcome to implement this. Note it's not clear to me at the moment how different desktops plan to use Wayland (no system compositor, shared system compositor, differing system compositors, same/differing Wayland socket naming conventions). I'll leave it to Kubuntu/KDE to decide the value in sharing the rest of the display manager codebase.


[1] The "system compositor" is a display server that switches between running sessions. The purpose of this is to control the graphics during the whole boot / session lifetime. It replaces handing over between display servers and VT switching.

Here's a discussion I half started as part of vUDS.

The switch to Mir in Ubuntu seems pretty risky for the existance of
Kubuntu, I wonder if other flavours have the same probable problem.

KWin dev has opinions on the subject
From the architecture section on that blog post:

 "Mir's architecture is centered around Unity. It is difficult to really
 understand the architecture of Mir as the specification is so full of
 buzz-words that I don't understand it [5]. From all I can see and
 understand Unity Next is a combination of window manager and desktop
 shell implemented on top of Mir. How exactly this is going to look
 like I do not know. Anyway it does not fit our design of having
 desktop shell and window manager separated and we do not know whether
 Mir would support that. We also do not know whether Mir would allow
 any other desktop shell except Unity Next, given that this is the main
 target. Wayland on the other hand is designed to have more than one
 compositor implementations. Using KWin as a session compositor is an
 example in the spec."

and on protocol

 "But it gets worse, the protocol between Mir server and Mir clients
 is defined as not being stable. In fact it's promised that it will
 break. That's a huge problem, I would even call it a showstopper....
 Given that the protocol may change any time and given that the whole
 thing is developed for the needs of Unity we have to expect that the
 server libraries are not binary compatible or that old version of the
 server libraries cannot talk with the latest client libraries"

Canonical was going to port LightDM to Wayland but now does not plan
to so someone else would have to do this.  KDE might be interested
but more likely will switch to SDDM.

For Kubuntu the options are:
- Use Mir - infeasable as upstream can't support it as described above
- Use Wayland with packages from Debian and hope we can make those packages
  live with Mir as best as possible
- End of Kubuntu

The second options is the one I'm expecting.  It's completely unknown
how much it means Kubuntu and other flavours will need to maintain X
and Wayland packages, hopefully not much (it's hardly our speciality)
and hopefully Debian and Ubuntu Desktop will support it enough.

I don't think there's a public timeline for Mir so we don't know when
this will hit us, presumably in the next year.

Other flavours I think are this:
Mythbuntu: not evaluated, hope to do so once NVideo and AMD provide drivers
Lubuntu: not evaluated, hope to use X and GTK
ubuntustudio: I've heard both that they use xfce based on xubuntu and
will follow them, and "aiming for users to choose whatever desktop
environment they want"

Any other flavours got an opinions?

Are there any misconceptions I have in the above?


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