Proposal: Change the default display manager

[ericsagnes]

NixOS version: 15.09.919.b3bba00 (Dingo)

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SLiM has a few issues that make it a suboptimal choice for being the default display manager.

User-unfriendly behavior

if a user wants to enable Xfce, he could think that adding the following to configuration.nix is enough as mentioned in the manual:

  services.xserver.desktopManager.xfce.enable = true;

But SLiM will propose a Xterm session by default.

To start Xfce, the user can:

• push F1 to change to almost invisible "session: xterm" to "session:xfce", not documented on the github mirror (and the official site is down).
• disable xterm so SLiM sees only the xfce session

  services.xserver.desktopManager.xfce.enable = true;
  services.xserver.desktopManager.xterm.enable = false;

It is abandoned

The official homepage is down, its development is abandonned since 2013, there is no available documentation and it is not fully compatible with systemd.

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Proposition

Change NixOS default display manager to something else.

• It should be lightweight, user-friendly and up to date.
• Users desiring to still use SLiM could enable it in configuration.nix.

Also, a mention in services.xserver.displayManager.slim.enable should be added to explain to set services.xserver.desktopManager.xterm.enable = false; if SLiM is not starting the desired desktop manager by default.

Candidates

It is hard to find lightweight, user-friendly and up to date DM, and none of the following candidates feels like a perfect replacement for SLiM (specially regarding being lightweight and the dependencies).
But they sure bring advantages that are worth to considering.
The following list is not definitive and if better alternatives exists they should be considered.

LightDM

homepage

• Cross-desktop - supports different desktop technologies.
• Supports different display technologies (X, Mir, ...).
• Lightweight - low memory usage and fast performance.
• Guest sessions.
• Supports remote login (incoming - XDMCP, VNC, outgoing - XDMCP, pluggable).
• Comprehensive test suite.
• Low code complexity.

Cons:

• The default greeter we have in nix, lightdm-gtk-greeter, has GTK dependencies.
• There are no greeter as lightweight as slim at the moment.

SDDM

homepage

SDDM is a modern display manager for X11 and Wayland aiming to be fast, simple and beautiful. It uses modern technologies like QtQuick, which in turn gives the designer the ability to create smooth, animated user interfaces.

Pros:

• support X11 and wayland
• seems easier to customize than LightDM

Cons:

• has Qt dependencies

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This kind of proposition can easily turn into a debate about personal preferences.
I hope the goal of this proposition, that is improving usability and being nice with first time NixOS users, is clear enough that we can avoid that.

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related issue: #4300

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updates:

• fixed desktop manager / display manager inversions
• fixed lightdm homepage broken link

[vcunat]

Well, yes, slim is a bit problematic.

[jgillich]

Agreed, I think LightDM would be a good default. SDDM, not so much.

[dezgeg]

Well, I don't think we can blame slim for arbitrarily choosing the xterm session, IMHO we should disable the xterm desktop manager by default. But I agree on the other points.

[codyopel]

Just to note, lightdm-gtk-greeter has a hard dependency on X.Org, and only supports cairo compiled with GLX. Cairo can be compiled with GLX, a X.Org specific OpenGL implementation, or GL ES which is a display server agnostic implementation (can't be compiled with both). If you want to support wayland or any display server other than X.Org, lighdm is not the best choice at the moment.

Maybe one of the alternative lightdm greeters doesn't have a dependency on X.Org, but I haven't checked. I am also not sure on lightdm itself; I believe it also has a hard dependency on X.Org.

[heydojo]

From what I have experienced of sddm; I can say that it could be a good candidate. However, I think that maybe making the default display manager selection as of right now is a little pre-mature.
The current live cd offerings are console and kde based and the recommended display manager for kde 5 is sddm. There is no value in changing the kde 4 cd to use anything other than kdm because it just works™ (and there are other areas of nixpkgs which need attention. )
The question of what display manager to use will likely come up if a decision is made to transition to kde 5 by default. At the moment kde 5 looks good, however there are a few small creases still to iron out (such as the bluetooth widget.)

What really concerns me about sddm are bug reports such as sddm/sddm#575 sddm/sddm#551 and sddm/sddm#537
the reports expose that sddm does not yet make enough proper checks when starting and does not error out properly when it falters.

So what I am trying to communicate is that I think it may be a bit too early to come to a decision about sddm as a default display manager because currently it would appear that features which would make it a robust enough default are yet to be added to the code base.

[jgillich]

Slim also depends on Xorg (and I don't see anyone getting rid of Xorg for a few more years anyway), SDDM is just not ready for prime time yet. GDM often doesn't work and is also quite outdated, KDM is dead. That leaves us with LightDM and LXDM; I strongly prefer LightDM because it supports multiple greeters.

[heydojo]

That leaves us with LightDM and LXDM

lxde was forked to lxqt. I think the focus of the lxde people is lxqt and not lxde any more. lxdm also depends on gtk2 iirc.
The recommended display manager for lxqt is sddm.

A display manager which has not been mentioned is https://github.com/linuxmint/mdm which is a fork of gdm by the linux mint people.

[ericsagnes]

I submitted a PR to make lightdm the default display manager as it seemed the best choice at the moment and it is a freedesktop project.
Thanks for all the inputs!

[edolstra]

It's not my experience thrat slim+xfce boots into xterm by default. But maybe somebody broke that recently. In any case, that would be a bug orthogonal to a decision whether to switch to another display manage.

[heydojo]

100% agree with @edolstra. If there is low hanging fruit there which retains the current default then lets address that.
Changing the default dm because of a bug would be a knee jerk reaction.