Blueprint changed by Richard Hansen: Whiteboard changed: - * Introduction - The protocol for interacting with logind in a multiseat environment is described at [1]. I define "basic multiseat support" to be everything documented there except steps 1 and 2 of "complete porting". Also see "Multiseat on Linux" [2]. [1] http://www.freedesktop.org/wiki/Software/systemd/writing-display-managers/ [2] http://www.freedesktop.org/wiki/Software/systemd/multiseat/ - ** Related future work + = Background = + + See: https://wiki.ubuntu.com/Multiseat + + = Related future work = The following are not considered to be part of this blueprint. They should be tackled separately. - - Steps 1 and 2 of "complete porting" in [1]. Those two steps are - required for automatic multiseat support (dynamically spawning and - terminating greeters as seats come and go). See bug #1190581. + - Steps 1 and 2 of "complete porting" in [1]. Those two steps are + required for automatic multiseat support (dynamically spawning and + terminating greeters as seats come and go). See bug #1190581. - - A tool to make it easier for users to assign devices to seats via - udev rules. - [EDIT Laércio de Sousa] What about loginctl (part of systemd)? We can do something like "loginctl attach seatXXX /sys/devices/path/of/device" to assign a device to a seat. We can also see the list of assigned devices with command "loginctl seat-status seatXXXX". Is loginctl currently packaged in Ubuntu? + - A tool to make it easier for users to assign devices to seats via + udev rules. - - Fixing X so that the logind multiseat wrapper [3] can be replaced - with appropriate command-line arguments. + Note that systemd's loginctl utility, available in Ubuntu since + 13.04 raring, has an "attach" feature that is intended to make it + easier to assign devices to seats. I have not tested this feature + to see how or even if it works. Getting this utility to work with + Ubuntu, or improving its functionality, is out of scope for this + blueprint. + + - Fixing X so that the logind multiseat wrapper [3] can be replaced + with appropriate command-line arguments. [3] http://cgit.freedesktop.org/systemd/systemd/tree/src/login/multi- seat-x.c?id=v206 - ** Terminology + = Requirements = - - display server: an instance of X, Mir, a Wayland compositor, etc. + - follow steps 1-4 of "minimal porting" and steps 3-4 of "complete + porting" + - support X, Mir, Wayland compositors, etc. - For other terms, see [4]. + = Work Item descriptions = - [4] - http://www.freedesktop.org/wiki/Software/systemd/multiseat/#definitionofterms - - ** Requirements - - - follow steps 1-4 of "minimal porting" and steps 3-4 of "complete - porting" - - support X, Mir, Wayland compositors, etc. - - ** Mir and Wayland - - I am not very familiar with alternative display servers such as Mir and - Wayland, but I believe their designs are similar enough to X with regard - to multiseat that all of the concepts in this blueprint also apply. - Thus, I will try to use the term "display server" instead of "X" when - the core idea is broadly applicable. - - * Background: Multiseat and Virtual Terminals - - ** Associating with a VT - - A display server can associate itself with a VT. For example, by - default X will discover an unused VT at startup and switch to it. When - it exits, it switches back. - - When the display server is associated with a VT, it will allow the user - to switch to another VT at any time (e.g., by pressing a Ctrl-Alt-F? key - sequence). When the user switches away, the display server releases - control over the input and video devices so that they can be used by - another virtual terminal. When the user switches back, the display - server re-acquires control over the devices. - - Note that a different display server could be running concurrently on - another virtual terminal. When the user switches to the other display - server, the virtual terminal system acts as a mediator, coordinating the - handoff of control over the input and video devices. This is how fast - user switching is currently implemented. - - ** Running independent of a VT - - A display server can also be invoked in such a way that it does not - associate itself with a VT. It simply acquires control over some or all - input and video devices. Because the display server is not associated - with a virtual terminal, fast user switching is not supported -- there - is nothing mediating the release and acquisition of control over the - devices. - - When a display server is not associated with a VT, it must not: - - probe to see what the current VT is - - attempt to switch VTs - - let a VT see the seat's input (keyboard, mouse) events - - Note that any devices not controlled by the display server would still - be available for use by the Linux virtual terminal system. - - ** VT limitations - - The Linux virtual terminal system only supports a single seat. This - makes configuring a multiseat X/Mir/whatever system difficult because - only one VT can be active at any given time. If one seat's display - server is running on vt7 and another seat's display server is running on - vt8, then only one of these two seats can be used at a time. To use the - other seat, a VT switch must be initiated. - - To make all seats usable at the same time, there are a few options: - - - Associate all display servers with the same VT. If any user at - any seat switches VTs, all seats will switch to the new VT. This - makes VT switching (and thus fast user switching) impractical. - - - Don't associate any display server with a VT. Fast user switching - is impossible in this case. Text-based console logins are only - possible if an input and display device are reserved for this - purpose. - - - Associate only one of the display servers with a VT. The other - display servers can't do VT switching, but the display server - associated with a VT can. VT switching on that one seat won't - affect the other seats. This is the approach favored and assumed - by systemd, and will be the approach taken by LightDM. - - If/when kmscon [5] happens, the Linux kernel virtual terminal - limitations will go away. kmscon will support multiple seats, which - will make it possible to support fast user switching on each seat. (The - system administrator will have to be careful to configure kmscon and the - display manager to agree on the definition of the seat in terms of - hardware devices.) - - [5] http://www.freedesktop.org/wiki/Software/kmscon/ - - * X specifics - - ** Associating with a VT - - To run X associated with a VT, no special arguments or configuration - settings are needed. Some useful arguments are: - - - 'vtXX' where XX is a virtual terminal number. This is used to - force X to use a particular VT, rather than try to discover an - unused VT. This argument is useful in combination with Plymouth, - where smooth handoff of the video device is desired. - - - '-novtswitch' prevents X from doing any VT switching on startup - and shutdown. LightDM passes this option to X because it does its - own VT switching (to facilitate smooth handoff from Plymouth to - X). - - ** Without a VT - - To run X without an associated VT: - - - the '-sharevts' argument is used - - - the '-novtswitch' is used (this might not be required) - - - the following configuration snippet is used to prevent X from - attempting any VT switches: - - Section "ServerFlags" - Option "DontVTSwitch" "True" - EndSection - - - the following configuration snippet is used to prevent input - events from going to a virtual terminal (see the evdev(4) man - page): - - Section "InputClass" - Identifier "prevent input events from going to the console" - Option "GrabDevice" "True" - EndSection - - The above may not be sufficient; a code audit should be done to ensure - that the above settings are enough to guarantee that X does not do - anything related to virtual terminals. - - ** Device selection - - When given the '-seat' argument, X will only use devices that have been - "tagged" (via udev rules) with the given name. All other devices are - off limits. This provides a convenient way to tell X which subset of - input and display devices it is permitted to use. See [6] for more - information. - - [6] - http://www.freedesktop.org/wiki/Software/systemd/multiseat/#udevrules - - * logind specifics - - - seat0 is the name of the "primary" seat. It always exists. Any - seat-specific hardware device not specifically designated to - another seat belongs to seat0. - - - As documented at [6], each seat must have a device tagged (via - udev) "master-of-seat". Without a device with this tag, logind - assumes the seat is not ready yet. When a master-of-seat device - appears, logind declares the seat available, signaling the display - manager that the seat is ready for a display server. This tag - should be applied only to the seat's video device so that the - video device is guaranteed to be ready when the display manager - launches the display server. - - * Work Item descriptions - - ** Package systemd-multi-seat-x + == Package systemd-multi-seat-x == See bug #1214146. - ** New setting: xdg-seat=<name> + == New setting: xdg-seat=<name> == The xdg-seat=<name> seat setting tells LightDM the name of the seat. The name defaults to "seat0". This name is used: - - in the XDG_SEAT variable (for PAM) - - in the -seat argument to X (if the seat is an X seat and not Mir - or something else) - - to determine how to handle VTs + - in the XDG_SEAT variable (for PAM) + - in the -seat argument to X (if the seat is an X seat and not Mir + or something else) + - to determine how to handle VTs If the name is unset, set to the empty string, or set to seat0, then: - - LightDM switches to an automatically chosen VT - - 'vtXX' is passed to X (if the seat is an X seat) where XX is the - number of the chosen VT - - '-seat seat0' is passed to X (if the seat is an X seat) + - LightDM switches to an automatically chosen VT + - 'vtXX' is passed to X (if the seat is an X seat) where XX is the + number of the chosen VT + - '-seat seat0' is passed to X (if the seat is an X seat) If the name is set to something other than seat0, then: - - LightDM does NOT do any VT switching - - 'vtXX' is NOT passed to X (if the seat is an X seat) - - '-seat <name>' is passed to X (if the seat is an X seat) - - LightDM creates a temporary xorg config file with the following - contents: + - LightDM does NOT do any VT switching + - 'vtXX' is NOT passed to X (if the seat is an X seat) + - '-seat <name>' is passed to X (if the seat is an X seat) + - LightDM creates a temporary xorg config file with the following + contents: - Section "ServerFlags" - Option "DontVTSwitch" "True" - EndSection - Section "InputClass" - Identifier "prevent input events from going to the console" - Option "GrabDevice" "True" - EndSection + Section "ServerFlags" + Option "DontVTSwitch" "True" + EndSection + Section "InputClass" + Identifier "prevent input events from going to the console" + Option "GrabDevice" "True" + EndSection - - '-config /path/to/above/config/file -sharevts' is passed to X (if - the seat is an X seat) + - '-config /path/to/above/config/file -sharevts' is passed to X (if + the seat is an X seat) If lightdm.conf defines two or more seats with the same name, an error message is logged and the second and subsequent seats with that name are ignored. - ** New setting: use-vt=auto|true|false|<integer> + == New setting: use-vt=auto|true|false|<integer> == (this is not strictly required, but would be a nice feature in case anyone needs to control VT associations) The use-vt=* seat setting tells LightDM which VT to associate with the display server (if any). - - use-vt=true: Switch to an automatically chosen VT, pass vtXX to X, - and do not pass -sharevts to X. - - use-vt=false: Do not switch VTs, do not pass vtXX to X, do pass - -sharevts to X, and generate a X config with "DontVTSwitch" and - "GrabDevice" enabled. - - use-vt=auto (default): If the name of the seat is seat0, this is - the same as use-vt=true. Otherwise (not seat0), this is the same - as use-vt=false. - - use-vt=<integer>: Same as use-vt=true, but use the specified VT. + - use-vt=true: Switch to an automatically chosen VT, pass vtXX to X, + and do not pass -sharevts to X. + - use-vt=false: Do not switch VTs, do not pass vtXX to X, do pass + -sharevts to X, and generate a X config with "DontVTSwitch" and + "GrabDevice" enabled. + - use-vt=auto (default): If the name of the seat is seat0, this is + the same as use-vt=true. Otherwise (not seat0), this is the same + as use-vt=false. + - use-vt=<integer>: Same as use-vt=true, but use the specified VT. - ** Write test cases + == Write test cases == - ** Set can_switch appropriately + == Set can_switch appropriately == - - call set_seat_can_switch() with the value of the - org.freedesktop.login1.Seat.CanMultiSession dbus property - - bug that needs to be figured out: If can_switch is true on a - seat, then when you log out LightDM won't respawn a greeter on - that seat. + - call set_seat_can_switch() with the value of the + org.freedesktop.login1.Seat.CanMultiSession dbus property + - bug that needs to be figured out: If can_switch is true on a + seat, then when you log out LightDM won't respawn a greeter on + that seat.
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