Multiple DRI card detection in compositor systemd units

[Hoosier, Matt]

I get the intuition behind the suggestion to aggregate using logind seats, as far as it goes. But it still seems to me that this just pushes the question back: how do you identify which card is which in order to assign it to a seat? Normally this is done using udev rules, right? Additionally, I'm working with some constraints that not all of the compositors are logind-aware. I realize that's not really your problem, and it doesn't really influence the merits of your suggestion.

The /dev/dri/by-path idea works, I suppose, if you have different physical graphics cards. In my case, that's not true. These are virtualized cards that the silicon vendor's DRM drivers use to expose different subsets of DRM resources as different cards. So there's only one /dev/dri/by-path card here. Think: DRM leases, but with the lessees popping out as card nodes rather than arranged dynamically using the drm ioctl()'s to manufature leases.

The use-case here is to allow separate DRM domains for each of several containers. It's not really desirable to try to funnel everybody's graphics through a common compositor that runs all the connectors.

Thanks for the thoughts.

-Matt

On 9/22/21, 10:29 AM, "Simon Ser" <contact at emersion.fr> wrote:

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    Maybe try creating multiple physical seats with logind, and start each
    compositor on its own seat? A physical seat is a collection of devices like
    DRM nodes and evdev device files.

    Also udev creates files in /dev/dri/by-path/, these should be stable across
    reboots. `udevadm settle` before a compositor start-up can wait for udev to
    finish its job.

    Out of curiosity, can you explain your use-case? Why do you need to start
    multiple compositors, each on its own GPU?


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