Created attachment 82364 [details]
Go further with "legacy" PCI bus probe on non-seat0 seats when no platform bus devices are found

I have a multiseat setup with a primary on-board Intel graphics device and a secondary Silicon Motion SM501 graphics card. My SM501 is configured via custom xorg.conf with siliconmotion ddx driver, so it exposes no framebuffer devices to e.g. systemd-logind. The same situation is seen e.g. for a NVIDIA graphics card with proprietary drivers.

Every time I configure my system to assign my SM501 card with a secondary seat, i.e. not seat0, my X server fails to start, returning a "No devices detected" error. Nevertheless, when I assign my SM501 card with seat0 and attach my Intel graphics device to secondary seat (via loginctl), both seats load successfully.

Investigating the Xorg-server sources, I've found something related to "platform bus". I don't know what is it at all, but whatever it means, my Intel graphics card with its open-source drivers implement it, but my siliconmotion driver doesn't.

In its current form, Xorg-server probes for platform bus devices as well as "legacy" PCI bus ones only for seat0. For other seats, it only probes for platform bus devices, hence it never reaches my SM501 card, returning that "No devices found" error.

I don't know if there's a good reason for probing only platform bus devices on non-seat0 seats, but this brings a problem for multiseat setups like mine.

I've wrote a small patch that solves my problem, going further with PCI bus probe, while tries to keep the current behavior if a platform bus device is found on secondary seats. Please consider applying it, or giving another solution.

Thanks in advance!

Related bug: https://bugzilla.redhat.com/show_bug.cgi?id=962907