simpledrm, running display servers, and drivers replacing simpledrm while the display server is running

[Jani Nikula]

On Thu, 09 May 2024, nerdopolis <bluescreen_avenger at verizon.net> wrote:
> Hi
>
> So I have been made aware of an apparent race condition of some drivers taking a bit longer to load, which could lead to a possible race condition of display servers/greeters using the simpledrm device, and then experiencing problems once the real driver loads, the simpledrm device that the display servers are using as their primary GPU goes away. 
>
> For example Weston crashes, Xorg crashes, wlroots seems to stay running, but doesn't draw anything on the screen, kwin aborts, 
> This is if you boot on a QEMU machine with the virtio card, with modprobe.blacklist=virtio_gpu, and then, when the display server is running, run sudo modprobe virtio-gpu
>
> Namely, it's been recently reported here: https://github.com/sddm/sddm/issues/1917[1] and here https://github.com/systemd/systemd/issues/32509[2]
>
> My thinking: Instead of simpledrm's /dev/dri/card0 device going away when the real driver loads, is it possible for simpledrm to instead simulate an unplug of the fake display/CRTC?
> That way in theory, the simpledrm device will now be useless for drawing for drawing to the screen at that point, since the real driver is now taken over, but this way here, at least the display server doesn't lose its handles to the /dev/dri/card0 device, (and then maybe only remove itself once the final handle to it closes?)
>
> Is something like this possible to do with the way simpledrm works with the low level video memory? Or is this not possible?

Related [1][2].

BR,
Jani.


[1] https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/10133
[2] https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11158


>
> Thanks
>
> --------
> [1] https://github.com/sddm/sddm/issues/1917
> [2] https://github.com/systemd/systemd/issues/32509

-- 
Jani Nikula, Intel