On Thu, 09 May 2024 09:06:29 -0400 nerdopolis <bluescreen_avenger@xxxxxxxxxxx> 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? Hi, what you describe sounds similar to what has been agreed that drivers should implement: https://docs.kernel.org/gpu/drm-uapi.html#device-hot-unplug That would be the first step. Then display servers would need fixing to handle the hot-unplug. Then they would need to handle hot-plug of the new DRM devices and ideally migrate to GPU accelerated compositing in order to support GPU accelerated applications. Simpledrm is not a GPU driver, and I assume that in the case you describe, the GPU driver comes up later, just like the hardware-specific display driver. Any userspace that initialized with simpledrm will be using software rendering. Ideally if a hardware rendering GPU driver turns up later and is usable with the displays, userspace would migrate to that. Essentially this is a display/GPU device switch. In general that's a big problem, needing all applications to be able to handle a GPU disappearing and another GPU appearing, and not die in between. For the simpledrm case it is easier, because the migration is from no GPU to a maybe GPU. So applications like Wayland clients could stay alive as-is, they just don't use a GPU until they restart. The problem is making display servers handle this switch of display devices and a GPU hotplug. Theoretically I believe it is doable. E.g. Weston used to be able to migrate from pixman-renderer to GL-renderer, but I suspect it is lacking support for hot-unplug of the "main" DRM display device. Thanks, pq > Thanks > > -------- > [1] https://github.com/sddm/sddm/issues/1917 > [2] https://github.com/systemd/systemd/issues/32509
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