Hi
Am 09.05.24 um 15:06 schrieb nerdopolis:
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 and here
https://github.com/systemd/systemd/issues/32509
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?
To my knowledge, there's no hotplugging for CRTCs.
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?
Userspace needs to be prepared that graphics devices can do hotplugging.
The correct solution is to make compositors work without graphics devices.
The next best solution is to keep the final DRM device open until a new
one shows up. All DRM graphics drivers with hotplugging support are
required to accept commands after their hardware has been unplugged.
They simply won't display anything.
Best regards
Thomas
Thanks
--
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Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
