[systemd-devel] Ordering after udev applied rules to `/dev/dri/card0`

[Andrei Borzenkov]

19.03.2020 19:47, Paul Menzel пишет:
> Dear systemd folks,
> 
> 
> I am using Debian Sid/unstable with systemd 245.2 and Weston 8.0.0.
> 
> I amtrying to start a graphical desktop as soon as possible. Currently,
> I use Weston, but unfortunately accessing `/dev/dri/card0` it gets a
> permission denied error. The Weston service unit is ordered after
> `systemd-logind.service`.
> 
> Weston now fails to start, because it gets a permission denied error
> accessing `/dev/dri/card0` [1][2].
> 
>     drmModeSetCrtc(backend->drm.fd, output->crtc_id,
> scanout_state->fb->fb_id, 0, 0, connectors, n_conn, &mode->mode_info);
> 
> Right before Weston starts, the permission and ownership are like below.
> 
>     $ ls -l /dev/dri:
>     insgesamt 0
>     crw------- 1 root root 226,   0 Mär 19 17:29 card0
>     crw------- 1 root root 226, 128 Mär 19 17:29 renderD128
> 
> After udev applied the rules, it looks like below, meaning users in
> group `video` are allowed to access the device.
> 
>     $ ls -l /dev/dri
>     insgesamt 0
>     drwxr-xr-x  2 root root         80 Mär 19 17:29 by-path
>     crw-rw----+ 1 root video  226,   0 Mär 19 17:29 card0
>     crw-rw----+ 1 root render 226, 128 Mär 19 17:29 renderD128
> 
> Is there a way to order a service in such a way, that it’s guaranteed
> that udev rules to devices were applied?
> 

After=device should work. udev announces device after all rules have
been processed.

> A small script applying permissions and ownership manually in
> `ExecStartPre=` seems to work around the (graphics) issue.
> 
> If it cannot be solved with ordering, what would you suggest?
> 
> [1]: https://gitlab.freedesktop.org/wayland/weston/issues/382
> [2]:
> https://gitlab.freedesktop.org/wayland/weston/-/blob/master/libweston/backend-drm/kms.c#L741
> 
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