Why does KMS drmModeSetCrtc() fail with permission denied while running in X11 session?

Question

The title already implies the question, so what is blocking the execution from the sample program in this repo https://github.com/dvdhrm/docs/tree/master/drm-howto

It's a simple program showing KMS in action, so I downloaded it, compiled it and gave it a run. This was the output:

michael@lord:~/Downloads/docs/drm-howto$ sudo ./modeset
[sudo] password for michael: 
using card '/dev/dri/card0'
mode for connector 59 is 1920x1080
ignoring unused connector 68
ignoring unused connector 71
ignoring unused connector 75
cannot set CRTC for connector 59 (13): Permission denied
exiting

Also note that I have a running X11 session on my machine. It seems X is already using the device /dev/dri/card0 and blocks it for all other programs.

Here is my video card:

michael@lord:~/Downloads/docs/drm-howto$ lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation UHD Graphics 620 (rev 07)

I have also added i915.modeset=1 into /etc/initramfs-tools/modules and rebuilt it with sudo update-initramfs -u so it loads the driver from inside my kernel and not from the xorg side.

There is also the possibility that my driver doesn't support KMS but lspci has shown that I only have an internal intel graphics card installed. So I sticked with the xorg core display driver package which is installed by default from xserver-xorg-core.

None of these changes have taken affect. The only way I can execute those examples is by killing the xorg session and let it run in tty.

Any ideas?

Holmium

Answer 1

It works as expected. Only DRM-Master is allowed to do modesetting, and there can only be one DRM-Master. X is already master.

You can become master using drmSetMaster() or implicitly by opening the the primary device node when there is no current master. (The example code becomes DRM-Master implicitly.)

See also the drm-kms(7) man page.

Zap

In the example, where/how did you insert the call to drmSetMaster()? I'm trying to run under X. – Enrapture

@RyanMcClue You can't. You need to be master, but X is already master. – Zap

How would a game become master then? Is direct rendering by a video game from a x11/wayland session not a covered use case? oO – Izzy

Great answer! I did not have to stop anything, I just switched to tty1, and during the switch I saw the video mode change and my monitor reinitialize. Then I could just run my master application and it worked (Raspberry OS Bookworm, Wayland). – Sterne

Answer 2

To execute any modeset application first disable the display server by

sudo systemctl stop gdm3

sudo systemctl stop lightdm

switch to command line through eighter CTL + ALT + F1, CTL + ALT + F2, CTL + ALT + F3

and execute $ sudo ./modeset

Eldoneldora

In fact I just used CTL + ALT + F2 and then ran the demo, it worked even without sudo. One thing is that it took me 10 seconds to switch back to X after the demo completed. – Grau