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I have a Raspberry Pi 4/Raspbian and am now attempting to open /dev/dri/renderD128 for talking to V3D DRM. This fails with EACCESS.

Looking into the filesystem:

$ ls -l /dev/dri/renderD128 
crw-rw---- 1 root render 226, 128 Aug  1 23:17 /dev/dri/renderD128
$ groups
pi adm dialout cdrom sudo audio video plugdev games users input netdev gpio i2c spi

This seems correct, since pi is not a user of render.

Reading up on this it seems that I should magically (this seems largely undocumented) get access to renderD128 through a mechanism called UACCESS.

The rules contain:

SUBSYSTEM=="drm", KERNEL=="renderD*", TAG+="uaccess"

so I assume, this should be configured correctly!?

Could anyone explain how this should work or how I can further debug that UACCESS mechanism?

  • It's different because whoever made your RaspPi distribution (you didn't tell us which one you were using) decided to make it this way. Any reason you can't make pi a member of the group render? – dirkt Aug 4 '19 at 20:47
  • Sorry, added the Raspbian. It feels weird to add pi to group render, when it isn't in the list of default groups for users. Still wondering, what the philosophy around this render group is. – abergmeier Aug 4 '19 at 21:13
  • Found some more details: salsa.debian.org/systemd-team/systemd/commit/… – abergmeier Aug 5 '19 at 6:28
  • Ah, it's systemd, so you should use their "uaccess" method (however this works, I haven't used it myself) if you don't what to add a user to a group. (And there's nothing strange in adding users to groups, it's a very basic sysadmin operation). – dirkt Aug 5 '19 at 11:25

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