ddcutils is unable to read/write all i2c devices and unable to control monitors

[TraceyC77]

Please confirm there isn't an open report for this issue

• I have searched through bug reports

Summary

ddcutil does not have proper read/write permissions for i2c devices, which causes the ddcutil utilities to fail. The user is unable to change brightness, for example.

Steps to reproduce

Possibly related: This bug in the ddcutil tracker
nvidia GPU on laptop connected to a dock. 2 monitors connected to the dock

1. Run ddctuil detect

Expected result

Monitors correctly detected and details listed

Actual result

ddcutil detect was unable to get information from the monitors.

❯ ddcutil detect
Invalid display
   I2C bus:  /dev/i2c-9
   DRM connector:           card0-eDP-1
   EDID synopsis:
      Mfg id:               SHP - Sharp Corporation
      Model:                
      Product code:         5399  (0x1517)
      Serial number:        
      Binary serial number: 0 (0x00000000)
      Manufacture year:     2021,  Week: 45
   This is a laptop display.  Laptop displays do not support DDC/CI

Invalid display
   I2C bus:  /dev/i2c-16
   DRM connector:           card1-DP-9
   EDID synopsis:
      Mfg id:               GSM - Goldstar Company Ltd (LG)
      Model:                LG ULTRAGEAR
      Product code:         30538  (0x774a)
      Serial number:        104NTQDH8932
      Binary serial number: 586932 (0x0008f4b4)
      Manufacture year:     2021,  Week: 4
   DDC communication failed. (getvcp of feature x10 returned Error_Info[EIO in ddc_write_read_with_retry, causes: EIO])

Environment

• Is system up to date?

Repo

Unstable

Desktop Environment

Plasma

System details

Operating System: Solus 4.7
KDE Plasma Version: 6.3.0
KDE Frameworks Version: 6.10.0
Qt Version: 6.8.2
Kernel Version: 6.12.12-313.current (64-bit)
Graphics Platform: Wayland
Processors: 16 × 11th Gen Intel® Core™ i7-11800H @ 2.30GHz
Memory: 62.5 GiB of RAM
Graphics Processor 1: Intel® UHD Graphics
Graphics Processor 2: NVIDIA GeForce RTX 3060 Laptop GPU
Manufacturer: Dell Inc.
Product Name: XPS 17 9710

2 DDC capable LG monitors attached to the Dell dock

Other comments

For common use cases, such as controlling brightness on monitors, see
https://commandmasters.com/commands/ddcutil-linux/

I had been trying to test a bug for KDE Plasma related to monitor brightness control
I'm pretty sure this had worked in the past.

Per the DDC FAQ I ran ddcutil environment which suggested the cause was my user not having RW permissions to the ic2 devices:

*** Primary Check 2: Check that /dev/i2c-* exist and writable ***

Current user: tracey (1000)

Checking /dev/i2c-* devices...

Devices /dev/i2c-* must exist and the logged on user must have read/write permission for those devices (or at least those devices associated 
with monitors).

Typically, this access is enabled by:
  - setting the group for /dev/i2c-* to i2c
  - setting group RW permissions for /dev/i2c-*
  - making the current user a member of group i2c

Alternatively, this can be enabled by just giving everyone RW permission
The following tests probe for these conditions.

Checking for /dev/i2c-* devices...
   crw-rw----+ 1 root root 89,  0 Feb 20 08:42 /dev/i2c-0
   crw-rw----+ 1 root root 89,  1 Feb 20 08:42 /dev/i2c-1
   crw-rw----+ 1 root root 89, 10 Feb 20 08:42 /dev/i2c-10
   crw-rw----+ 1 root root 89, 11 Feb 20 08:42 /dev/i2c-11
   crw-rw----+ 1 root root 89, 12 Feb 20 08:42 /dev/i2c-12
   crw-rw----+ 1 root root 89, 13 Feb 20 08:42 /dev/i2c-13
   crw-rw----+ 1 root root 89, 14 Feb 20 08:42 /dev/i2c-14
   crw-rw----+ 1 root root 89, 15 Feb 20 08:42 /dev/i2c-15
   crw-rw----+ 1 root root 89, 16 Feb 20 08:42 /dev/i2c-16
   crw-rw----+ 1 root root 89, 17 Feb 20 08:42 /dev/i2c-17
   crw-rw----+ 1 root root 89, 18 Feb 20 08:42 /dev/i2c-18
   crw-------  1 root root 89, 19 Feb 20 08:42 /dev/i2c-19
   crw-rw----+ 1 root root 89,  2 Feb 20 08:42 /dev/i2c-2
   crw-------  1 root root 89, 20 Feb 20 08:42 /dev/i2c-20
   crw-------  1 root root 89, 21 Feb 20 08:42 /dev/i2c-21
   crw-rw----+ 1 root root 89,  3 Feb 20 08:42 /dev/i2c-3
   crw-rw----+ 1 root root 89,  4 Feb 20 08:42 /dev/i2c-4
   crw-rw----+ 1 root root 89,  5 Feb 20 08:42 /dev/i2c-5
   crw-rw----+ 1 root root 89,  6 Feb 20 08:42 /dev/i2c-6
   crw-rw----+ 1 root root 89,  7 Feb 20 08:42 /dev/i2c-7
   crw-rw----+ 1 root root 89,  8 Feb 20 08:42 /dev/i2c-8
   crw-rw----+ 1 root root 89,  9 Feb 20 08:42 /dev/i2c-9
Device /dev/i2c-19 is not readable and writable.  Error = EACCES(13): Permission denied
Device /dev/i2c-20 is not readable and writable.  Error = EACCES(13): Permission denied
Device /dev/i2c-21 is not readable and writable.  Error = EACCES(13): Permission denied

WARNING: Current user (tracey) does not have RW access to all /dev/i2c-* devices.

....
Configuration suggestions:
   Issue:
      Current user has RW access to some but not all /dev/i2c-N devices.
      If there is RW access to at least the /dev/i2c-N devices for connected monitors,
      this is not a problem.
      Remaining suggestions assume RW access is still to be established.
      
   Issue:
      Group i2c does not exist.
   Suggestion:
      Create group i2c. To create group i2c, use command:
         sudo groupadd --system i2c
      Assign /dev/i2c-N devices to group i2c by adding a rule to /etc/udev/rules.d
      Add the current user to group i2c:
         sudo usermod -G i2c -a <username>
      After this, you will have to logout and login again.
      The changes to the user's group list are not read until a new login.

[TraceyC77]

I found these, which are properly installed by the ddcutils package

❯ fd rules | rg ddc
usr/lib64/udev/rules.d/60-ddcutil-usb.rules
usr/lib64/udev/rules.d/60-ddcutil-i2c.rules
usr/share/ddcutil/data/60-ddcutil-usb.rules
usr/share/ddcutil/data/60-ddcutil-i2c.rules

[TraceyC77]

There's been a new major release of ddcutil. I'll see if I can update the package and if that fixes this issue