libddcutil behaves differently/fails after a hotplug compared to ddcutil

[digitaltrails]

I did a hotplug of the Beng BegQ T2200HD, after which libddcutil correctly detected the new monitor, but an attempt to use libddcutil getvcp failed returning DDCRC_INTERNAL_ERROR. The command-line ddcutil worked fine, both with detect and setvcp.

I traced this back to src/ddc/ddc_displays.c inside

ddc_validate_display_ref(Display_Ref * dref, bool basic_only, bool 
require_not_asleep)

Where it appear that the drm_connector was NULL:

else if (drm_enabled && !basic_only) {
              if (!dref->drm_connector)    <----------------------- NULL
                  ddcrc = DDCRC_INTERNAL_ERROR;

Suspecting the driver had not initialised some aspect of drm, I then hacked the call to ddc_validate_display_ref to force basic_only, and then the setvcp worked.

I then rebooted figuring drm would then be correctly initialised. Sure enough the basic_only hack was no longer required, a getvcp via libddcutil worked just fine.

I don't think you can trust the drm structures if the Nvidia driver is in play. I suppose this could be covered off with documentation. Or maybe an option to completely switch off drm.

[rockowitz]

This looks like another instance of stale value in /sys/class/drm//edid, causing dref->drm_connector not to be set. Probably simplest to use ddcutil environment --verbose to confirm. Execute the command after a hotplug and also when the system is started with the monitor plugged in.

[digitaltrails]

Attachments: 1) Already connected and nvidia-settings enabled ddcutil environment --verbose 2>&1 | tee boot-nvidia-config.log

boot-nvidia-config.log

2) Reboot and hotplug, enable Beng via nvidia-settings then ddcutil environment --verbose 2>&1 | tee hotplug-nvidia-config.log

hotplug-nvidia-config.log

sdiff boot-nvidia-config.log hotplug-nvidia-config.log |less

...
   Display 2                                                       Display 2
      I2C bus:  /dev/i2c-4                                            I2C bus:  /dev/i2c-4
         DRM connector:                         card1-DP-2    <
         /sys/class/drm/card1-DP-2/dpms         On            <
         /sys/class/drm/card1-DP-2/enabled      disabled      <
         /sys/class/drm/card1-DP-2/status       connected     <
         Driver:                                Unknown                  Driver:                                Unknown
         I2C address 0x50 (EDID) responsive:    true                     I2C address 0x50 (EDID) responsive:    true 
         I2C address 0x37 (DDC)  responsive:    true                     I2C address 0x37 (DDC)  responsive:    true 
         Is LVDS or EDP display:                false                    Is LVDS or EDP display:                false
         Is laptop display by EDID:             false                    Is laptop display by EDID:             false
         Is laptop display:                     false                    Is laptop display:                     false
         /sys/bus/i2c/devices/i2c-4/name        NVIDIA i2c ad            /sys/bus/i2c/devices/i2c-4/name        NVIDIA i2c ad
         PCI device path:                       /sys/devices/            PCI device path:                       /sys/devices/
      EDID synopsis:                                                  EDID synopsis:
         Mfg id:               BNQ - UNK                                 Mfg id:               BNQ - UNK
         Model:                BenQ T2200HD                              Model:                BenQ T2200HD
         Product code:         30502  (0x7726)                           Product code:         30502  (0x7726)
         Serial number:        2C803527SL0                               Serial number:        2C803527SL0
...