P2421DC, MST, Source Input Autoselect

Question

InformationMonitor_1: P2421DC, Firmware M3C103Monitor_2: P2421D,   Firmware M2C102ConfigurationSource 1: [Desktop PC MST Compliant]   DP Out-> Monitor_1 DP In -> Monitor_1 DP Out -> Monitor_2 DP InSource 2: [Precision 5520 MST Compliant] USB-C Out -> Monitor_1 USB-C In -> Monitor_1 DP Out -> Monitor_2 DP In *When MST is enabled all displays are functioning as expected in the daisy chain.  Problem Source Input Auto Select is not functioning correctly in MST Mode. Troubleshooting Steps Case 1: MST = DISABLED, Auto Select = ON, Auto Select for USB-C = Prompt for Multiple Displays To test whether or not source input auto select was working at a base level, I disabled MST. Monitor 2 was just duplicating but that was besides the point for the test case.  Auto select worked as expected for all tested scenarios. Test 1: Monitor 1 DP Selected. Source 1 and 2 asleep. Source 2 (USB-C) woken. Monitor 1 switched to USB-C [SUCCESS] Test 2: Monitor 1 USB-C Selected. Source 1 and 2 asleep. Source 1 (DP) woken. Monitor 1 switched to DP. [SUCCESS] Test 3: Monitor 1 DP Selected. Source 1 and 2 awake. Source 1 (DP) put to sleep. Monitor 1 switches to USB-C [SUCCESS] Test 4: Monitor 1 USB-C Selected. Source 1 and 2 awake. Source 2 (USB-C) put to sleep. Monitor 1 switches to DP [SUCCESS] Case 2: MST = ENABLED, Auto Select = ON, Auto Select for USB-C = Prompt for Multiple Displays Test 1: Monitor 1 DP Selected. Source 1 and 2 asleep. Source 2 (USB-C) woken. Monitor 1 switched to USB-C [SUCCESS] Test 2: Monitor 1 USB-C Selected. Source 1 and 2 asleep. Source 1 (DP) woken. Monitor 1 displays 'No Source for USB-C'. [FAIL] Test 3: Monitor 1 DP Selected. Source 1 and 2 awake. Source 1 (DP) put to sleep. Monitor 1 switches to USB-C [SUCCESS] Test 4: Monitor 1 USB-C Selected. Source 1 and 2 awake. Source 2 (USB-C) put to sleep. Monitor 1 displays 'No Source for USB-C'. [FAIL] Case 3: MST = ENABLED, Auto Select = ON, Auto Select for USB-C = NO Test 1: Monitor 1 DP Selected. Source 1 and 2 asleep. Source 2 (USB-C) woken. Monitor 1 displays 'No Source for DP' [FAIL] Test 2: Monitor 1 USB-C Selected. Source 1 and 2 asleep. Source 1 (DP) woken. Monitor 1 displays 'No Source for USB-C'. [FAIL] Test 3: Monitor 1 DP Selected. Source 1 and 2 awake. Source 1 (DP) put to sleep. Monitor 1 displays 'No Source for DP' [FAIL] Test 4: Monitor 1 USB-C Selected. Source 1 and 2 awake. Source 2 (USB-C) put to sleep. Monitor 1 displays 'No Source for USB-C'. [FAIL] Case 4: MST = ENABLED, Auto Select = ON, Auto Select for USB-C = YES Test 1: Monitor 1 DP Selected. Source 1 and 2 asleep. Source 2 (USB-C) woken. Monitor 1 switched to USB-C [SUCCESS] Test 2: Monitor 1 USB-C Selected. Source 1 and 2 asleep. Source 1 (DP) woken. Monitor 1 displays 'No Source for USB-C'. [FAIL] Test 3: Monitor 1 DP Selected. Source 1 and 2 awake. Source 1 (DP) put to sleep. Monitor 1 switches to USB-C [SUCCESS] Test 4: Monitor 1 USB-C Selected. Source 1 and 2 awake. Source 2 (USB-C) put to sleep. Monitor 1 displays 'No Source for USB-C'. [FAIL] Thoughts It seems I may have discovered a firmware bug. Others have stated for other monitor models that Auto Select is disabled when MST is enabled. This makes sense to protect for inputs that are not MST compliant like HDMI or VGA. For the P2421DC, Auto Select is not disabled or greyed out when MST is enabled. My desire is for Aut0 Select to function for the DP and USB-C when MST is enabled and Auto Select for USB-C is set to NO. Any help would be greatly appreciated! Also sorry for the long winded post.

Hemmingway · Answer

Diagram included for clarity.

Hemmingway · Answer

This is the response I received when I submitted a support ticket:

The P2421DC is working as designed. For previous models, when user enables MST on, the auto-select will be grey out. For P2421DC model they are transition models. Due to it only add cable detection, when user plug the video cable into the platform. It will change the video output accordingly. Kind of partial auto select. But if the user enables MST On with cable remain plugged and go to sleep. This won’t work. This model, P2421DC will be EOL by next Feb. 2022. There will not be a firmware update supporting this feature as it may cause some other compatibility issues to appear.

In our testing we noticed the U2421E worked better on auto-select, so if still available for purchase then the customer can buy one to test. There may be other Dell models that work but IPS did not test all models for this investigation.

My response:

After waiting for more than three weeks this is very disappointing feedback. I disagree that the P2421DC is working as designed. If Auto-Select was not intended in MST mode it should have been greyed out. As you know it is not and partially works based on cable detection. As you say, this monitor is EOL in Feb 2022. This means that it is still available for product support which should include bug identification and full firmware support. The comment that supporting this feature might cause other problems is not a good excuse. This is what validation testing is for.

@DELL-Chris M do you have any thoughts or internal pull on this topic?

DELL-Chris M · Answer

The response you received is directly from the Displays team to the Support teams. We cannot counter their decision.

Hemmingway · Answer

Update I have acquired the next model year P2422HE, which product support recommended as working. After plugging everything in with the same setup above as Monitor 1, MST mode and Auto-Select work completely in all scenarios. This would be acceptable, except that somehow the next model years is 1080p, where the P2421DC is 1440p. Hence this is not a viable solution for my use case. It is still very disappointing that the display team will not fix a bug that has been clearly identified on an expensive monitor that has not reached its EOL date in 2022. The firmware on P2422HE shows that it can be done. Further frustration is that there is no 2022 model year 24' QHD monitor that replaces P2421DC.

Hemmingway · Answer

Does anyone know if the firmware for the newly released monitor P2423DE would be compatible with P2421DC?

Monitors

P2421DC, MST, Source Input Autoselect

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