Can you please verify which L4T version you are using? E.g., “head -n 1 /etc/nv_tegra_release”.
Looks like the Dell is listed as DFP-1. I’ll work on the assumption that the other monitor (DFP-0) is the one in question. Do know that having both monitors connected at the same time may cause behavior you would not see if only the one monitor is connected. The information from ModeDebug should still be valid, but it would be useful to know if the mode select behavior you noted above is with just the non-Dell connected, and if it is connected to the same port.
I am also working on the assumption that you want a 1920x1080 mode.
This is the final list of accepted modes for the non-Dell:
[ 11.380] (II) NVIDIA(GPU-0): --- Modes in ModePool for CMN (DFP-0) ---
[ 11.380] (II) NVIDIA(GPU-0): "nvidia-auto-select" : 640 x 480 @ 60.0 Hz (from: NVIDIA Predefined)
[ 11.380] (II) NVIDIA(GPU-0): "640x480" : 640 x 480 @ 60.0 Hz (from: NVIDIA Predefined)
[ 11.380] (II) NVIDIA(GPU-0): "640x480_60" : 640 x 480 @ 60.0 Hz (from: NVIDIA Predefined)
[ 11.380] (II) NVIDIA(GPU-0): "640x480_60_0" : 640 x 480 @ 60.0 Hz (from: NVIDIA Predefined)
[ 11.380] (II) NVIDIA(GPU-0): --- End of ModePool for CMN (DFP-0): ---
Obviously 1920x1080 isn’t there, so the next step is to see why. There are a large number of modes which are being rejected for this same reason, but specifically for 1920x1080 I see this:
[ 11.378] (WW) NVIDIA(GPU-0): Validating Mode "1920x1080_60":
[ 11.378] (WW) NVIDIA(GPU-0): Mode Source: X Server
[ 11.378] (WW) NVIDIA(GPU-0): 1920 x 1080 @ 60 Hz
[ 11.378] (WW) NVIDIA(GPU-0): Pixel Clock : 138.50 MHz
[ 11.378] (WW) NVIDIA(GPU-0): HRes, HSyncStart : 1920, 1968
[ 11.378] (WW) NVIDIA(GPU-0): HSyncEnd, HTotal : 2000, 2080
[ 11.378] (WW) NVIDIA(GPU-0): VRes, VSyncStart : 1080, 1083
[ 11.378] (WW) NVIDIA(GPU-0): VSyncEnd, VTotal : 1088, 1111
[ 11.378] (WW) NVIDIA(GPU-0): [b] H/V Polarity : +/-
[ 11.378] (WW) NVIDIA(GPU-0): Mode is rejected: Only modes from the NVIDIA X driver's
[ 11.378] (WW) NVIDIA(GPU-0): predefined list and modes from the EDID are allowed
[ 11.378] (WW) NVIDIA(GPU-0): Mode "1920x1080_60" is invalid[/b].
...
[ 11.379] (WW) NVIDIA(GPU-0): Validating Mode "1920x1200_60":
[ 11.379] (WW) NVIDIA(GPU-0): Mode Source: X Server
[ 11.379] (WW) NVIDIA(GPU-0): 1920 x 1200 @ 60 Hz
[ 11.379] (WW) NVIDIA(GPU-0): Pixel Clock : 154.00 MHz
[ 11.379] (WW) NVIDIA(GPU-0): HRes, HSyncStart : 1920, 1968
[ 11.379] (WW) NVIDIA(GPU-0): HSyncEnd, HTotal : 2000, 2080
[ 11.379] (WW) NVIDIA(GPU-0): VRes, VSyncStart : 1200, 1203
[ 11.379] (WW) NVIDIA(GPU-0): VSyncEnd, VTotal : 1209, 1235
[ 11.379] (WW) NVIDIA(GPU-0): H/V Polarity : +/-
[ 11.379] (WW) NVIDIA(GPU-0): [b]Mode is rejected: Only modes from the NVIDIA X driver's
[ 11.379] (WW) NVIDIA(GPU-0): predefined list and modes from the EDID are allowed
[ 11.379] (WW) NVIDIA(GPU-0): Mode "1920x1200_60" is invalid.[/b]
Simply put, the driver doesn’t like your 1920x1080@60 modes. I know 1920x1080@60 is itself well accepted most of the time, so there is a high probability that the pixel clock is an issue.
Can someone from NVIDIA suggest if modifying the mode tables in the kernel source would allow this, or if the modes are simply out of the safe range for the required pixel clocks? Long ago I remember a case where the pixel clock was basically in range, but due to floating point conversion ended up being rejected.