Re: not able to drive ibm t221 3840x2400 screen with 81.78 (was ok with 76.76)
Based on your bug report, X is doing exactly what you've configured it to do. You're requesting 1216x2400 on DFP-0, not 3840x2400. DFP-1 is on a single link TMDS controller, and is therefore incapable of doing the 2624x2400 mode that you've specified in your Metamodes.
Perhaps what you meant is that you were getting a total of 3840x2400 across both DFPs in 1.0-7676?