I cannot set the ALIGN_COMMA_MSB attribute to TRUE to override the default "FALSE" setting. When can I use ALIGN_COMMA_MSB?
By default, ALIGN_COMMA_MSB is set to FALSE for all primitives except GT_CUSTOM, as the presence of a comma in the wrong byte lane will cause resynchronization. This causes the RXFIFO to reset, which ultimately leads to data loss.
The purpose of ALIGN_COMMA_MSB is to align bytes into words in the FPGA fabric.
As long as you ensure that you do not send commas in a byte lane other than the MSB, you can use the GT_CUSTOM primitive with ALIGN_COMMA_MSB set to TRUE.