If you’re aiming for 1920×1080, there’s very little space for horizontal back&front porches. The default L5 mode (1536×240) has samplerate of 2046, so (2046-1536)=510 pixels is available for backporch, sync and frontporch. However, following active area adjustments are applied internally to achieve 1920×1080 format:
1. H.synclen is reduced by 120 (150->30 in default case), effectively increasing H.frontporch to 247
2. Required active area expansion to both sides is calculated: In default case (1920-1536)/2=192
3. H.backporch and H.frontporch are reduced by the amount defined in (2). In default case H.backporch 234->42, H.frontporch 247->55
4. V.active is reduced by 24, V.backporch is increased by 12.
Maximum valid H.backporch parameter (with default H.samplerate & H.synclen) would thus be 42+55+192=289, otherwise H.frontporch would get negative.
If you use 1600×1200 format and 320×240 mode, then all H.values are internally multiplied by 5 (so you can get effective H.backporch of 305) and only (1) of the steps above is applied. You can then manually do V.active-=24 and V.backporch+=12 if you want 1600×1080 output instead.