I saw that,
1. Under these circumstances, it is acceptable for the Manager to then perform a NONSEQ or IDLE transfer that then effectively terminates the undefined length burst
2. When a master uses the BUSY transfer type the address and control signals must reflect the next transfer in the burst.
But I'm confused about this case:
HBURST is changed to INCR4 at T3, is this legal?
This is definitely NOT legal !
You can only change "control" signals such as HWRITE, HSIZE, HPROT, HMASTLOCK and HBURST at the start of a new burst, which means either when issuing a NONSEQ or IDLE transfer on HTRANS.
You cannot start indicating HBURST=INCR4 during a BUSY transfer because a BUSY is only used DURING a burst.
So in the sequence in your question, HBURST can change to INCR4 during time T4, but T3 (the BUSY) is part of the ongoing burst in T1 and T2, so HBURST must remain at INCR in T3.