The DATA_FC method provides the finest allocation granularity at the expense of logic.
As with PACKET_FC and RCB_FC, start with two registers, CPLH_PENDING and CPLD_PENDING (loaded with zero at reset).
1. When the user application needs to send an non-posted request, determine the potential number of CplH and CplD credits it might require:
NP_CplH = ceiling[((Start_Address mod RCB) + Request_Size) / RCB]
NP_CplD = ceiling[((Start_Address mod 16 bytes) + Request_Size) / 16 bytes]
(except I/O Write, which returns zero data)
2. Check these:
CPLH_PENDING + NP_CplH < Total_CplH
CPLD_PENDING + NP_CplD < Total_CplD
3. If both inequalities are true, transmit the non-posted request, increase CPLH_PENDING by NP_CplH and CPLD_PENDING by NP_CplD.
4. At the start of each incoming completion, or when that completion begins at or crosses an RCB without ending at that RCB, decrease CPLH_PENDING by 1. The number of RCB crossings can be calculated by:
RCB_CROSSED = ceiling[((Lower_Address mod RCB) + Length) / RCB]
Lower_Address and Length are fields that can be parsed from the completion header. Alternatively, you can load a register CUR_ADDR with Lower_Address at the start of each incoming completion, increment per DW or QW as appropriate, then count an RCB whenever CUR_ADDR rolls over.
5. At the start of each incoming completion, or when that completion begins at or crosses at a naturally aligned credit boundary, decrease CPLD_PENDING by 1. The number of credit-boundary crossings is given by:
DATA_CROSSED = ceiling[((Lower_Address mod 16 B) + Length) / 16 B]
Alternatively, you can load a register CUR_ADDR with Lower_Address at the start of each incoming completion, increment per DW or QW as appropriate, then count an RCB whenever CUR_ADDR rolls over each 16-byte address boundary.
This method is the least wasteful but requires the greatest amount of user logic. If even finer granularity is desired, you can scale the Total_CplD value by 2 or 4 to get the number of completion QWORDs or DWORDs, respectively, and adjust the data calculations accordingly.