The RCB_FC method allocates and de-allocates blocks of credit in RCB granularity. Credit is freed on a per-RCB basis.
As with PACKET_FC, start with two registers, CPLH_PENDING and CPLD_PENDING (loaded with zero at reset).
1. Calculate the number of data credits per RCB:
CplD_PER_RCB = RCB / 16 bytes
2. When the user application needs to send an non-posted request, determine the potential number of CplH credits it might require. Use this to allocate CplD credits with RCB granularity:
NP_CplH = ceiling[((Start_Address mod RCB) + Request_Size) / RCB]
NP_CplD = NP_CplH × CplD_PER_RCB
3. Check these:
CPLH_PENDING + NP_CplH < Total_CplH
CPLD_PENDING + NP_CplD < Total_CplD
4. If both inequalities are true, transmit the non-posted request, increase CPLH_PENDING by NP_CplH and CPLD_PENDING by NP_CplD.
5. 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 and CPLD_PENDING by CplD_PER_RCB. Any completion could cross more than one RCB. 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.
This method is less wasteful than PACKET_FC but still gives an RCB granularity. If a user application transmits I/O requests, the user application could adopt a policy of only allocating one CplD credit for each I/O read and zero CplD credits for each I/O write. The user application would have to match each tag for incoming completions with the type (Memory Write, I/O Read, I/O Write) of the original non-posted request.