RCB_FC Method - 3.4 English

Versal Adaptive SoC CPM Mode for PCI Express Product Guide (PG346)

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3.4 English
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.