linux/Documentation/xtensa/atomctl.txt
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   1We Have Atomic Operation Control (ATOMCTL) Register.
   2This register determines the effect of using a S32C1I instruction
   3with various combinations of:
   4
   5     1. With and without an Coherent Cache Controller which
   6        can do Atomic Transactions to the memory internally.
   7
   8     2. With and without An Intelligent Memory Controller which
   9        can do Atomic Transactions itself.
  10
  11The Core comes up with a default value of for the three types of cache ops:
  12
  13      0x28: (WB: Internal, WT: Internal, BY:Exception)
  14
  15On the FPGA Cards we typically simulate an Intelligent Memory controller
  16which can implement  RCW transactions. For FPGA cards with an External
  17Memory controller we let it to the atomic operations internally while
  18doing a Cached (WB) transaction and use the Memory RCW for un-cached
  19operations.
  20
  21For systems without an coherent cache controller, non-MX, we always
  22use the memory controllers RCW, thought non-MX controlers likely
  23support the Internal Operation.
  24
  25CUSTOMER-WARNING:
  26   Virtually all customers buy their memory controllers from vendors that
  27   don't support atomic RCW memory transactions and will likely want to
  28   configure this register to not use RCW.
  29
  30Developers might find using RCW in Bypass mode convenient when testing
  31with the cache being bypassed; for example studying cache alias problems.
  32
  33See Section 4.3.12.4 of ISA; Bits:
  34
  35                             WB     WT      BY
  36                           5   4 | 3   2 | 1   0
  37  2 Bit
  38  Field
  39  Values     WB - Write Back         WT - Write Thru         BY - Bypass
  40---------    ---------------         -----------------     ----------------
  41    0        Exception               Exception               Exception
  42    1        RCW Transaction         RCW Transaction         RCW Transaction
  43    2        Internal Operation      Exception               Reserved
  44    3        Reserved                Reserved                Reserved
  45
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