OvmfPkg/XenBusDxe/TestAndClearBit.c

   1 /** @file
   2   Implementation of TestAndClearBit using compare-exchange primitive
   3
   4   Copyright (C) 2015, Linaro Ltd.
   5   Copyright (c) 2015, Intel Corporation. All rights reserved.<BR>
   6
   7   SPDX-License-Identifier: BSD-2-Clause-Patent
   8
   9 **/
  10
  11 #include <Base.h>
  12 #include <Library/SynchronizationLib.h>
  13
  14 INT32
  15 EFIAPI
  16 TestAndClearBit (
  17   IN INT32  Bit,
  18   IN VOID   *Address
  19   )
  20 {
  21   UINT16  Word, Read;
  22   UINT16  Mask;
  23
  24   //
  25   // Calculate the effective address relative to 'Address' based on the
  26   // higher order bits of 'Bit'. Use signed shift instead of division to
  27   // ensure we round towards -Inf, and end up with a positive shift in
  28   // 'Bit', even if 'Bit' itself is negative.
  29   //
  30   Address = (VOID *)((UINT8 *)Address + ((Bit >> 4) * sizeof (UINT16)));
  31   Mask    = 1U << (Bit & 15);
  32
  33   for (Word = *(UINT16 *)Address; Word &Mask; Word = Read) {
  34     Read = InterlockedCompareExchange16 (Address, Word, Word & ~Mask);
  35     if (Read == Word) {
  36       return 1;
  37     }
  38   }
  39
  40   return 0;
  41 }